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Post by nginmu on Oct 25, 2023 18:12:01 GMT -8
I went googling for info on how to build an analogue DC watt meter and one of the results led me here. Gary had posted a design for a watt meter that had similarities to what I was looking for.
I discovered he had sadly passed but I decided to register anyway because his spirit's clearly still here and I think I would have liked him.
Well met, fine sirs.
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Post by volkswagenvan on Oct 25, 2023 21:06:34 GMT -8
Hi nginmu and welcome! Thanks for the kind words! Yes,... Gary was a very likeable guy who enjoyed meeting new people with the same interests. The Watt Meter is here... True RMS Watt Meter chemelec.com/Projects/Watt-Meter/Watt-Meter.htmBest of luck on your project and don't be afraid to ask questions. Thare is always someone around here to help if we can. Jerry |
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Post by nginmu on Oct 26, 2023 7:31:17 GMT -8
Thanks for the welcome.
Yes that's the one!
I was thinking about modifying the circuit, so that it was capable of measuring something like 0 - 300 DC Watts within the ranges 0 - 30A and 0 - 50V. The voltages and currents would only vary slowly and never go negative.
Such a simple measuring device seems impossible to find these days, everything seems to have a digital display run by a microcontroller now. I positively love the look & feel of the old analogue meter driven purely by laterally thought design.
I'm good with project construction but not as good with circuit design, so any help and suggestion as to what and how to modify to achieve my range aim would be really good.
Thanks :-)
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Post by volkswagenvan on Oct 26, 2023 9:59:12 GMT -8
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Post by nginmu on Oct 26, 2023 14:41:01 GMT -8
Thanks, that's perfect. Wow he already simplified the original circuit a lot, by the looks of it. Does anybody happen to know if I'm on the right track, thinking the LM394 is acting as a Gilbert Cell?
If so.. wonder if an ordinary matched pair of suitable NPN transistors would do the job.
Out with the soldering iron and the random parts tub I think!
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Post by volkswagenvan on Oct 26, 2023 16:03:39 GMT -8
Experiment and let us know.
I have never tried to build one myself. Interesting stuff anyways!
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Post by nginmu on Oct 27, 2023 2:38:33 GMT -8
Not sure whether I should be googling 'gilbert' or 'long tailed pair'.. but I think it's something along those lines. Yes will post back if I get somewhere with it. Thanks again for finding the National Semis book.
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Post by nginmu on Oct 27, 2023 6:34:08 GMT -8
I roughed up a quick simulation to try and understand the operation of the circuit and tweak values. I've tried to make it work so that if I use a 300 microamp galvanometer, easy to find and cheap on eBay, the units on the printed scale of microamps will correspond perfectly with the measured Watts and all I'll have to do is cover up the 'uA' with a sticky label marked 'W'.
Had some trouble getting linearity & scaling to behave but seem to have gotten somewhere near through trial and error. I'm sure someone who understood the circuit better from a theory point of view, could explain how to calculate more appropriate resistor values than I've managed basically by guesswork. The LM394 seems to be obsolete; I discovered the BCM847BS which is a single package SOT363 matched NPN pair & ordered one on a breakout board, once it comes, will start soldering..
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