Technical Tips

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.

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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.

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Discussion:-

• • You will need a DAQ system that has the following features
  • Differential analog input channels to the ADC
  • Single ended analog output channels from the DACs

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.

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.

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 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 )

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: I have driven a Mitsuibishi variable speed drive with a USB to 422 converter directly (needed terminator resistors)
  • I have also driven a Siemens Air Conditioning Damper Servo Motor off a Laptop using a USB to 485 converter and a remote RS-485 6021 analog output module that drove the damper servo set point.

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.

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.

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.

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.

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.

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.

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.