Can't get DVM to measure & power from same source--SOLVED! - Page 1
Forum section: electronics / projects

author: Analog Kid
author_rank: Super Contributor
timestamp: February 23, 2025, 09:49:43 pm
content: OK, thisiswas frustrating.I'm still working on my transistor beta tester. The beta tester part works fine; it gives a close-enough "right answer" (checked against a couple of DMMs that have that function). Not looking for a high-precision instrument here; close enough is close enough.Here's the beta-test circuit, tested and ready to go:Rather than have to connect an external meter to measure the beta (in mV), I wanted to incorporate a digital panel meter in the project, to make it self-contained.I understand that I cannot power the panel meter from the same source (9V battery in this case) used to power the beta-tester circuitry. So I'm following this plan:To that end, I built a small inverter to provide an isolated power supply for the beta-test circuit; the panel meter would be powered directly from the battery.Here's the inverter, tested and ready to go:My assumption is that since the inverter uses a transformer, the output of it should be isolated from the battery supply for all purposes, right? Which should make the panel meter work.Except that it doesn't. When I hook all this stuff up, the meter shows an overrange condition ("-1"), even though the actual voltage it's measuring is well within range. No go.When I power the meter and the beta-tester from 2 separate 9-volt batteries, everything works fine: I get the "right answer" (the beta, to within a few mV) on the panel meter.The meter seems to somehow "know" that it's operating off the same supply as the beta-test circuitry. But how?I've tried reversing the meter and beta-test circuit power (running the panel meter from the inverter instead): still no go.So my first suspicion was (maybe still is) that the problem is high-frequency ripple on the supply to the beta-tester. To that end I've stuck in bunches of capacitors to try to filter it out (inverter frequency is ~75 KHz, so I figure that fairly low-value caps in the neighborhood of 0.1μF to 0.01μF should do the trick). But no matter how many caps I stick in various places, the problem persists. I've currently got a 2-stage low-pass filter directly on the output of the inverter, and it doesn't help.Does anyone have any experience with these panel meters? Mine is, I think, the bog-standard type, a made-in-China module that comes set to 200mV. I rejiggered it to read 2000mV full scale, which works fine (changed the divider resistors). I've tried two of them and neither one works here, though they're both working fine.What am I missing here? I hate the thought of having to use two batteries with this thing. That would be the obvious solution, but every fiber of my body resists the idea ...BTW, hat tip to PCB.wiz, whose design this was (the beta-tester part) that I modified.

author: JoanBS
author_rank: Contributor
timestamp: February 23, 2025, 10:08:06 pm
content: I recently built a portable milliohmmeter and had the same problem. I wanted to use a 200mV digital voltmeter (LCD, because of its lower consumption) and finally decided to use 2 independent 9V batteries.However, with an identical voltmeter, but with LED display, it worked perfectly with a single battery.

author: PlainName
author_rank: Super Contributor
timestamp: February 23, 2025, 10:32:09 pm
content: QuoteIf you measure voltage from panel meter (and battery) negative to beta tester positive, what do you get?

author: Analog Kid
author_rank: Super Contributor
timestamp: February 23, 2025, 10:38:59 pm
content: Quote from: PlainName on February 23, 2025, 10:32:09 pm5.5 volts!Dayum! Where's that coming from?Do you see any fault in my design?Scratching my head ...That can't be leakage across the transformer.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 23, 2025, 10:45:58 pm
content: Yes it can; primary <--> secondary measures ~3KΩ. That must be the problem.Looks like time to wind another xfmr. This time with insulation between pri. & sec. I guess the varnish on the wire isn't enough ...

author: Analog Kid
author_rank: Super Contributor
timestamp: February 23, 2025, 11:48:17 pm
content: Quote from: PlainName on February 23, 2025, 10:32:09 pmYou, my friend, are a genius. You asked the right question.That was egg-zactly the problem. Leakage across the transformer.All it took was a little bit of scotch tape around the primary.I was actually able to salvage the transformer; I unwound the secondary, put on the tape and rewound it.It works! Meter needs calibrating, but other than it it seems good to go.

author: PlainName
author_rank: Super Contributor
timestamp: February 24, 2025, 12:08:53 am
content: Glad it was a simple fix

author: Analog Kid
author_rank: Super Contributor
timestamp: February 24, 2025, 02:42:41 am
content: Moral of the story:If anyone plans on winding their own transformer (hey, it's fun! and cheap), you might want to put down a layer of insulation on top of the primary before winding the secondary. Don't rely on the insulation of the magnet wire (varnish) to do the job.I used regular scotch tape; probably a good coat of nail polish would also work.

author: oilburner
author_rank: Contributor
timestamp: February 25, 2025, 10:33:35 pm
content: hey, show us a picture of your transformer! : ) thanks

author: Analog Kid
author_rank: Super Contributor
timestamp: February 25, 2025, 10:54:03 pm
content: Quote from: oilburner on February 25, 2025, 10:33:35 pmAs you wish:It's wound on a rectangular ferrite bar that was salvaged from ... something. Wound with 34 ga. magnet wire.The wire on the left is the primary center tap.The thicker wires at the ends are for sticking into a breadboard for testing.