can OPAMP ua741 output directly be connected to a relay? relay requires 100mA i/p current?

if not, HOW can this current be supplied to the relay? by using a transistor??

3 Answers

  • 1 decade ago
    Favorite Answer

    No, the 741 can only supply 10 mA or so.

    You can use a transistor to switch the relay. There are two ways of doing this. Unfortunately Y!A does not allow diagrams.

    (1) Common emitter configuration. Emitter tied to ground, relay wired from V+ to collector, base resistor between 741 and base to limit base current. This relies on your 741 being powered from + and - supplies and your relay being rated at the voltage of a single supply. The reason for that is that the 741 output cannot swing low enough to switch off the transistor in a single-supply circuit. There are ways around this like using forward-biased diodes or a reverse-biased zener in line with the base resistor.

    (2) Common-collector. Collector to V+, emitter to relay, base to 741 output. Simpler but you lose some power in the transistor and you don't get the full voltage swing on the relay coil.

    The above assume a NPN transistor. Don't forget to put a rectifier diode reverse-biased across the coil to dissipate the energy of the coil on switch-off.

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  • 1 decade ago

    No, it can only source or sink a few milliamps.

    Use an external transistor such as a BC337. This has a hFE (gain) of at least 100. So connect a resistor from the op-amp output to the base that will allow about 1mA of current to flow when the op-amp output is high (remember when calculating its value that the op-amp output will not go all the way to +Vcc, and the base-emitter juction of the transistor will drop about 0.7V.) The emitter of the transistor goes to 0V. The collector of the transistor goes to one end of relay coil, and the other end of the relay coil goes to +Vcc. You will also need a 1N4007 diode connected "backwards" across the relay coil (i.e. so it's normally not conducting), to protect the circuit from the back EMF generated in the relay coil when the transistor switches off.

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  • Bert K
    Lv 7
    1 decade ago

    No. It can supply about 1.7 ma.

    Boosting the output with a transistor is the usual way to drive heavier loads.

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