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Anyone good with electronics? - odd issue with digital voltmeter
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PostPosted: Fri Jan 22, 2021 2:50 am    Post subject: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

Putting this in the off-topic section because I've seen that there are some folks on here who have a better understanding of electronics than I. I'm hoping they might be able to explain why I get a false reading on a digital panel voltmeter in a project circuit that I've built...

One of my other hobbies is collecting watches that use an obsolete type of electronic movement regulated by a tuning fork. (ESA 9162-based movements, in case you're interested.)

Over the years, I've learned to service them myself by re-using parts scavenged from 'faulty' watches that I've picked up cheap.

To ensure accurate timekeeping, the movements need testing at a range of voltages from 1.0 to just under 2.0V. I've been using a variable voltage power supply that outputs 0-3V. I built it several years ago based on this design http://www.pocketwatchrepair.com/catt/pwr-supply.php

The power supply runs from a 12V 'wall wart' power supply and outputs 0-3V.

It uses an LM317 and an LM7805 voltage divider.

The only change that I've made to the design is to have only one set of output terminals (I don't needed the others my application) and to connect a 0-50micro-Ampere ammeter and a 0-30V digital voltmeter to read the current and voltage directly at those terminals.

The ammeter always reads correctly but despite trying several voltmeters, I've never found a voltmeter that works on this supply. They all add 5.1V or thereabouts to the actual output voltage. If I measure the output voltage with my multimeter, it shows 0-3V output, but the built-in voltmeter shows 5.1V - 8.1V.

I've tried a variety of three and four-wire voltmeters powered by the 12V supply to the tester and they all add 5.1V to the actual output voltage.

If I test the voltmeters independently of the circuit, they read voltage correctly (e.g. they read the voltage of an old AA battery on my desk as 1.2V). Connected back in to the circuit, they show the voltage as 6.2V.

Why? Confused The only thing I can think is that the voltmeter is only reading the output from the LM317 voltage regulator (the LM7805 is a 5V fixed-voltage regulator), but I don't understand why. The multimeter clearly shows the actual output is the combined effect of both regulators.

Is there anything I can do to fix that, other than just subtract 5.1V from the displayed voltage on the voltmeter.

I'd rather omit my multimeter from the testing process because the connectors have a habit of pinging off the power supply terminals just when I don't need them to. Rolling Eyes

The voltmeters I've been trying are powered by the 12V
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PostPosted: Fri Jan 22, 2021 12:41 pm    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

If the impedance of the voltmeter is a lower than that of the circuit driver you could well be measuring the 5V added to the output, which will not be present on load. I think I made this same mistake back in the 1980s, when they failed you in the lab and made you do it again weekends.

A cheap voltmeter will have a low input impedance so that the voltage sensitive circuit can be less sensitive and thus cheaper.

The fix is no Fluke, or perhaps the opposite.
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PostPosted: Sat Jan 23, 2021 3:08 pm    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

Abscate wrote:
If the impedance of the voltmeter is a lower than that of the circuit driver you could well be measuring the 5V added to the output, which will not be present on load. I think I made this same mistake back in the 1980s, when they failed you in the lab and made you do it again weekends.

A cheap voltmeter will have a low input impedance so that the voltage sensitive circuit can be less sensitive and thus cheaper.

The fix is no Fluke, or perhaps the opposite.


Isn't impedance a parameter of AC circuits (the issue that leads to Power Factor problems)?
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PostPosted: Sat Jan 23, 2021 3:16 pm    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

It’s a measure of load , but you usually use it to talk generally about circuits which can be DC or AC. You could say load here since I think it’s just a dc device
‘Sorry to obfuscate, Abscate.
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PostPosted: Thu Jan 28, 2021 2:54 am    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

Thanks Abscate. Your reply has led me to read around voltmeters and resistance.

I found information here... https://www.allaboutcircuits.com/textbook/direct-current/chpt-8/voltmeter-impact-measured-circuit/ ... that corroborated what you've said.

I figured that means I'd need to increase the resistance at the voltmeter to get it to record correctly, but was concerned that would have an affect on voltage and current at the output terminals.

Then I found this post on Quora...https://www.quora.com/Why-does-an-ideal-voltmeter-have-infinite-resistance. If I understand it correctly, it says that current will follow the path of least resistance, so putting a voltmeter of infinite resistance in parallel across the output terminals will have no effect on the current or voltage at those terminals as the current will bypass the voltmeter? (The voltmeter will show the potential difference across the terminals but not influence current or voltage?)

So:- do I just need to add resistors in series with the voltmeter (with the voltmeter connected in parallel across the output terminals) to get it to read correctly? If so, how do I calculate how much resistance to add (the total value of resistors)?

Or alternatively; if I buy a different voltmeter, how do I identify whether it has sufficient resistance already to read correctly?
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PostPosted: Thu Jan 28, 2021 5:51 am    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

You want to look for the input impedance spec on your voltmeter.

Here’s my Fluke manual

Snip

When measuring voltage, the Meter acts approximately like a 10 MΩ (10,000,000 Ω) impedance in parallel with the circuit. This loading effect can cause measurement errors in high-impedance circuits. In most cases, the error is negligible (0.1% or less) if the circuit impedance is
10 kΩ (10,000 Ω) or less.
For better accuracy when m

Unsnip

How accurate a measurement do you want in your power supply?
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PostPosted: Thu Jan 28, 2021 9:00 am    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

Abscate wrote:
You want to look for the input impedance spec on your voltmeter.

Here’s my Fluke manual

Snip

When measuring voltage, the Meter acts approximately like a 10 MΩ (10,000,000 Ω) impedance in parallel with the circuit. This loading effect can cause measurement errors in high-impedance circuits. In most cases, the error is negligible (0.1% or less) if the circuit impedance is
10 kΩ (10,000 Ω) or less.
For better accuracy when m

Unsnip

How accurate a measurement do you want in your power supply?


Thanks. I need 0.01V across a range from 1 to 2.5V and micro-ampere accuracy.
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PostPosted: Thu Jan 28, 2021 4:22 pm    Post subject: Re: Anyone good with electronics? - odd issue with digital voltmeter Reply with quote

I have shot atleast 4 of the digitail meaters across the room.....my memory dont seem to work the same at those new meaters do....( cheep harbrefright meaters)
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