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  • One of the serious problems in relay operated circuits is the relay clicking or chattering during the on/off of the relay driver transistor. This problem is severe if the input circuit is a light/temperature sensor. During the transition of light/temperature levels, the relay clicks which may cause sparking of contacts. By using a simple tip, this problem can be avoided.

    Below is the circuit of a relay driver using the NPN transistor BC 548. The relay is connected between the positive rail and the collector of the transistor. When the input signal passes through the I K resistor to the base of the transistor, it conducts and pulls the relay. By adding a 470 uF electrolytic capacitor at the base of the relay driver transistor, a short lag can be induced so that the transistor switches on only if the input signal is persisting. Again,even if the input signal ceases, the transistor remains conducting till the capacitor discharges completely. This avoids relay clicking and the offers clean switching of the relay.

    Relay Driver Circuit

    transistor relay driver circuit schematic

    transistor relay driver circuit schematic

    Another 470uF capacitor is added parallel to the relay coil which maintains steady current through the relay coil so that relay clicking can be avoided if the power supply varies momentarily.

    IN 4007 diode eliminates back e.m.f when the relay switches off and protects the transistor. LED indicates the on status of the relay

    attentionThe author D Mohankumar is not an active member anymore. Please take into consideration that the presented information might not be correct.
    
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    8 Responses to "Transistor Relay Driver Circuit"

    1. The transistor will be destroyed or short lived if switched on with such a high capacitor at its collector…I think the author has to learn about adding hysteresis to drivers.

    2. Rajavel-Bengaluru says: on February 1, 2011 at 12:22 pm

      Very poor design.High MFD cap in base must destroy the transistor.Like eletronics is a childplay.I am join with you Siva.

    3. Capacitor with relay will be discharged through relay so no worries. However capacitor at transistor base may be a problem, may require a bleeder resistor, not sure, not an expert.

    4. The capacitor on the base will not harm the transistor in any way. It only serves to allow a time delay of when the relay will turn off when the input signal is lost.

    5. Thanx

    6. I don’n understand input signal,what is it?

      • The input signal is a voltage in the order of 3 to 12V or so– generally non-critical. Just keep in mind that the base current should be about 5 to 10% of the collector current, so the 1K input resistor value is dependent upon relay coil and LED current (collector current).

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