Technical Tips
Technical...Application Notes

 How to tell if your SLA Batteries are at the end of their life
 UPS Specs -- Be careful
 UPS and PC Hibernation
 Grounding USB Systems
 NiMh versus SLA batteries
 Pressure Transducers --Try Honeywell Silicon models
 ADC Analog Input Pull Down or Pull Up Resistors
 RS-232, 422 and 485 ... Laptops without Serial Ports and PCMCIA
 Measuring Temperature ... With Low Cost USB DAQ Modules
 
 Measuring Mains AC Current ... With Low Voltage DAQ Modules
 Measuring Mains AC Voltage ... With Low Voltage DAQ Modules
 Laptops -- Solving a Laptop problem -- Noisy In-built Audio Systems
 Using LabVIEW with several MCC USB DAQ Modules
 LabVIEW Background DAQ with MCC DAQ
 Batteries ... Making SLA Batteries last longer
 AC Surge Protection ... Protection for All AC Powered Systems
 OPTO 22 ... Protection for PLC and RTU's I/O systems
 Choosing your UPSs
 How to remove analog input offset signals from a DAQ systems
 Using PC DAQ systems with Differential Inputs
 
 Calibration in PC Based DAQ Systems
 Power Supplies for Micro based systems
 UPS Systems
 DAQ Systems






How to tell if your SLA Batteries are at the end of their life

SLA Batteries -- Sealed Lead Acid Batteries have a finite number of discharge and recharge cycles in the situation they are put in. Things like load on discharge and temperature of site has a big impact on battery life time.

So how can you tell if your SLAs are close to life end.

1/ One of the simplist way after the discharge cycle, is to watch the recharge cycle, if it is prolonged and the battery gets hot the SLA is close to the end.

On a UPS or a Charger this can be done simply by watching the charge LEDs. Some UPSs and Chargers show the state of the charge -- where it is up to by colour, if you have meters this is even easier.

A tell tale is if the charger cannot take the SLA to an equaliser voltage of say 14.5VDC easily the battery is failing.

2/ Conversely, if after discharge, the charger takes the battery to full charge in a short time, the SLA is not taking a charge and has a high impedance, however usually this will show up in the discharge cycle with the battery voltage dropping substantially on discharge due to the high impedance.

3/ If you have the time, watch the discharge cycle and see how long it lasts. With a UPS or alarm this is easy to do, just pull the AC power off the system and watch the time until the UPS drops the load. Over the life of the battery this should be quite constant only dropping 20-25% before the battery enters its rapid degradation period and fails.

4/ A battery impedance meter will tell you quickly but they are expensive.

5/ If you have a string of batteries a quick indicator is to measure each batteries voltage on charge, if there are great differences, then their are batteries in there that have failed or are failing. High voltage indicates a battery with high impedance usually, low voltage indicates a cell or cells have failed.

The same is true for discharge cycles. The one with the lowest voltage is the weak one.

MasTec has a large supply of SLA in stock and can supply most times that day or next day for your needs. Ask us to quote on your systems.

We will also fit SLA batteries to your system at site or at our service center.

Last point, if you have failed SLAs or other batteries -- Please dispose of these sensibly -- call a metal recycler, do not them put in the rubbish. Lead is a nasty metal to get in to the environment.



This month we are going to discuss UPS specifications.

There are always some new distributors for UPSs coming on stream and you need to be very careful in reading their specifications on their UPS.

This is important to know how to compare UPSs and know what the specs mean.

Most of these newer power distributors are not really knowledgeable, they have just added UPSs to their product line as an extra sale item and the sales people in these organisations do not really know about UPS power at all.

The main buying criteria for a UPS is how much power can it put out and secondly how long can it do this.

Take for example one low cost 1700VA UPS that is being sold in NZ right now, it seems an excellent buy but read the specs carefully and you will find out that this UPS is actually a 1000 watt UPS with a PF of .6. (power factor)

This is an old way some manufacturers continue to use to trick the competition, even though computer power supplies and monitors are now running on PF .8 or greater.

So if you take the same UPS, 1700VA unit, and rearrange the spec a little, it really is a 1400VA UPS, not 1700 at all. (1000/.7) = 1400 approx or even (1000/..8) = 1250VA

So now you need to compare it with UPSs with a PF of .7 and are 1400VA. Suddenly what seems like a good buy is OK or ordinary but this is how sales are made to inexperienced buyers.

So please be very careful when buying power systems, you can almost bet that the sales person doesn't know these points either.

MasTec UPSs, except for some older design, Alpha and PK models, are all PF.7 or greater specifications now.

Next we look at batteries and runtimes.

Many of these over speced UPSs fail in runtime also because they have lower battery capacity, just to keep the price down. Batteries make up about 30% of the cost of the UPS and putting a little battery in, will save some dollars.

So for instance, I noticed that one 2000VA UPS had qty four 12VDC 7 amphr batteries. This is 330 watthrs of battery ( 12*7*4)

So if we take a UPS like this and load it up to its 1200 watt limit, how much time do you get? Well only about 50% of the energy in the batteries will come out if you are very lucky, so we have 165 watthrs and the inverter is usually 90% efficient if you are lucky, so really only 150 watthrs can come out as AC.

Runtime is 150/1200*60minutes or 7.5 minutes and that is on new batteries, give them a year and you are down to 5 minutes, certainly not enough time to shutdown a server.

So again be very careful what you buy.

Ask our staff to help with all of this and get what you require to do a good job.

Hope this helps.


UPS and PC Hibernation

This is a note we developed back a few months and others are starting to ask for it.

How do you make a PC and UPS hibernate when the AC fails and then auto recover when the AC power returns.

Note this is different than a UPS that will autostart when the AC power returns. For instance the Power Bank IP series will restart when the AC power returns.

Hibernation is a software based system where on AC Power Failure, the work on the PC is saved as a mirror file and then the PC is powered down and then the UPS actually goes to sleep, not fully depleteing its battery. On AC Power Restore the UPS powers up, starting the PC adn thePC then restores the software and continues running where it left off.

Link to Hibernation Note


Grounding USB Systems

Well I thought I has seen it all until last month I was commissioning a Smartworks project that used USB DAQ modules from Dataq.

We don't list Dataq yet, a page will be up shortly. Very nice product.

What happened was I found the DAQ system was very unstable, in fact it would not install and run on a Dell PC at all, it just kept crashing the driver and a reboot would bring the same result. Naturally I thought it was the driver, so did the usual new driver etc, no difference.

I changed PCs and had better results, but still the DAQ unit continued to fall over now and then, like run for an hour and then stop, and then run for 10 seconds the next, very strange.

In desperation, not really just to see what would happen, I took a ground line or earth line from the USB module directly back to the PC chassis and bingo there it was stable and fine and has been since then.

So, don't trust USB cables to make good grounds, and it varies greatly from PC to PC, the Dell was poor a Taiwan General PC was stronger.

Now admittedly this was a tough site, industrial, lage power factor, loads of harmonics on the AC lines and better still I was driving a VSD and large electric morot only meters from the PC.

Once grounded well the DAQ unit was fine, I suspect RF noise, Ground Noise, Ground loop etc.

 


NiMh versus SLA batteries

We recently built a mobile DAQ system for a client and the unit was powered by SLA or Sealed Lead Acid Batteries.

Our rational for the SLA use was ease of use, easy to charge, reliable, lower cost and robust.

However, after they had been in the field a while, we receive a call saying the SLAs were having difficulty holding charge and seem to be loosing capacity quickly.

We then realised that the SLAs were getting hammered due to very deep disharges, the field team were letting the SLAs go down very deep and we had no auto shutdown on the device to save them from this.

The manuals and experience tell us that SLAs fail very quickly if discharged deeply often and this is what was happening.

It turns out the real reason was not just that, but the batteries had been speced with enough capacity for a day and a little more, allowing for recharge each night. As the capacity fell this criteria could not be reached easily and the batteries were progressively discharged deeper each day until they couldn't even do a days work. Then a short charge was made to finish the day and the number of discharge cycles grew rapidly, leading to the rapid end of the life.

A decision was made to use NiMh technolgy, as the battery holder size couldn't be enlarged for a larger SLA. In doing this swap of technology the cost was higher but the capacity of NiMh is vastly greater for the same volume and lead to the use of a small size battery, still with larger capacity with excellent results.

Now there is enough capcity for the device and recharges are quicker if required.

The specs say the charge/dsicharge cycle count is much higher than SLAs and it has lead to and excellent soliution.

I can see the day, when the costs are right, UPSs and many other devices shrinking in size as NiMh technology takes over from SLA for light power work.

If you are doing mobile products with SLAs, do have a look at NiMh, it may slove problems similar to the above one.


Pressure Transducers --Try Honeywell Silicon models


MasTec has been using Honeywell Silicon pressure transducers sine 1994 with great success.

If you need to do Absolute, Gauge or Differential pressure measurement with better than 1% accuracy then the lowest cost way is the extensive range from Honeywell.

The trick is signal conditioning these units. Some models are temperature sensistive and need some correction to get the best results.

Fortunately, Honeywell has some excellent low cost circuits that fix up these low cost transducer to make they very usable. MasTec will shortly have a range of PCBs for this purpose also.

For a very quick and easy solution that is remarably accurate, use a Data Forth 5B38-07 signal conditioner module on a DIN Rail backplane. This method doesn't have temperature correction built in but for short term measurement is excellent and better than 1% is possible over almost the complete range of the transducer.

We use a two point calibration and y=mx+b calibration with excellent results.

See the Honeywell Sensor transducer page new for this month.



ADC Analog Input Pull Down or Pull Up Resistors


When building ADC systems, it is a good idea to put pull down or pull up resistors in the lines of each channel of the ADC.

I prefer pull downs, I use a 1 or 10meg resistor ( this depends on the output impedance of the channel signal source , you do not want to load the source).

If you have pull downs or pull ups, you can write software that will detect if the channels are connected or not. It is very rare that a analog source is zero or maxed out and by reading the ADC channels at boot time you can report to the user what is connected.

This technique is like the standard open Thermocouple pull up resistor which in most cases is 10 megohms and if the TC opens the channel will be pulled to max input and the software catches it as an open TC.

On many analog systems with long analog fed lines to each channel, I will put a 1 or 10 meg resistor as a pull down right at the plug into the ADC unit and put across the resistor a small ceramic cap, say .01 mfd as an RF filter.

Last point -- having ADC lines just open floating is quite dangerous, as the ADC multiplex may receive a large static discharge shock which may compromise it.

Even new ADC systems that have an ADC Chip for each channel, are vunerable to an open channel lead, even with all the silicon protection.

Tie the leads to ground or the PSU +ve rail and it will safe guard them.




RS-232, 422 and 485 ... Laptops without Serial Ports and PCMCIA


So you brought one did you, one of those new Laptops that is USB based only ( NO Old RS-232 Serial Ports) and you need RS-232, 422 or 485 for a project. Don't sweat.

We have had this problem in several projects over the last few months and we solved it very well with the ADAM series USB to 232, 422 or 485 converter.

Some recent Laptop Projects:

Advantech, Adlink and Topscc all make very successful models which we stock.

Nice feature:- they are self powered from the USB port (which is better than the RS-232 to 422 or 485 models)

So no power supply is required, rapid install and very easy to use, transparent, they come up in XP as a Comms port of course.





Measuring Temperature ... With Low Cost USB DAQ Modules

You want to build a low cost temperature monitoring system using a low cost USB DAQ module.

MasTec has been doing this type of product for several years using the MCC USB-1208LS and the LabJack U12 and you can to, very easily.

We use a Semiconductor Temperature Probe we make, with an LM35 or LM34 semi conductor chip and stainless steel probe well. read more connected to the USB DAQ analog input port.

The LM series temperature chips can be powered by the USB DAQ units 5 volt supply.

This example is unsing single ended analog Inputs with a fixed gain, but if you want to do some fancy programming and wiring you can configure the DAQ units with differential inputs which have programmable gains by connecting the LM output to the +ve of the differnetial input and connect the DAQ units Analog output to the -ve of the differnetial input and supply an offset to counter offset the LM output so as to expand the range and gain to increase the accuracy and resolution considerably.

If you want us to contact you about these points click this link




Measuring Mains AC Current ... With Low Voltage DAQ Modules

You want to build an AC Mains Current Monitoring System using a low cost USB DAQ module.

This was received by MasTec from Steve Brewster, in response to last mounths AC voltage article. Thanks Steve.

With regard to current monitoring:

I believe it is necessary to use a terminating resistor with a current transformer or the secondary voltages can become quite high if the secondary winding is left unterminated. The output currents are usually 0 to 5 amps for whatever range input current is chosen, so if you want to measure 100 amps, get a 100 amp current transformer, and terminate the secondary with a 0.2 ohm secondary terminating resistor of 5W rating (would be best) and would give 0 to 1 V out. (or 0.02 ohm for 100mVolts )

If you want us to contact you about these points click this link




Measuring Mains AC Voltages and Current ... With Low Voltage DAQ Modules

You want to build an AC Mains Voltage Monitoring System using a low cost USB DAQ module.

And you are scared to build a voltage divider to make the 230 VAC low enough for the DAQ unit.

Well, another way to go, is to use a low cost transformer.

I find a simple 6.3 VAC mains transformer suitable as the voltage divider and safety isolator. At 230 VAC they put out +/- 8.9 VAC pk/pk. Even at 255 VAC they are under the +/- 10 V pk-pk for the DAQ unit.

If you do gets mains that goes over 255VAC, use an Australian 240 VAC, transformer, not 230 VAC, as it is in NZ. Just choose a transformer with a 5VAC secondary or switch the DAQ unit range to +/-20 VAC and use a 12 VAC transformer, even at 270 VAC you will be less than the +/- 20 range.

The transformer is wired up so as to feed a single ended +/- 10 VDC input range channel on the DAQ unit. Use a LabJack or MCC 1208 etc. and you can make a very low cost voltmeter.

With a little extra software you can make a Harmonics Analyser etc. These small transformers are quite flat up to about 10KHZ, even up to 20KHZ, they are very similar in size and construction to Audio transformers used to drive speakers in tube radios years ago. If you want to you can calibrate the transformer and apply a simple polynomial correction when you run the DAQ system.

Now for current, use a current transformer that will give out the correct voltage and if they don't use a transformer again to step down. A little software and you have an excellent current monitor also.

Hope that is useful. The main advantage is the safety, with great isolation, it allows you to use a single ended DAQ unit safely with out a whole heap of questions about have you got phase and neutral right etc.

If you want us to contact you about these points click this link




Laptops -- Solving a Laptop problem -- Noisy In-built Audio Systems

This discussion was spurred by many people, all with the same Laptop problem.

Problem:-- When a Laptop that is running off the AC Power Supply and is connected to a recording or play back desk, often the digitisation noise on the ADCs and DACs is higher than when running the Laptop off batteries.

This is a common problem. The same noise problem may also be there with the Laptop standalone, you can usually hear this with a good set of head phones.

Try this, plug the laptop into its power supply, connect up the Recording Desk or Playback Desk, play some music and then disconnect the power supply while it is running and let the laptop continue running off the battery. The base noise may change considerably. If it does you have the same problem many are having.

This added noise can be from several sources, Ground noise, DC noise, Charger noise and RF noise.

Laptop power supplies are generally nasty little beasts, that make nasty DC and lots of RF.

How to fix this noise problem:-This is not definitive, there may be more to do, to solve it.

Here are some suggestions for you to try.

Isolate the problem:- Is the noise from the power supply ground??
Plug the power supply into the laptop and run up some music or recording work on the desk, then turn the Laptop power off at the wall but leave the power supply connected to the wall and run the laptop off its battery.
Is the noise still there? Yes =Ground Noise usually.
Unplug the Power Supply from the wall, does the noise go?? Yes = definitley ground noise.

Isolate the problem:- Is the noise from the power supply DC or RF??
Plug the power supply into the laptop and run up some music or recording work on the Desk, then unplug the laptop dc cable side to the laptop and run the laptop off its battery.
Is the noise there? Yes= DC supply ripple or RF noise.

Isolate the problem:- Is the noise RF from the power supply ??
Plug the power supply into the laptop and run up some music or recording work. Get an AM and FM radio and place it close to the power supply. Tune across all the AM and FM bands.
Is there interference noise on the radio? Yes= RF Noise maybe.

Isolate the problem:- Is the noise from the power supply charger??
Take the battery out of the Laptop. Plug the power supply into the laptop and run up some music or recording work.
Is the noise there? Yes= The Laptop charger may be making the noise.

If the problem is Ground Noise:-
What you can do to remove the ground is get "a ground cheater connector" that will lift the ground off the power on the laptop PSU. It should still be safe as most laptop PSUs are double insulated from the case. Actually many laptop PSUs have no ground, just two AC pins -- phase and neutral. If this fixes it, you are home free. The audio system can now be the ground for the laptop. Just make sure the Laptop is always connected to the Audio system and unplug the Laptop power from the wall before moving it around.

Another way to fix Ground Noise:-
The cheapest and easiest way to isolate the laptop off ground is to use an isolation transformer, to let the laptop float on the Audio recording or playback systems ground.

If the problem is RF Noise:-
We can supply a RF quieter Laptop Power Supply or you can wrap the power supply in foil and ground the foil and try again. Be carefully with this as the supply may get hot, most do not have airholes and are sealed but they will get hot in foil.

If the problem is DC power supply ripple or noise:-
We can supply a quieter Laptop Power Supply or you can build a filter for the DC side of your supply. In a small box bring the DC from the PSU in and then on a small board build the filter, use a torroid inductor, .1 mfd high energy cap as a high frequency filter and then place a good size Cap 1000mfd or so across the DC line after the toroid to act as a reservoir to take out the noise. You may like to add a Tantalum about 1mfd to take out mid range noise.

If you want us to contact you about these points click this link



Measurement Computing ... Using LabVIEW with several USB DAQ Modules

All MCC USB DAQ products can use this Software Tip.

Continuing on from last months discussion about using USB DAQ modules inside LabVIEW in Background Tasks.

Just in case you don't know the difference between fore and back ground tasks.

In background the DAQ unit can collect data without the computer (CPU) having to sit and wait for data, the data collection goes on in background time using a double buffer in RAM and the program can read this at will and usually at slow rates by loading large buffer loads at a time into a foreground task for processing. This method allows the computer system to collect DAQ data and do computation, watch the operator interface, service other types of I/O all at the same time. It is multi tasking but the DAQ tasks are in background.

Note :- In LabVIEW -- any foreground DAQ VIs will wait until the Data Collection is complete, locking the computer to that one VI or task, it the CPU sits, waiting for data and possibly for a long time, if the data collection is long. Background DAQ gets around all of this.

Now with USB there is no DMA, direct memory access, as there is with PCI or ISA slot DAQ cards. So this means that with USB DAQ the CPU does have to shovel the data across the cable so it is not entirely CPU free DAQ, but the main point is the background VIs allow the CPU to free itself from the background VIs or tasks and do other work and it is this feature we are using for this example.

So why are we interested. Well we have a project coming up shortly that requires many USB modules to be connected to a PC, possibly 6 to 8 modules, total I/O count is about 80+, and is mix of analog, digital and counters.

The analog inputs are in groups that are going to run at different sampling rates, some in KHZ, some hundreds HZ and others very slow just HZ, the digital in and out have to be real time live and in the foreground and all of this happening at the same time.

So we did some background investigation work to see how far you can push USB with Multiple MCC DAQ modules.

We started by using the same LabVIEW program discussed in the last month's Newsletter and with some additional code, a multi module USB background DAQ VI was built.

To test the software, we used two MCC USB-1208LS modules for analog DAQ and we were able to make each USB-1208LS run in background at full 1KHZ sample rates through one USB 1 port, using a USB 1 hub and we thought that was pretty cool.

The CPU % was about 8% which was quite good, considering all that was going on down the USB bus and the USB Driver, to make it all work cleanly. There are a few little tricks but it is quite straight forward.

Now if that wasn't enough, at the same time it was possible to run in a foreground VI or Task, the analog and digital I/O in another USB-1208LS module without hiccups. So we had three USB modules running into a hub and then into one USB port.

The MCC driver seems to be solid and some how allows all this to happen on one USB port. In our final version for the client we anticipate using a PC with 4 separate USB ports and bring the high speed data through a USB 2 port.

If you want us to contact you about these points click this link


Measurement Computing ... LabVIEW Background DAQ

All MCC DAQ products can use this Software Tip.

This work has been tried on both USB and PCI DAQ products from MCC.

The reason for the development of this Tip, came from some work MasTec was undertaking recently where we seemingly ran into a brick wall and could not make the Background VI set in the Universal Library for LabVIEW run correctly.

We asked MCC for some help, after describing the problem and we got back from their excellent support staff the next day, two super examples of how to do it, so we cleaned one of them up and we are now offering it to you and anyone else that may need this sort of useful software technology.

Description:-

You want to collect analog data in backgound, for some reason -- in our case because we needed the CPU in foreground to do other work asynchronously with the background task. In fact we had several foreground tasks that were loosely coupled together and running on different time lines, in fact they were driven by events rather than time.

So click on this link to get to the MCC DAQ LabVIEW Forum and Down load area. Background Data Collection VI

If you want us to contact you about these points click this link


Batteries ... Making SLA Batteries last longer



Panasonic, Exide, CSB and GCB

First point about SLA= Sealed Lead Acid Batteries, they only have so many charge and discharge cycles built in.

The number of cycles is determined often by how deep the discharge is on each cycle.

So a simple rule is to put in bigger batteries than you require, so you can still get the energy you require during the dicharge but without a deep discharge cycle.

It is quite easy to work out battery capacity, battery volts * amphrs= watt hrs. You will not get all these watthrs out of a battery ever, the larger the discharge current the lower the capacity of the battery goes due to loss of energy due to internal resistance battery heat. The bigger the battery, usually the lower the internal resistance is, hence you win more capcity if you have larger batteries due to less heat dissipation on discharge.

Next key decider on battery life is the charging method and the end voltage set point. We hve found that SLAs will last almost twice as long if you charge them to 13.6 VDC rather than 13.8-14 VDC. So lower the charge stop set point, you do not loose a lot of capacity doing this.

The rule with SLAs on charging levels is to be gentle, charge to quickly you make the battery hot from both internal resistance heating and also chemical heating.

And this bring us to the last major decider on battery life, temperature. If you put a battery system on concrete floors and in a cool room you will probably double their lives over being inside a warm/hot UPS/Inverter/Instrument. We know that SLAs do not last as log in the Pacific Islands as in NZ due to temperature. If you can, have external batteries in a cool area on concrete, as long as if doesn't freeze. Also keeping the equipment in a cool room will make them last and perform better also.

Consult the battery manufacturers spcification sheets, you will be educated rapidly.

If you want us to contact you about these points click this link


AC Surge Protection ... Protection for All AC Powered Systems



TrippLite Surge Protection

AC surge protection is essential inside all buildings in NZ now. There is so much expensive equipment in houses and buildings, all hoping for clean AC power, but alas it is not that way, so save your gear from being done over by rogue AC.

At the MasTec office in East Tamaki Auckland, we regularly get wacked with over voltage surges and also a strange condition where there are 1-2 second AC Power Holes and then a high voltage surge after it.

The reason we know this, we have UPSs and line conditioners on every computer and they all switch on these conditions.

Above is a picture of the best surge protector available made by TrippLite.

For those who understand, here is a short note of the protection circuits in these devices.

These have a circuit breaker for overload, then MOVs (metal oxide varistors) across Phase-Neutral, Neutral-Ground and Ground-Phase. Then across the phase-neutral a high voltage cap for noise suppression.

Then after these initial circuits, they have for each pair of AC outlets, a ferrite donut inductor/choke with caps and MOVs. This circuit isolates each AC Outlet bank-pair from the other pair, in case there is noise coming out of a faulty device on the opposite outlet pair. True power protection both ways, from the mains adn from devices.

MasTec also has line conditioners that regulate the voltage of the mains to stop brown outs and over voltage.

The next line of defence after that is UPSs and we carry a large range for every application.

If you want us to contact you about these points click this link


OPTO 22 ... Protection for PLC and RTU's I/O systems



This may sound rather odd to say protect PLC and RTU's I/O systems. Aren't they meant to be industrial tough anyway??

Well, talk to most controller engineers and they will tell you that in many situations they are not, and why risk having to buy an expensive I/O section if there is a low cost protection systems available off the shelf.

OPTO 22 I/O Protectors are safe and isolate to 4000VAC, actually tested to 8000. Life time gaurantee.

Download a note covering all the options.

MasTec has many thousands of OPTO 22 protected I/O points running in NZ.

This is your lowest cost protection available, and we stock in depth all models.

Don't get caught again.

If you want us to contact you about these points click this link


Choosing your UPSs

 

Discussion:-

After selling UPSs now for 13 years, we have noticed, there is a tendency for most UPS purchasers to buy lowest cost, buy close to maximum rating or buying just enough for now.

Possibilities:-

MasTec sells $50 UPSs suitable for a single non critical PC workstation with an LCD display. Is this type of UPS suitable for the accounting computer? Maybe not. Maybe a higher rated UPS is suitable with bigger batteries to allow ordeally shutdown without out loosing any work, in fact maybe complete the page you are on before you shutdown.

For servers, usually a Sinewave UPS is adviced, not necessarily an On Line UPS but a Puresinewave, as a sinewave UPS is better at driving a complex load, a server, an ethenet switch, maybe an Opengear IT Infrastructure system etc.

UPS types are: Backup, Line Interactive, On Line, Redundant, Parallel.-- some with extrnal battery for long runtime.

UPS styles are: Rack Mount, Floor Mount, Wall Mount, PC Disk Bay Mount

Sizing UPSs:-

Think of the application.

Is this always going to be single Workstation UPS? or will there be two accounting computers this year??

Is this always going to be single Server UPS? or will there be another server this year??

It is lower cost usually to buy a bigger UPS than small ones, the price drops away on large ones.

So buy bigger if you know the future needs will grow.

Sizing UPSs:-

Never run a UPS close to max rating continuously. It might happen that the UPS loads 100% at start up but at run time is 70%. This is good.

It will do this for the UPS if you buy with a reasonable rating.

Longer life on the UPS
Longer life on the UPS batteries.

Longer runtimes == more security

Less maintenance
Lower running temperature

So think about what the UPS is for: it is security and buying wisely will give good returns.

If you want us to contact you about these points click this link


How to remove analog input offset signals from a DAQ systems

Discussion:-

You will need a DAQ system that has the following features

Required:-

You have converted 4-20mA signals to voltage signals using shunt resistors across the ADC analog inputs and you now have voltage levels of 1-5 VDC feeding into the analog input on the ADC.

You want to make it so the ADC actiually see the range as 0-4 VDC without using expensive conditioner modules etc.

Here is the Fix:-

Feed the 1-5 VDC signal into the +ve terminal of the differential input of the ADC MUX. Ground the 4-20mA signal source to the DAQ card analog ground terminal.

On each of the -ve differential input on the ADC MUX, connect an analog output -- from the DAC.

Set the input signal on all ADC input channels to 4mAs and scan the input voltage of all the channels through the ADC with the DAC output set to 0 volts and then keep the ADC conversion numbers in an array.

When it comes time to take reading in the future without the ADC seeing the offset voltages, before taking a reading with the ADC, set the DAC output to the value in the array indexed to the channel. Doing this removes the offset of 1VDC to 0 VDC.

The reason is the differnetial amplifier in the ADC does a subtraction for you, offset in +ve terminal -offest on -ve terminal = 0 VDC.Simple trick really but handy.

Note the 1-5VDC will only be approximate as each channel will be different. This method will take in to consideration the 4mA input variations. To actually get a truly calibrated system you will also need to take into consideration the variations in the 20mA signal levels and fix this.

This can be done by another initial scan.

Set the input signal on all ADC input channels to 20mAs and scan the input voltage of all the channels to the ADC with the DAC set to 0 volts and then keep that numbers in another array or make a matrix for 4 and 20mA levels.

If you want us to contact you about these points click this link


Using PC DAQ systems with Differential Inputs

Discussion:-

First time users of DAQ system almost always have problems setting up the differential inputs to work correctly.

Almost all PC DAQ systems, whether PCI, PCMCIA, USB boxes and other devices have a differential input mode. However they are ground reference differential inputs and this is where most people get unstuck. The differential amplifier in the DAQ systems must have a circuit where it can obtain return bias current from the differential floating source ( Thermocouple, transducer, circuit etc) referenced to its ground circuits, otherwise the differential amplifier's output will just be floating, referenced to nothing particular ( could be unknown static voltages, bias voltages etc ).

This is how the phone call to us usually goes, " Yes connected up the Diffferential signals blah blah blah and the output values off the ADC are just floating all over the place.

Here is the Fix.

Each differential input must be equipped with a bias resistor referenced to analog common, usually about 100K Ohm.

The chart below shows how to connect most single and differential input DAQ systems, including NIs NRSE DAQ amps.

If you want us to contact you about these points click this link


Calibration in PC Based DAQ Systems

Which Way to Calibrate

After many years of building computer measurment systems, (about 37 years now), it has become clearer and clearer over the years to calibrate the complete sub sections of the measurement system all together. To do this use a software method rather than calibrate each sub section individually on the bench using a calibration test rig and then enter into the software the calibration data for each sub section.

Now to expand on that idea. Apply several known calibrate signal to a transducer or measurement system, (eg say a load cell, several known certified calibrated loads ) and then using the PC do the calibration using the DAQ system in the computer using a specific built calibration function or routine in the software. By doing this type of calibration technique, it will take in to account all the subsections gains and offset factors. From this you can develop if linear a Y=mX+b equation to correct all data received from the transducer or if non linear a polynomial equation to correct any data received.

Using this method the offsets in the transducer, signal conditioner, cabling, DAQ mux and amps and also the ADC are all linked together for the one time calibration.

For most linear systems, it is always best to do at least 3 point calibration, for non linear systems, 5-7 if possible for good polynomials.

For linear systems it is then not necessary to actually zero the signal conditioning systems and you can use a PC controlled auto zero system in software, that just reads the input at zero input and what ever the offset is, subtract that and then apply the data received through the y=mX+b formula.

This is not true for non linear systems, always zero a non linear system at the signal conditioner so the conditioners, amps DAQ etc are working in a known calibrated way.

If you want us to contact you about these points click this link


Power Supplies for Micro based systems

 

Use a Good One

That is the idea in a nut shell. Micros, especially high clocking rate, modern CPUs require clean, steady DC power.

In the last 14 years of selling micros, almost the only times our models do not work out of the box, is when a noisy, usually low cost power supply is connected.

Reliability and Stability

So if this is true, and you actually get them to work on the bench with a low cost supply, please do characterisation on the complete system including the power supply you will ship with the device before marketing and selling.

When someone in Australia plugs in your device and it is 35-40 degrees, you may get the phone call.

The most common cause of problems is low cost switched mode supplies, linears hardly ever cause problems for small loads.

Also never skimp on bypass or filter caps, a few cents scattered around correctly, solves many of these issues.

We can supply good power supplies for you designs, we have a large range of units from PowerTech, Innovative Energies, Artesyn, Mean Well and other reputable and long standing manufacturers.

If you want us to contact you about these points click this link


UPS Systems

A Very Good Start

Before you connect your new UPS to the computer etc. Always Do This. Charge the UPS battery, by plugging the UPS into the AC mains and leave it charging for at least 8 hours. This will give the UPS battery an excellent start in it’s service for you.

Gently with the Batteries

After the charge, connect the computer to the UPS. Next, power up the UPS and then the computer, then turn the AC mains power off at the wall. The computer will now run off the UPS battery. Verify that everything on the computer runs correctly and then turn back on the wall AC mains. Don’t run the computer too long off the battery, always be gentle to the battery and it will be your life line when you need it. Don’t see how long the computer will run off the battery and let it run until it stops, if it runs for 5-10 minutes that should be sufficient for any shut down procedure except large servers. Battery life is determined by several factors but the biggest factor is how badly abused they are, deep discharges will shorten the life.

more...

If you want us to contact you about these points click this link

 


DAQ Systems

Differential and Single Ended Configuration Confusion

Most DAQ systems that have configurable Differential and Single Ended input MUXs, come configured for differential mode. This is a trap for many users, as they wire up thinking they are hooking to a singled ended ground referenced systems and then discover the digitised data drifts around uncontrollably due to the spare open floating differential input. So check this. The configuration software will allow you to select either Diff or SE. So with MCC products use Instalcal to config and NI use MAX etc.

Offsets on analog inputs in DAQ System

If you are having difficulty getting full resolution out of your DAQ system because the input signals have an offset, use this little trick. It only works if you have a differential input mux and a bipolar analog output DAC and a single ended input signal. Tie the analog output to the other side of the differential inputs and null the offset off to zero using the DAC output, you will then get full resolution. If each input has a different offset, tie all the -ve diff inputs to the DAC and build an offset table in software for each channel so when each channel is selected the offset is now nulled to zero and you get full resolution.

If you want us to contact you about these points click this link

  © MasTec Ltd