Fellow engineer Sanjay R. Chendvankar of Mumbai, India, recently offered a novel way of reducing current through a relay coil (see “Driver Saves Power In Energized Relay”). His idea utilized an unused normally closed pair of contacts of that relay to reduce the driving voltage to the relay once it was energized. The design works very well, but what do you do when an unused pair of contacts is not available?
It is well known that an energized relay coil will maintain contact closure even after the driving voltage is reduced somewhat. For many years, we have been employing a very simple approach to reduce the relay’s driving voltage without relying on unused contacts or sophisticated circuitry. All we do is add two common parts: a resistor and capacitor in parallel.
The resistor/capacitor pair interrupts the usual emitter lead to ground connection of the relay driver transistor (see the figure). When the drive transistor is off, R2 ensures that the capacitor is discharged. When the relay command turns on the drive transistor, that uncharged electrolytic capacitor temporarily appears as a dead short, causing maximum current to flow through the relay coil and closing the relay contacts without chatter.
As the capacitor charges, however, both the voltage across and the current through the relay coil decline. The circuit reaches steady state when the capacitor has charged to the point that all the current through the relay coil is moving through R2. The contacts will still remain closed until the drive voltage is removed.
In this example the resistor value is roughly twice that of the relay coil resistance: a 390-Ω resistor for a 200-Ω relay coil. The capacitor was chosen to be 150 µF at 25 V, creating a combination that works well for a supply of 12 V to the coil. If the drive transistor’s emitter is simply connected to ground, the relay circuit draws around 58 mA. With the resistor/capacitor addition the steady state current is only 20 mA, a nice savings in power. The additional two parts cost only a few cents, and no extra relay contacts are needed.