Why can't I use a 4N25 for an analog isolator

ian field

Joined Oct 27, 2012
I have a sensitive an unusual dash dimmer switch on my vehicle. 1999 Miata. Unlike most dimmers it does not simply connect the dash lights through a variable resistor to ground.

Because of the complexity of the circuit I need to isolate the circuit optically in order to add an aftermarket radio and gauges [ to dim the lights in the radio and gauges along with the dash]

I have researched this forum and seen posts that suggest that a 4N25 optical isolator can not be used for an analog circuit, just switching circuits. My question is: Why not?

Attached is my proposed circuit, if I am way off base let me know. And if you have an alternate circuit design it would be appreciated.

Several manufacturers have published appnotes describing linear use of opto couplers. Probably HP, Sharp certainly did but I think they dropped out of opto, so you may have to trawl archives. Siemens & TFK are also worth a look.

Basically; you drive the PD of 2 optos in series - the transistor section of one opto provides your application signal, the other is included in the nfb loop of the amplifier driving the PDs.