File:Economy relay coil drive for battery-powered devices.png
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English: Driving a relay coil from a battery-powered microcontroller at the rated voltage of the relay may be too inefficient. At the rated voltage, the smallest relays consume 100 mW; a relay with 6A, 8A or 10A contacts will probably consume 400 mW to 1W, etc. However, this energy is only necessary to turn the relay on. Once it is on, it will stay on until the voltage across the coil decreases to a very low release voltage value. It is usually safe to assume that the operate (turn-on) voltage is 3/4 of the rated voltage (e.g. 9V for a 12V relay), and that the release voltage is less than 1/4 of the rated voltage. Always check the datasheet for specific values and terms of application. Knowing this, you may construct a simple double switch to save energy losses. Switch T2 engages as soon as control voltage goes high, providing the required turn-on voltage and current. After a fraction of a second, the capacitor charges up and T2 closes. Switch T1 remains open for as long as the control signal is high, providing the continuous low holding current (1/4 .. 1/3 of the rated coil current). When the control signal goes low, the capacitor quickly discharges via a reverse diode and the microcontroller's output port. The approach also works with bipolar switches like the ULN2003. |
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| Source | Own work |
| Author | Retired electrician |
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I, the copyright holder of this work, hereby publish it under the following license:
 
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| current | 00:50, 10 February 2021 | | 2,258 × 879 (86 KB) | Retired electrician (talk | contribs) | {{Information |Description={{en|Driving a relay coil from a battery-powered microcontroller ''at the rated voltage of the relay'' may be too inefficient. At the rated voltage, the smallest relays consume 100 mW; a relay with 6A, 8A or 10A contacts will probably consume 400 mW to 1W, etc. However, this energy is only necessary to turn the relay on. Once it is on, it will stay on until the voltage across the coil decreases to a very low release voltage value. It is usually safe to assume that t... |
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