On Tue, 24 Sep 2002, Chris Loiacono wrote:

*>> Subject: Re: [EE]: optically isolated mains voltage acquisition
*>
*>Peter,
*>This looks too simple to work well. But then again, the same applies to me.
*>:)
*>Are you in a good enough mood to offer a little more explanation of how this
*>and the CdS idea work???

The transistor circuit is an adaptation from a book schematic that uses an
opamp and two ocs. The transistor circuit can be considered a inverting
'Norton' opamp for this analysys. Its gain reduces any nonlinearity in the
transfer characteristic of the OC, by this gain (beta). The second OC
should be run at a comparable voltage differential for the output to be
accurate but it should be close under all conditions.

If ocs with simple photodiodes would be available and inexpensive then
this gymnastic would be unnecessary.

The CdS + LED circuit relies on the respective curves of CdS resistance vs
light and LED emittance vs current. The device I built used a green led
and a 12.7mm dia. CdS photoresistor in a black tube with black epoxy
sealing (Devcon I think). It was better than 2% over one order of
magnitude.

Peter

PS: The o.p. could use a capacitive divider too. This gives voltage
reduction + galvanic separation in one shot. A standard 2nF/1kV X2 ceramic
cap has Zc ~= 1.5Megs at 50Hz. Just what the doctor ordered to get 2Vpp
out using an inverting opamp with Rfb ~= 7.5k, or use a proper
differential divider with 2nF/200nF from each circuit (L and N) to circuit
gnd and amplify the difference voltage in an opamp.

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