sizing a freewheeling diode for a coil

I am controlling the coil of a relay using a switching transistor. To protect the transistor I intend to use a freewheeling diode in parallel with the coil. I'm not sure of the issues in specifying the diode.

My impulse is to simply use a 1N4007

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(1000V peak repetitive reverse voltage, 1.0A average rectified forward current) because it is common, cheap, and seemingly the most heavy-duty of the 1N400x line. Would that be a good choice for about any PCB-mounted relay?

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

Tam

Reply to
Tam/WB2TT

"Matt"

** Normally called a "kick back" diode - IME.

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

** Long as the diode can pass the same average current need to drive the coil and sustain the DC voltage across it - it should be OK.

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

........ Phil

Reply to
Phil Allison

I've known quite small relays to 'take out' 1N4148s, 914s etc.

Graham

Reply to
Eeyore

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.

Reply to
John Popelish

I hear people say this, but I have never seen a case of it. I have used 1N4148 diodes for 200 mA coils for years.

Reply to
John Popelish

I've had it. 1N4148s gone to a short circuit. Never found out why.

--
Tony Williams.
Reply to
Tony Williams

It's the energy storage, proportional to inductance and square of the current, that matters. A 200 mA coil on a reed switch has much lower stored energy than a 50 mA coil on a frame relay. When you're using more current than the (average) rating on your diode, it only succeeds if the temperature spike doesn't melt anything.

Reply to
whit3rd

ESD? Were the contacts connected to "external" signals?

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John Devereux
Reply to
John Devereux

But the diode will still only see the 50mA current, even if the stored energy is *enormous*. It will just take a bit longer for that current to decay when the driver is switched off (and this will be independent of the diode rating).

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John Devereux
Reply to
John Devereux

They were multiplexing resistor values to simulate a thermistor, into a unit that was about 3ft away.

The relay coils were 24V/1k. I still don't see any reason why 3 1N4148s in a bank of 8 should all go s/c, but they did. The only reasonable explanation was possibly a bad batch of diodes.

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Tony Williams.
Reply to
Tony Williams

maybe you should be using faster switching diodes? also, I do know if the relay chatters it can heat up diodes.

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Reply to
Jamie

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.

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"I\'m never wrong, once i thought i was, but was mistaken"
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Reply to
Jamie

But the temperature rise in the diode is not proportional to the stored energy, but is related to how fast that energy is dumped into the diode. A very large inductance energized at

50 mA may take a long time to discharge, but that gives a long time for heat energy to escape the die, also.

By the way, 200 mA is not higher than the average current rating of the 1N4148. See "IF" on page 3:

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Reply to
John Popelish

Jamie wrote: (snip)

I think this can be a problem if the coil is driven by another relay contact, rather than a solid state switch. If a dry contact re closes while the diode is conducting (as during contact bounce), there can be a rather large reverse current spike.

Reply to
John Popelish

But the relatively low frequency involved by the driving relay will keep that average power really low. Also this will suppose that the coil current hasn't decayed down to zero which is unlikely a normal working condition as well for an all relay system.

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Thanks,
Fred.
Reply to
Fred Bartoli

=20

=20

p=20

My point was that the most stressful moment n the life of a=20 coil diode might occur with a dry contact driving the coil.=20 At contact release, if there is not a clean break, the=20 first opening diverts full coil current through the diode,=20 but as the contacts slide apart, there can be a very abrupt=20 re application of supply voltage, in reverse, across the=20 conducting diode. This can cause a very strong reverse=20 recovery current that dumps, not well defined coil current,=20 but essentially unlimited supply current, through the die.

Reply to
John Popelish

1N4007
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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.

Reply to
ekrubmeg

An application note from a relay manufacturer that I read a while ago suggests a small signal diode in series with a zener.

Apparently if just a diode is used, the emf due to collapsing field can pass enough current round the diode/coil circuit to cause faltering contact separation and contact burn. There are other serious issues but I'd have to search out the appnote to remind me what they are.

The zener should be wired so it would forward conduct with the transistor on and the small signal diode in series pointing the other way to prevent that happening, a guesstimate of Vz might be about 60% of the transistor's breakdown voltage.

Reply to
ian field

Jamie wrote in news:EUj8i.22$ snipped-for-privacy@newsfe04.lga:

You f****ng retard.

Reply to
JackShephard

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