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Old 22nd Jan 2019, 5:31 pm   #11
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
Default Re: Comark Early 1905 Insulation Meter Checks

Comark 1905.
I stand corrected, Yes, Jimmc101 looked at the circuit more carefully than I did, and the feedback does indeed reduce the voltage under load, and he has calculated the voltages on the various ranges. Many thanks.

I quote from the manual: OUTPUT VOLTAGE
If the "cal" control has been set correctly, the output voltage is within 5% of the nominal at infinity ohms. Due to action of the stabilising circuit, the voltage falls linearly from this point to zero at zero ohms.

That feedback also protects the expensive 10 ua meter. With the cost and rareity of that meter, I can see why for the later models, Comark used Op-amps and a 500ua meter.
And if I mention it now, the Avo Valve tester people will be wanting to use it, suitable shunted, to replace a failed Avo 30ua meter.

wme_bill
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