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Selecting transistor for switching relay ? Answered

Hi guys,
I wanna switch Relay(6 Volt DC Input ,250 V AC output ) using the output of HT12D IC (I used BC547 independently and also wit  CL00 Transistor) but relay is not switching properly ,Please guide me with a way to select  the transistor?????


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steveastrouk

3 years ago

What's the coil current rating ?

Did you put a diode across the relay ?

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electrical snipersteveastrouk

Answer 3 years ago

No i dnt put any diode across the relay (it s only for protection right , it does nt have any relation with the working of relay(tat is with my problem)

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-max-

3 years ago

Assuming that at 6V, the relay coil pulls 100mA, I think most any transistor will do the job. BC547 However only has an absolute maximum collector current of 100mA. So i would say, given all the other choices for transistors, that it is not ideal. Something as jelly bean as a 2N2222 should do the job, But your HT12D according to the datasheet can only sink and source up to 1.6mA on the output pins. That might also be an issue.

Try using the NPN transistor in "common emitter" mode instead of "common collector." That should increase the input impedance of the base of the transistor so that it is easier to drive with a weak signal. If that does not work, try using a MOSFET instead (in common source mode of course!), and always include a catch diode across the relay coil to prevent transient voltage spikes. Also with MOSFETs, you should use like a 1-10 megaohm resistor between the gate and source.

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steveastrouk-max-

Answer 3 years ago

The 2N2222 won't have enough Hfe to drive the coil, if your 100mA is correct

And, depending on the OPs supply volts, the mosfet probably needs to be a logic level one.

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-max-steveastrouk

Answer 3 years ago

Good points! I had a sneaking suspicion that the gain might be an issue with the BJT transistors, hence mentioning to use it in emitter follower mode, so that the input impedance is higher and easier to drive. What is the HFe gain of the PN2222 or 2N2222? My guess is that it would have to be at least 62.5, correct?

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-max-steveastrouk

Answer 3 years ago

Thanks! Yeah, that's probably not high enough. But in emitter follower mode, would a drop from 6V across the coil to even 5V prevent it from working? My 12V relays will function down to around 9V if I remember right, and down to much lower voltages if I modify them (replace the spring with a softer one).

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steveastrouk-max-

Answer 3 years ago

It was more a general comment on the emitter follower configuration. HOWever, we STILL don't have the actual coil parameters or the supply volts, so all we're doing is speculating.

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steveastrouk-max-

Answer 3 years ago

Don't forget emitter follower mode loses you a Vce voltage drop from the rail, and the voltage on the emitter is always Vbase - Vbe.