4N32 Relay Driver 2

[RICH535]

I want to use a 4N32 opto to drive a 5v relay coil (200 ohms) from a 24v source. Can I just divide down the 24v with a resistor tied from the 4N32 collector to the 24v and take the output from there with the 4N32 emitter to ground?

 

[Noway2]

There are two potential issues with your approach, but the answer depends on the relay coil current. 1) the opto has a property called current transfer ratio, which means that the transistor side can only source / sink a certain percentage of the LED current. This means that, depending on the device chosen, you may not be able to directly drive the relay from the opto. 2) you need to consider the power limitations of the devices. If you drop from 24V to ~5V, you will need to be sure you don't exceed the power requiremetns of the resistor, which would easilly happen whem dropping 24V.

A better apporach would be to use the opto to drive a switching transistor that in turn controls the relay. This would avoid problem number 1, but you would still need to have a solution to provide the proper voltage. Perhaps you could use a small 3 pin Vreg or a zener diode. The relay will be somewhat tollerant of the input voltage, but it will still need to be about 5V.

 

[OperaHouse]

It is never a good idea to pump too much current into an opto isolator. As they heat up, transfer ratio drops even lower. I never count on getting more than 20% transfer ratio.

 

[RICH535]

I breadboarded a circuit using a 2N3904 transistor with a 78L05 regulator (+5V) and it worked great. Thanks for your help Norway2 and OperaHouse!