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duncanlowe · 30th Oct 2020, 8:05 pm

The back EMF from a relay coil is indeed an issue, and for many years a flywheel diode was recommended, and it was possible to buy relays with built in diodes. But certainly in some fields, the relay manufacturers have moved away from this and instead have a resistor. I don't know the detailed reason why but it's somehow related to the opening characteristic of the contacts where a diode affects the time or speed of opening and ends up causing earlier contact wear.

Regarding a circuit, a 555 would work but bear in mind they have a funny little quirk, in that the output is totem pole (that is active high and low) and during change of state both transistors are on at the same time albeit very briefly. Not only can that cause them to be quite generous creators of interference, but on a low impedance supply the dissipation can be surprisingly high. The first time I smelled 'that' smell was when I used a 555 for my own design of intermittent wiper control, which was fine on a power supply but lasted just a few minutes in a car before emitting all of the magic smoke.

Oh, and I'd also say a FET was a good bet for driving relay coils. Lower dissipation generally (depends on RDSon of course) and the intrinsic body diode clamps the back EMF (but consider the first paragraph contardiction here).

30th Oct 2020, 8:05 pm	#30
duncanlowe Nonode Join Date: Jan 2016 Location: Stafford, Staffs. UK. Posts: 2,532	Re: 12VDC power switching circuit The back EMF from a relay coil is indeed an issue, and for many years a flywheel diode was recommended, and it was possible to buy relays with built in diodes. But certainly in some fields, the relay manufacturers have moved away from this and instead have a resistor. I don't know the detailed reason why but it's somehow related to the opening characteristic of the contacts where a diode affects the time or speed of opening and ends up causing earlier contact wear. Regarding a circuit, a 555 would work but bear in mind they have a funny little quirk, in that the output is totem pole (that is active high and low) and during change of state both transistors are on at the same time albeit very briefly. Not only can that cause them to be quite generous creators of interference, but on a low impedance supply the dissipation can be surprisingly high. The first time I smelled 'that' smell was when I used a 555 for my own design of intermittent wiper control, which was fine on a power supply but lasted just a few minutes in a car before emitting all of the magic smoke. Oh, and I'd also say a FET was a good bet for driving relay coils. Lower dissipation generally (depends on RDSon of course) and the intrinsic body diode clamps the back EMF (but consider the first paragraph contardiction here).