Relay with quarter consumption

Although electronics is advancing by leaps and bounds, classic mechanical relays are often found even in the most advanced devices. Of course, also in radio communication devices such as TCVR, PA, tuners, remote antenna switches, and the like. Virtually zero loss on switching contacts is a major advantage over various semiconductor switches. The only disadvantage of relays is their consumption.

Principle of reducing relay consumption

Everyone who has ever constructed something with a relay knows that the voltage at which the relay closes is much higher than the voltage at which the relay drops out. We will achieve a reduction in power consumption by supplying it with slightly higher voltage than the voltage at which the relay drops out after pulling in the relay. This will significantly reduce the total power consumption.

Especially when operating from batteries, we will appreciate the possibility of longer operation using this simple circuit with six components. After applying the supply voltage Un for a period determined by the combination of R1C1, the transistor opens, providing almost full supply voltage to the relay. For direct current, the charged capacitor will shortly represent infinite resistance and the transistor will close. Now the current will flow only through the relay and resistor R3, which will limit the flowing current. Diode D is intended to protect the transistor from induced voltage arising fromthe relay armature release.

The value of resistor R3 is determined by calculation:

R3=(Un-Uod+2V)/Iod [ohm;V,V,A]

Un is the supply voltage
Uod is the voltage at which the relay armature drops
2V is the reserve for fluctuations in Un and below
Iod is the current at which the relay armature drops

Bill of materials

The price of used parts is about 10 (in a word: ten) crowns, and it can drop to almost zero if we use older parts from an old radio, TV... Capacitor C1 should have a small leakage current, for example tantalum or high-quality electrolytic capacitors are suitable. The second critical component is the transistor. It should not have a high gain and the maximum collector current must of course be greater than the relay current.

Another way to reduce relay consumption

Another option is to use a capacitor connected in parallel with a resistor in the voltage supply to the relay. If capacitor C1 is not connected, we connect a resistor trimmer (about 1kohm) instead of R1 and increase its resistance until the relay drops out. Then we measure its resistance with an ohmmeter and replace it with a fixed resistor of lower resistance value so that the relay does not accidentally drop out even when the supply voltage fluctuates.

We connect a capacitor with a capacity of about 100uF to the circuit without voltage. We connect the power supply. If the relay closes, we disconnect Un, replace the capacitor with another one of smaller capacity (68uF). This way, we experimentally determine the minimum capacitor capacity needed for the relay to reliably close.