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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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Old 19th Apr 2021, 11:50 pm   #21
Radio Wrangler
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Default Re: Valve tester meter protection?

Quote:
Originally Posted by resurgance View Post

VT160 FSD = 30uA @ 3250ohms
MkII = 440uA @ 100ohms
539b = 115uA @1500ohms

And there is voltage present at the meter terminals which is the bit I am fuzzy on as this totally relies on the selector switch position right?
No, that's wrong. The meter scaling can show volts, mA, mA/V and arbitrary indications like set AC. But the meters with the pointers at full scale simply pass 30uA, 440uA and 115uA. They don't know how to do anything else. The meters and any added protection, live downstream of all the range and mode switches.

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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