Trough opto voltage issue

[Pin Muncher]

I am trying to sort an old issue I have with my RFM, the trough jam opto is bad, I replaced the transmitter and receiver and still the same issue.

For some reason there is 13.5V on the Q1 Tx opto instead of 1.5V

The opto board has 7 resistors that drop the 13.5V to 1.5V via a 270 Ohm Resistor.

The resistor for Q1 the trough jam Tx opto measures the correct resistance but when power is applied 13.5V is still read instead of dropping to 1.5V like the others.

So, is this the issue and the resistor has failed.

All the other switches in the same column or row all work correctly

I don't have a spare but will be going to maplins tomorrow.

 

[Moonraker]

It is possible that the resistor is failing under load so swap it to rule it out.

 

[Pin Muncher]

I'm assuming that's what it is, I'm off to Maplins

Cheers

David

 

[pinballmania]

The resistor doesn't drop the voltage. It sets the forward current. Opto transmitters have a forward voltage drop of about 1.7v. You have a 12v supply. The specified max forward current is 100mA , then R=V/I. R=(12-1.7)/0.1 = 103ohms minimum. Setting the resistance to 220ohms decreases the forward current to 46mA.

In short, I expect the led is either installed backwards, so blocking current, or open because it is faulty by excess heat. Both can be checked with a multimeter.

 

[Pin Muncher]

LED was installed backwards it's now all working as usual.

Anyone need a spare 270 ohmn resistor

Thanks for your help

David