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Old 16th Feb 2017, 4:17 pm   #21
MrBungle
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Default Re: Thurlby PL310 various issues (brown unit)

Been caught out by that before. I had a Uni-T which had ~3G impedance in mV range. Good but annoying! At that impedance you have to start worrying about cables and stray capacitance and guard rings and stuff. Ick.

Sorry PM2524. It's 10M on all DC voltage ranges - I checked it carefully as I use it with an RF probe regularly.
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Old 16th Feb 2017, 5:44 pm   #22
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Default Re: Thurlby PL310 various issues (brown unit)

Hi again,

I admit that I'm still confused. In post #8 I see C9 (2 times 33uF in parallel) prior to the switch, and 3 diodes (D11, 17 and 18) after it. I have the covers off one of these right now, and I have exactly the same. Sometimes I've seen a single 47u for C9, for what that's worth.

Realistically, it's only the middle of the 3 diodes (D11) that could be acting as a capacitor to give the results you observe. Looking at the datasheet of the 1N5401, the junction capacitance is in the region of 30pF. That forms a very short TC with a 10M DVM - doesn't seem likely unless it's faulty in some way.

So, I think there's something else at play here.

Having just checked one of mine, I can get "ghost" voltages of around 20 to 50mV DC with the DC output switched off. It doesn't matter what voltage the PSU is set to (which seems to rule out leakage in the switch). If I turn off the AC, the ghost voltage actually rises to 100mV or so, but that's a red herring.

Basically, it's just noise. I've hooked up a 'scope in parallel with the DMM (Fluke 87V). The 1M of the scope input is enough to make the DMM say 0V, but switching the probe to x10 mode makes it come back.

On the 'scope screen, I have noisy 50Hz pickup which is about 350mV pk-pk. Switching the DMM to AC reports 123mV RMS, which agrees nicely with the 'scope. The waveform is asymmetric around 0V; in other words there's a DC offset there, which is what the Fluke reports in DC volts mode.

So, no mystery here. 50Hz is everywhere. I also get a load of 100kHz if I turn on my main bench lamp (a pair of T5 tubes fed from an electronic dimable ballast).

So have a look with a 'scope and see if you're getting the same. And if so, you might find that you don't need anything like 22k to get rid of it - as I say, the 1M of the 'scope is enough for me. Obviously, every situation is different, but I'd rather keep the accuracy of the metering than bodge it. And also, when you connect the output to a real circuit, that'll damp down the pickup anyway. In other words, it's not a real problem - rather it's an observation problem.

Hope this helps,

Mark
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Old 16th Feb 2017, 6:52 pm   #23
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Default Re: Thurlby PL310 various issues (brown unit)

Fair points. I'll poke it with a scope later when I'm back home.

I'll try a 1M as well - if that is sufficient then it will do the job to my satisfaction. You're right it's not a real problem but I prefer the principle of least surprise and this is one of those things that is a surprise periodically when you've forgotten about it for a few months
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Old 16th Feb 2017, 7:11 pm   #24
mhennessy
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Default Re: Thurlby PL310 various issues (brown unit)

Just done another check. The problem is there even with the PSU switched off completely, but goes away when you remove the AC feed to the unit. I'd suggest that it's caused by capacitive coupling within the unit - between the mains wiring and the low voltage wiring. You might be able to improve things by rearranging the cables.

As I said earlier, "birds nest"
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Old 16th Feb 2017, 7:14 pm   #25
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Default Re: Thurlby PL310 various issues (brown unit)

Interesting. Will try that as well.

Agree with bird's nest. The newer unit is much nicer!
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