Pros and Cons of Optocoupler + Transistor vs SSR for DC

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1

Hi

It seems SSR's are quite expensive for switching 24V DC loads, so was thinking of using an optocoupler + NPN transistor for 24V DC loads with currents up to about 2Amps.

What are the Pros and Cons between these 2 options?

Thanks.

2

piashaw:
Hi

It seems SSR's are quite expensive for switching 24V DC loads, so was thinking of using an optocoupler + NPN transistor for 24V DC loads with currents up to about 2Amps.

What are the Pros and Cons between these 2 options?

Thanks.

Well most SSRs can't switch DC loads on and off anyway, they only work with AC powered loads.

For switching DC loads with the voltage and current you mentioned I would use a "logic level" N-channel MOSFET transistor.

Lefty

3

Thanks lefty,

Why is this better than a TIP120 for example?

Are you suggesting to run this directly from the arduino or would you still runj it through an optocoupler?

4

piashaw:
Thanks lefty,

Why is this better than a TIP120 for example?

Modern power mosfets have less voltage drop across them when fully turned on then npn/pnp transistors. So less heat generated, more efficient, etc.

Are you suggesting to run this directly from the arduino or would you still run it through an optocoupler?
Could be, depending on what the actual load device you are controlling. If it's a particularly 'noisy' type load then an opto isolator can help isolate the load ground from the arduino ground, but that is not always required.

5

FETs take no gate current. Power transistors tend to have low current gain and so can load the output driving them. A 2 amp load might need a 200ma drive current to ensure saturation. You would have to calculate the power dissipation of both methods to know which is best. Fets are the usual way.