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Post by radioholic on Jul 22, 2022 15:54:27 GMT 12
Any comments on the attached article would be appreciated. How to protect an analog ammeter with a diode.pdf (151.29 KB) I used this method to protect the moving coil ammeter on my home-made valve tester. I have overloaded it accidentally a few times and it has survived - so far, so good! |
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Post by Philconut on Aug 8, 2022 14:00:38 GMT 12
Hi James,
I can't open the file?
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Steve
Society Members
vintageradio.co.nz
Posts: 732
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Post by Steve on Aug 10, 2022 12:56:57 GMT 12
I can open it ok - John, I've emailed it to you
I can't add much to the technical aspect of the document - but given how many Radio Corp VO meters have replacement movements in them (there is a service supplement on it somewhere in my notes from an annoyed factory engineer who got sick of sending out replacements) - its probably a good idea! 
Cheers, Steve
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johnr
New Member
Posts: 10
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Post by johnr on Apr 16, 2023 9:59:04 GMT 12
The article is correct, but of course the specific solution it comes up with is only an example. To protect any given meter requires you to know the fsd of the meter and its internal resistance. Once known/measured then you can follow the method of the article to determine the correct value of resistor for your particular case.
However for some (if not many) applications the addition of a resistor in series with the meter will introduce significant error. And example is any of the AVO valve characteristic meters where the 30-odd microamp movement cannot be protected this way without considerable complexity (see various articles by others over the years on these meters. Same for analogue multimeters which don't already have the diodes fitted.
For something you are making yourself this usually won't be a problem as you can design within reason for a meter of any series resistance.
regards
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