OpAmp, Buffer and Relay

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AtomSoft

Well-Known Member
Ok Ill explain what i have and what i want to do ... let me know if im crazy..

I have a 3.3v MCU, 5v Relay

I want to use an opamp to jump my 3.3v to 5v and buffer it since there are usually opamp pairs in an IC... then with the buffered pin control a npn or pnp to control a relay.

Would this be too complex for something simple? Or does it sound about right? Or should i use a level shifter?
 

just use the microcontroller to a 330 ohm to 1k ohm resistor to base of NPN. Then emitter of NPN to ground, collector to relay coil. Other end of relay coil to 5v supply
 
gophert said:
just use the microcontroller to a 330 ohm to 1k ohm resistor to base of NPN. Then emitter of NPN to ground, collector to relay coil. Other end of relay coil to 5v supply
Click to expand...

I will try this asap. Thanks. I just dont feel like burning anything up today lol
 
atferrari said:
And a friendly diode to catch de spike, in parallel with the coil, anode to Vcc?
Click to expand...
Depending on the relay load, the diode may slow down the speed at which the relay opens. Best practice is a to have a resistor in series with the diode, of about 5 -10 times the relay resistance. The voltage spike will be 5 - 10 times the supply voltage, so 25 - 50 V, so still well within the rating of readily available transistors. the relay current will drop 5 - 10 times faster than with a diode on its own.
 
Quick image grab from the innergoogle to show what is described in post #2. The resistor and transistor sizes depend on the relay coil. Voltage? Current? Resistance? Power?

ak

 
I recognise that picture



You could also use a small FET, I've just used four 2N7000's (with four 1N4148's across the relays) to feed four 5V relays from an ESP32. I could have used NPN transistors - but I've got a load of 2N7000's at work
 
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