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Old 10th May 2010, 07:07 PM   #1
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Default Ooops, I bought AC voltmeters and needed DC

I bought some AC 450V analog volt meters but on arrical discovered that I should have gotten direct current.

I opened one of them and It seems it have an internal rectifier. First a 2W 180k resistor, then two parallel, opposing diodes with one smaller resistor each.

So, there seems to be a sort of rectifier integrated in the meter.

But, what happens to the range when I remove the rectifier?
Is it a stupid question? It it just a matter of adding resistors until I get the 450V range back?
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Old 10th May 2010, 07:19 PM   #2
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Usually an ac meter has lower sensitivity so it will draw more current. Is the scale linear? Most ac meters have the scale crowded near the bottom although yours are high enough range that they probably are linear.

You don't have to remove the rectifiers; it should work on dc but you may need to recalibrate.
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Old 10th May 2010, 07:23 PM   #3
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Jepp the scale is linear.
its one of these:
High Quality Round Analog Volt Panel Meter AC 450V on eBay (end time 16-May-10 06:05:24 BST)
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Old 10th May 2010, 07:42 PM   #4
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There is no mention of the burden but I'd imagine it would draw a mA or as much as 5 mA at full scale.

So apply a known voltage and adjust the internal multiplier until it reads correctly. Use good quality resistors or you might have some temperature drift.

And be careful; these voltages are hazardous!

The most difficult part would be changing the dial to read DC instead of AC.
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Old 10th May 2010, 07:51 PM   #5
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Thanks for the input. Ill use the tube amp I am building for reference voltage, with a multimeter, and see if I can find the right resistor values and adjustments for using the scale thats on it. I will use it in the 250V area.

Go slow, one hand in back pocket, the secret sign of the DIYaudio guild!
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Old 10th May 2010, 08:22 PM   #6
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Problem solved!
There was a tiny potmeter in the circuit. It had sufficient range to adjust the voltage.
Thanks for info and psychological support Bob!
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Old 10th May 2010, 11:15 PM   #7
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Couldn't be easier! Good luck.
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