This is translated archive post from my polish language blog. I will be successively post translated version of some of my archives.
We ended up purchasing an AVT2857 kit to mount it as a voltmeter and ammeter for the power supply. It is really interesting circuit, indeed, a construction that shows on how difficult it is now to have a good electronic designer in Poland. I study the kit instruction briefly and starting to swear. It has a reference voltage of 2,047V from the voltage divider(!!!). It measure voltages within the measuring range from another voltage divider…. oh crap. The prepared tape transformer for the power supply, together with the bridge, came back to the drawer, and in the second drawer I beginning to search for a stabilized power supply, because of the voltage drops on the bridged transformer. I swear badly.
Fortunately, I used to have an external HDD enclosure. The enclosure has a power supply, pretty good, +5V and +12V outputs, 2A on each…. Enclosure lies down and hesitates, so after thinking a little, I grab a modeler grinder and I started cutting the casing. This is how I solved the topic of powering this voltmeter circuit. I can start to calibrate the voltmeter circuit, and since I have ALDA brand, model DT-830B multimeter from the early 90s, I did not expect any trouble ;).
And I fall back to the reference voltage. Someone came up with the “brillant” idea that 2.047V will be a good reference, where popular multimeters in our country (read cheap) have a measuring range of up to three significant numbers up to the 2V range, so I won’t be able to calibrate it precisely. So I wonder, was the creator of this kit sane? Probably that is how he ended using random selection of elements (but cheap), instead of thinking… I struggled, I was carefully turning the calibration potentiometer until the display jumped from 2.01 to 2.00, and I made a little movement down. This should do the trick, setting reference voltage in truly redneck style. The next step is to calibrate the individual range inputs from the divider. In the manual, I read:
“(…) provide supply voltage of known value, e. g. 5.00V to the measuring input. It can come from an adjustable power supply or an external voltage divider. At input 23 (PC0) You should set 1:10 of the provided voltage – in this case it will be 0.50V. At input 24 (PC1) , by using PR4 potentiometer, we should set voltage to provide 1:100 of the measuring voltage , in this case 0.05V. The calibration of the differential input is performed in a similar way. On inputs 26 (PC3) and 27 (PC4) potentiometers PR5 and PR6 set the standard voltage at 0.50V-1:10, which in this case should be set to UA input and then UB input”.
Is it that simple? I don’t think so. There was an opportunity to test the power supply, but after carrying out the calibration, folding the system, the differences between ALDA and the kit were gigantic. And then I read that although it is possible to reduce the inaccuracy of the reference voltage, by setting the reference point programatically, but:
“We must remember, however, that we will not be able to obtain ideal readings across the entire range of voltages because of the existing non-linearity of the A/C converter”.
That was killing me…
I was thinking a little on this information, finally I decided to buy a decent multimeter. My choice was Kyoritsu KEW1009, mainly because of price, and I preferred to buy it rather than popular Chinese UNI-T. Of course, Kyoritsu is also Chinese product, as the sticker on the bottom of the meter tells us, but the company is generally Japanese, and the bottom of the meter tells us its designed and qualified in Japan. I ordered it at tme.eu and I can truly recommend this company as a reliable supplier.
I check the reference voltage with new meter and it revelas, that I set the voltage on the divider with an error as low as -0.004V, despite the missing third significant digit on my ALDA meter. Excellent results! And yet the kit multimeter continued to show nonsense, only this time I had confirmation from two independent sources. What an intolerant circuit that is! I have been studying the manual and again I was amused:
“Calibration can also be made during normal operation. The only voltage that we have to adjust with an additional voltmeter is the reference voltage of the A/C converter. The remaining adjustments are made on the basis of the module display. We do not need to measure the voltage of the divider with an additional voltmeter. This method is faster and may be more accurate than the one presented above”.
Oh dear. Why force people to make unnecessary effort when calibration can be taken in minutes, with a few movements of a screwdriver? What sadist wrote that manual? It’s obvious, that when it come to provide supply by the voltage divider, actual values differ from those which are calculated. So, after such adventures I gained a fairly well calibrated voltage meter. Connecting several light bulbs connected in series made it possible to calibrate the ammeter and I enjoyed a working power supply with voltage and amp measurements.
Unfortunately, these few minutes of operation at a load of 0.26A showed that without cooling it would not work well on long runs, because of heat. And since I still had a free +5V output from the stabilized supply for the multimeter and a lot of power backup at +12V, I decided to use the built-in thermostat for which I bought the whole AVT kit first. But this is a story for another episode.