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| Relay commonly available on electronic stores |
Relay Driving is one of the most important skills in robotics. Basically, a relay is an electrically operated switch. They are used to allow a low power circuit to switch a relatively high current/voltage on or off. For a relay to operate, a suitable pull-in and holding current should be passed to its coil. Generally, relay coils are designed to operate from a particular voltage often 5V or 12V. To energized a relay, a current of typically 25-70mA should pass to its relay coil. The function of the relay driver is to provide enough current needed by the relay.
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| Relay diving basic circuit |
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Figure above shows an NPN transistor used to drive a 12V-relay operating from a 12-V supply. This is the simplest of relay driving. Series base resistor R1 is used to set the base current for Q1, so that the transistor is driven into saturation (fully turned on) when the relay is to be energized. That way, the transistor will have minimal voltage drop, and hence dissipate very little power as well as delivering most of the 12V to the relay coil.
Working with R1 is not that hard. Say RLY1 needs 50mA of coil current to pull in and hold reliably, and has a resistance of 240W so it it draws this current from 12V supply. The transistor will need enough base current to make sure it remains saturated at this collector current level.
To work this out, we simply make sure that the base current is greater than this collector current divided by the transistor’s minimum DC current gain b. So if the transistor has b of say 100, we need to provide it a base current of at least 50mA/100 = 0.5mA. In practice, you’d give it roughly double this value, say 1mA of base current, just to make sure it does saturate. So if your control signal Vin was switching between 0V and +5V (as in 74series IC, flip-flops or microcontrollers), you’d give R1 a value of 4.3kW (using 0.7V as Vbe) to provide 1mA of base current needed to turn on both Q1 and the relay. This value of R1 was obtained using basic knowledge on transistor (KVL equation @ the base-emitter loop).
The simple transistor driver circuit discussed is very low in cost, and are generally fine for driving most relays.
