Renewable Energy Monitor

These four techniques extend component life and reduce power consumption in relay circuits.

1. Use a flyback diode

Advantage: Protects your relay control circuit from the 'flyback' or 'inductive kick' voltage generated by a relay's collapsing magnetic field as the relay is switched off.

Well-known and a must-have. Place a diode across the relay coil windings. IE, parallel to the coil and reverse biased in normal use. IE, so the diode does not normally conduct.

In practice:

Diode sizing isn't much discussed. But bear in mind that the issue we are dealing with is a very short-term high current that will flow in the forward-conduction mode of the diode. Diodes are well able to withstand higher-than-rated forward current provided the current is short-duration. So the diode generally does not have to be chunky. Think 1N4148 through to 1N4001.

Position the diode as close to the relay coil as possible, not the electronics controlling the relay coil

Use a diode with low (ie 'fast') reverse voltage recovery time (t_rr). Schottky diodes have no (t_rr) but if using a Schottky, do choose one with a suitably high reverse voltage breakdown specification.

There are many ways of tuning flyback response for particular results.

Further reading:

http://en.wikipedia.org/wiki/Flyback_diode

http://whites.sdsmt.edu/classes/ee322/class_notes/322Lecture4.pdf

http://www.electronicrepairguide.com/ultra-fast-recovery-diode.html

http://en.wikipedia.org/wiki/Schottky_diode

2. Add a snubber across the relay contacts

Advantage: Extends relay contact life, especially in DC applications

A snubber absorbs the sparks that occur when relay contacts open. A resistor and capacitor in series, they absorb the current caused by collapse of the [electro-]magnetic field around electrical conductors (the relay contacts in this case) and allow it to flow into the capacitor and then to bleed away via the resistor.

When no snubber is present, the collapsing [electro-]magnetic field produces high voltages which arc across the relay contacts, slowly melting the contacts away.

In practice:

100R and 100nF (0.1uF) work fine for most applications.

Position the snubber as electrically close to the contacts as possible.

3. Reduce relay 'hold-on' current

Advantage: Reduces the power used by a relay that is being held in 'coil energised' state.

Considerably more power is needed to 'pull on' or energise a relay's coil than is required to 'hold on' or keep the coil energised. But most relay drive circuits continue to supply full power to the relay coil after energising the coil. In reality the relay does reduce the power it consumes after latch but its hold-on power consumption can be further reduced to save power without losing the relay's 'latch'.

Assuming the relay pulls on 50mA but holds on with 25mA, we can current limit the relay to 25mA with a resistor in series with the relay. We parallel a capacitor with the resistor to allow the initial pull-on current to flow unhindered.

In practice:

RC values vary with the relay. Use the lowest value capacitor and resistor that reliably energises and holds the relay. I found 560R in parallel with a 1,000uF 35V electrolytic capacitor worked fine for an old, 12V junkbox relay.

The capacitor enables the driver circuit to deliver all its avaialable current with no significant reactance from the relay coil. As a result, relay pull-on is usually faster than normal, but the driver circuit may be over-driven (though that is unlikely).

If movement shakes the relay out of hold it will not automatically re-energise (because its driver is only offering hold on voltage). So this technique can't be used in circuits that may be knocked about.

4. Use a bypass capacitor

Advantage: Reduces unpredictable switching

Switching high current DC on and off produces voltage and current transients in a relay's power supply lines. And also in the lines powering the relay's control circuitry. A high value - 100uF - capacitor stabilises relay power lines against current variations to protect circuitry against these transients.

In practice:

Select a capacitor with a voltage rating at least double the highest voltage that will be seen on the power lines