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Incorrect Readings from Multimeter & DC Adaptors

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Incorrect Readings from Multimeter & DC Adaptors

Postby Temp » Mon Apr 12, 2021 1:15 pm

Hi Folks,

I've got a digital multimeter that has a good number of decent reviews online. However, I've been testing a number of old DC adaptors (for a headphone amp) and I'm consistently getting over-voltage readings (6v shows 9-10v, 12v reads as 16v). The odd thing is that numerous batteries and computer USB outputs appear to measure just fine (i.e accurate, and well within the documented tolerances).

Obviously, I'd rather it was the multimeter and not the mains supply (the house had a new consumer unit and ring main < 5 years ago). Other functions on the multimeter also appear to be accurate, and aside from an unsolved noise issue (which I might post a new thread about), there haven't been any noticeable problems with the electrics in the house.

My UPS reports a mains voltage of between 234 and 252 volts, which, although seemingly quite varied, is apparently also within the normal operating range.

I'd be grateful for any suggestions as to what might be going on here. Cheers.
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Folderol » Mon Apr 12, 2021 1:34 pm

The meter is almost certainly telling the truth!
Most of the older (cheap) PSUs were considerably overvoltage until significantly loaded down.
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Martin Walker » Mon Apr 12, 2021 1:53 pm

Folderol wrote:The meter is almost certainly telling the truth!
Most of the older (cheap) PSUs were considerably overvoltage until significantly loaded down.

Where 'significantly' was usually a load corresponding to the current quoted on the adaptor.

So, for instance, an adaptor displaying 24 volts 350mA could well measure 30 volts or more with no load, but around 24 volts when supplying 350mA.


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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Hugh Robjohns » Mon Apr 12, 2021 2:03 pm

Temp wrote:I've been testing a number of old DC adaptors (for a headphone amp) and I'm consistently getting over-voltage readings (6v shows 9-10v, 12v reads as 16v).

This is quite normal for an unregulated supply. The voltage will come down closer to its rated value when power is being drawn from it.

The odd thing is that numerous batteries and computer USB outputs appear to measure just fine (i.e accurate, and well within the documented tolerances).

Yes... the USB outputs are regulated, and batteries generally provide what they say they should when fresh.

My UPS reports a mains voltage of between 234 and 252 volts, which, although seemingly quite varied, is apparently also within the normal operating range.

It is indeed. The current UK spec allows a tolerance of -6% to +10% from a nominal 230VAC. That equates to a minimum of 216.2 Volts and a maximum of 253.0 Volts. Clearly, your local supply transformer is tapped towards the higher end of the range.

I'd be grateful for any suggestions as to what might be going on here.

Not understanding the nature of typical unregulated wall-wart power units... ;-)
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Temp » Mon Apr 12, 2021 2:15 pm

:headbang:

Thanks so much guys - that all makes total sense and puts my mind at rest. Now where's the 'shame-on-me-for-not-knowing-such-things' emoji? :oops: :silent:

Cheers!
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Luke W » Mon Apr 12, 2021 2:19 pm

No shame in not knowing something.

It's usually being unwilling to ask or listen that's the problem. :lol:
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Sam Spoons » Tue Apr 13, 2021 8:36 pm

Just a small cautionary tale WRT unregulated PSUs. As a guitarist I have several power supplies and various guitar poedals, I made the mistake of plugging an unregulated PSU (from a Boss Micro Cube practice amp) into my Peterson StroboStomp tuner which promptly fried the pedal. Most pedals will tolerate a fair bit of over-voltage but not all. My Boss pedal PSUs are well regulated but the MicroCube one is not measuring 13VDC with no load so if you have unregulated PSUs be careful to use them only with the gear they came with.
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Re: Incorrect Readings from Multimeter & DC Adaptors

Postby Martin Walker » Tue Apr 13, 2021 10:10 pm

Sam Spoons wrote:if you have unregulated PSUs be careful to use them only with the gear they came with.

Wise advice, although if you lose an unregulated PSU one trick is to replace it with a Universal AC Power Adapter of similar current capability that offers switchable voltages.

Image

Make sure that it's set to the appropriate polarity for your device, and then turn it on at its lowest voltage setting and plug in your device - if it doesn't work then increase the voltage to the next higher setting and stop once its loaded voltage is sufficient to power your device.

I did this with a DIY synth with its own built-in DC regulators and they ended up running really cool as they received no excess voltage to dissipate.


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