Why is this transistor required

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You don't provide enough information for a definitive answer - for which I would need the part numbers or specs of the relay and the opto-coupler.

However, typically a normal opto-coupler will have a coupling of between 10% and 50% - which means that for 5mA in the LED you can expect a collector current of between 0.5 and 2.5mA maximum. The relay will probably need a current of about 50mA to turn on.

Some opto-couplers are specified for much higher gains so may be able to reliably operate the relay but it is likely that using a standard low cost opto-coupler and  a transistor will be cheaper and offer a greater operating margin.

Another thing to look out for in the coupler spec is the maximum output current - which may not be enough to operate the relay.

For future reference, when you ask a question on a forum like this the quality of the answer will depend on the information you provide - please try to include part numbers for all the key components.

MK

Looks like it's probably switching the relay it's connected to.

These are the datasheet values for the opto coupler. Hope it helps:-

Collector-base voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 V

Collector-emitter voltage (see Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 V

Emitter-collector voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 V

Emitter-base voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 V

Input-diode reverse voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 V

Input-diode continuous forward current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 mA

Input-diode peak forward current (tw ≤ 1 ns, PRF ≤ 300 Hz) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 A

Could the transistor be in the circuit just to save the opto coupler ic. I mean if due to some reason the opto coupler gets damaged, it would be more expensive to replace the ic but if we just use a transistor that gets destroyed, we need to pay less money to fix it....What do you think??

Yes Sir but I want to know why cant we directly connect the relay to the Optocoupler. Is this circuit more reliable or something?

Ah yes, I see what you mean. Possibly the switching current for the relay coil is higher than the optocoupler can safely provide, so the transistor is there to do the job instead.

You haven't given me the key opto-coupler parameter - what's the coupling coefficient (current transfer ratio) ?

Take a look at the data sheet for Farnell 2323137 a Farichild  FOD817FOD817 on page 4 they give the key parameters in the table"DC Transfer Characteristics there will be a similar table in the data sheet for the device you have

MK

Yup, its a current buffer.  The output Q and the extra Q form a Darlington pair.  When one Q is NPN and the other is PNP, it is a Sizlacki pair.  Output Qs of run-of-the-mill optocouplers have low ampacity (how I hate that word!) and are susceptible to spikes.

The simple answer is the opto coupler is only rated for 50mA while the relay is either 72 or 90mA.

Under some circumstance you may be able to drive it (although it may not pull in correctly), but these have issues being drivn by a Raspberry Pi.

The 3v3, series resitor and LED n the input side combine to be just on the edge of triggering even with the transistor.

The answer is to add a second 1K across the resistor (R5 in the schematic).

Mark

There should be some notion of design margin.  Even if, say, the optocoupler Q is rated at 30mA and the relay coil is rated at 30 mA, the buffer Q should still be employed.  There is such a thing as specsmanship.  There is such a thing as design (process) variation.  There is such a thing as not being a stunod.  For every 10 degree K more heat (temperature rise, heat is not measured in Kelvins) you put into a device you halve its lifetime.

We don't have to race to the bottom of being cheap if our applications are valid.