sizing a freewheeling diode for a coil

I just finished a project that uses 12V, 30 ma relays. Used 1N914 s.

Tam

"Matt"

Freewheeling diodes are associated with DC motor drives and swiching regulators.

The cheapest possible diode are commonly used - like 1N4148s or 1N4001s.

........ Phil

The absolute minimum repetitive surge current rating for the diode is the steady state current for the coil, under the highest supply voltage. If the diode is rated for a continuous current equal to or greater than the coil current, the brief inductive quench can't possibly overheat the die.

The minimum reverse voltage rating for the diode is the highest possible supply voltage. Some extra voltage capability seldom costs much. But a lot of extra voltage capability may have a down side. High voltage diodes generally turn on and off slower than lower voltage devices and may recover with a vicious snap that generates high frequencies. So a 1000 volt diode is probably not quite as good as a 100 volt diode for a 24 volt coil.

If you're not worried about noise that maybe fine how ever, my self, I like to use low voltage diodes which tend to switch faster.

Matt. a high voltage diode is unnecessary because your high voltage spike from the coil is in the forward direction so it only needs to cover your working voltage. Like some have suggested, I don't think you need a 1A average but again I would be afraid to use a 914, I always considered that a signal diode, I didn't even know it was still available.

Thanks for all the helpful replies.

It seems that the overall reliability of the device is best served not by a single diode, but by a transient voltage suppressor. The diode is a little better at protecting the transistor that switches the coil, but apparently it can cause a not-so-clean unmaking of the relay, leading to welding or erosion of the contacts in the case of a heavily-loaded normally-open relay. This is described in

If the contacts operating the relay become dirty or the control transistor begins to oscillate, then you will get repetitive surges at quite frequent intervals. It is better not to rely upon the single-pulse surge current rating of the diode.

A diode with a continuous rating equal to (or greater than) the maximum coil current will give peace of mind under all possible conditions of operation and mis-operation.

I did not mention the single pulse surge current rating, but the repetitive surge surge current rating. But as with any limiting case, it is a good idea to include some safety factor. By the way, if the coil is driven by rapid pulses, its current will not reach the full, DC, steady state value, but will bounce between some lower limits.

Agreed. Especially if the diode is in a good thermal situation.