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Oscillator coil with common wires?
Oscillator coil with common wires?
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Old 15th July 2019, 08:12 AM   #41
Elvee is offline Elvee  Belgium
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I will test something else: a full-wave rectifier and a 200µA meter.

It will work, with a bit of luck, but 4 Ge or schottky will be required, because two drops of ordinary diodes will not be tolerated.
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Old 15th July 2019, 10:53 AM   #42
Elvee is offline Elvee  Belgium
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The 200µA meter + 4 Ge or schottky works, and the scale is quite usable, even for ESR>10ohm.
The best remains 50µA + 2 schottky, but the difference isn't huge.

Here is the updated circuit:


Oscillator coil with common wires?-editedesr6-png


Oscillator coil with common wires?-esr6-jpg
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Old 15th July 2019, 01:35 PM   #43
PhantomBox is offline PhantomBox  Venezuela
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I found another two VU meters, from an old tube reel recorder.
They are the same 200µA. However, when I drop the resistor to 47K ohms, I get a full scale deflection.
Now I will get some small Schottky diodes and a few caps...
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Old 15th July 2019, 09:39 PM   #44
Elvee is offline Elvee  Belgium
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Don't forget that Ge "crystal" diodes are also suitable (and even better than most schottky's) and could be easier to salvage from old equipments.
They include many two-digits 1N types, like 1N34, 1N70, european types, OA90 to 95, AA119, 143, etc, plus japanese 1Sxx, soviet types, etc
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Old 16th July 2019, 02:11 AM   #45
PRR is offline PRR  United States
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"ALL" VU meters are 200uA. Unless they are very cheap 1mA units with a custom driver.

Needle-type VOMs come in $10 500uA and $50 50uA basic movement.

This one is 50uA for $33, and you don't even have to open it up.
https://www.amazon.com/Tekpower-TP72.../dp/B00MI6T0IW
(Oddly, it says 60uA on the switch?)

Another key clue is the fine print on the scale: "2K/V" is 500uA, "20K/V" is 50uA. If they don't actually expose a 50uA range on the switch, you can bust it open and connect direct to the movement.

This appears to also be a 50uA meter:
https://www.amazon.com/Tekpower-TP82.../dp/B00SLVPJSI

Last edited by PRR; 16th July 2019 at 02:15 AM.
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