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Topic: Does this relay need a transistor? (Read 907 times) previous topic - next topic

woodnotoil

http://www.allelectronics.com/make-a-store/item/SRLY-19/1A-SOLID-STATE-RELAY-3-8VDC-CONTROL/-/1.html

The coil charges at 2-8vdc.  I have read a lot of people saying transistors need to be used with charging coils, but it seems to me the 5vdc coming off the arduino would be enough.  Am I right that I need no transistor?  I am turning an HVAC pump on and off on the other side of the relay.  Anyone forsee any problems with that?  Thanks for the help I am real new to all this!   :-/

Grumpy_Mike

Normally relays need a transistor because of the current they take not the voltage.
However, in this case it is a solid state relay, which is not a relay at all and so takes next to nothing in terms of current so no you don't need a transistor with this. :)

woodnotoil

Perfect!  Just what I needed.  Thanks :)

westfw

Just a small correction.  Solid state relays normally use some sort of LED in the "control side", and thus take current similar to an LED.  This particular relay looks like it takes about 20mA, which doesn't need a transistor when connected to an AVR/Arduino, but it's a pretty close thing, and it WOULD need some sort of driver on many microcontrollers with less drive capability...

Grumpy_Mike

Quote
Just a small correction.


Sorry Westfw but this is not correct.

The LEDs in relays of this type are low current. Typically this would require less than 3mA so it can be used with most micro controllers. I haven't been able to find the data sheet of this particular one but three other data sheets of a solid state relay of this capacity have all confirmed the low input current requirement.

retrolefty

Direct connection to an Arduino output pin is fine for driving the input to this relay. You do however need to make sure that the relay output current rating is OK for the size of motor you are going to be running.

So check the current requirements for you HVAC motor, including it's starting surge current ratings and see if this 1 amp relay rating will work.

There are solid state relays that will handle just about any AC voltage and current rating that one might require, however price does increase with higher current ratings.

Lefty

woodnotoil

Looks like the pump draws 87W and 0.75A at its highest speed, so the relay would work.  However, I think I will get a 3A relay just incase I want to add anything to it in the future I will have room.  I assume this one http://www.allelectronics.com/make-a-store/item/SRLY-20/3-AMP-SOLID-STATE-RELAY/-/1.html will be alright.  It is solid state, the coil charges between 3-32Vcd, it is 3A, and 120Vac.  Please confirm that my assumption I still need no transistor is correct.  Thanks for the input.

Grumpy_Mike

Yes that is still capable of being driven without any transistors.

Just a point on the load, it's not so much the running current but the switch on stall current of the motor. This is the surge you get in the current when you first switch it on.

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