Optocoupler With Relay

[Suraj143]

Hi All

I want to drive a relay with optocoupler.I couldn't find any high current optos.So I coupled a transistor.

What I need to know is Do I need both R2 & R3 What values do I need?

 

[aljamri]

you may drive the relay using the transistor alone. In my case I'm using 2N2222A and having no problem. In this case you can remove the two resistors. In any case, you need one more resistance between the Emitter and ground

 

[ljcox]

What is the resistance of the relay coil?

As aljamri wrote, you only need 1 resistor in the R3 position as the opto is a current source.

I have not looked at the transistor's data sheet, but I expect that 22k should be adequate.

 

[mneary]

You would use R2 or R3 (only need one of them) to make sure that small leakages in the OC1 don't leave the transistor partly on. It can be around 10k, it's not critical. R1 is used to limit the current in case OC1 is too powerful. Although the PC120 is unlikely to need current limiting, R1 is still a good idea.

D1 is required to catch the back EMF from the relay when you want to turn it off.

You should not add a resistance between Emitter and ground.

 

[Suraj143]

Ok i'm going to use only R3 = 10k.

In this case (omiting R2) doesn't that R1 & R3 act as a voltage divider?ratio will be [12x10]/14.7.will this base voltage burn the transistor?there is no current limiting!

 

[mneary]

Re-check your calculations. Solve for the current in R1.

 

[ljcox]

Ok i'm going to use only R3 = 10k.

In this case (omiting R2) doesn't that R1 & R3 act as a voltage divider?ratio will be [12x10]/14.7.will this base voltage burn the transistor?there is no current limiting!

As I wrote in a previous post, the opto is a CURRENT source, so you don't need R1.

I also asked what is the resistance of the relay coil.

 

[aljamri]

You should not add a resistance between Emitter and ground.

Do you mean that the coil resistance is enough ?

 

[mneary]

Do you mean that the coil resistance is enough ?

If it is a 12V coil and a 12V battery, no additional resistance should be added.