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I have a comparator: AD8651 that I am trying to use to switch on my relay (relay is rated at 5V). I have a 5V supply that I am using and I am just using a function generator to see the response. For this situation I am "comparing" to 2.5 V and my function generator is going at 100 Hz from 0 to 5 (I have this set up on a breadboard on my desk).

I am trying to figure out the proper use of the hysteresis and load. So I tested my relay and it has ~30 Ohm resistance between it so I put a 30 Ohm resistor on my output, and when I do that, my 5V does nothing. The comparator just spits out 0 V (well actually like 20 and 50 mV).

If I use a large load, like 100k (or 1 MOhm on the termination of scope)then I get 0 and 3.8V. But with the load that drops to zero. I am kind of assuming that my supply cannot draw enough power to handle the 30 Ohm load? Can I compensate with the hysteresis?

schematic

simulate this circuit – Schematic created using CircuitLab

yankeefan11
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3 Answers3

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Your comparator can't supply enough current for the relay. You can only get about 3mA from the comparator so you will need to add a transistor to activate the relay. It's still possible that your power supply can't provide enough current but you didn't give enough information to comment on that.

Elliot Alderson
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  • My supply is 5V and up to 3A, so I thought I would be alright, but seems like not. I do have a transistor around here, so I will do that. Thanks – yankeefan11 Jul 26 '18 at 11:40
  • The supply's 3A rating is the limit it can provide. It does not mean the comparator itself can pass through the same 3A. If it can, I don't think it would do so for very long. – Richard the Spacecat Jul 26 '18 at 12:59
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Wrong comparator:

This one is high speed 10mA

5V relay is possibly 80 mA.

Also you have it wired wrong.

See answer : Non-TTL debouncing

Tony Stewart EE75
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First of all, I think it's a bad idea to switch a relay at 100 or 1kHz. The contacts will wear out very quickly. Instead, you can use a transistor.

Generally speaking, OpAmps are unable to drive relays, so we use driver IC, such as ULN2003. This can solve the problem, assuming that your power supply can provide the power needed.

Also, Analog provides in page 8 of the datasheet, a way to utilize a hysteresis loop. They mixed up inverting and non-inverting inputs, but you can get a good idea for how to implement it.

thece
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  • I wasn't actually switching the relay, just looking at the output on a scope with a 30 Ohm load. As for the reason I am using the Op Amp, it is what we have around, so I am just trying to design something from that. – yankeefan11 Jul 26 '18 at 11:41
  • My bad, I misunderstood the part where you said you put a 30Ω resistor.

    In any case, like everyone said, it is not possible to drive the relay with this OpAmp. You have to use a transistor, a driver or an other OpAmp. And see page 8 of the datasheet, where they present a hysteresis configuration.

     – thece Jul 26 '18 at 12:43