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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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19th Apr 2021, 11:50 pm | #21 | |
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: Valve tester meter protection?
Quote:
30uA times 3250 Ohms is 97mV 440uA times 100 Ohms is 44mV 115uA times 1500 Ohms is 172mV All are below the threshold of common Schottky diodes. Silicon diodes with their higher threshold will give less protection. So I'd go for Schottkies. and put a pair in, connected in parallel with each other and the meter, and pointing in opposite directions. Low voltage types will be fine as each will protect the other. 200mA rated ones should be enough, but 1 amp may feel safer. If a schottky clamps at 300mV this means (V squared)/R the heating power is 27 microwatts into 3250 Ohms 900 microwatts into 100 Ohms 60 microwatts into 1500 Ohms. You could research the wire gauge of the meter coil, but then there's thermal mass and a concentration od turns to account for. Anyway, the powers represent a dramatic reduction in terms of what can be there when someone gets things wrong. The trick is that the diodes are protecting the meter, and therefore the meter defines what protection it needs. We can ignore all the switching and ranges. 900uW is a comfotrable listening level into a telephone earpiece, and they are wound with very fine wire. 27uW gets us down into butterfly fart power levels. David
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