sizing a freewheeling diode for a coil

[JackShephard]

That's possible, I've had that happen before where
the leads pulled out enough to loss connection.
it was a batch of diodes with bad bonding
on the leads

You fucking retard.

 

[Jamie]

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

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.
[/QUOTE]
That's possible, I've had that happen before where
the leads pulled out enough to loss connection.
it was a batch of diodes with bad bonding
on the leads

 

[Jamie]

JackShephard looks in the mirror.

"You fucking retard"

How many times have I told you to stop talking to your
self? I'm glad you're finally coming to terms of what you are.

Are you now ready to join the human race?

 

[JackShephard]

How many times have I told you to stop talking to your
self? I'm glad you're finally coming to terms of what you are.

Are you now ready to join the human race?

You're mother is a retard.

 

[Spehro Pefhany]

maybe you should be using faster switching diodes?

1N4148s have a maximum reverse recovery time of 4ns. 1N400x diodes are
around 1,000 times longer.

also, I do know if the relay chatters it can heat up
diodes.

Do tell.. how would that work?


Best regards,
Spehro Pefhany

 

[Matt]

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.

I think you mean this one:

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

 

[Paul Hovnanian P.E.]

http://www.theregister.com/2007/05/04/aacs_crack/

 

[James Arthur]

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

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.
[/QUOTE]

I've seen 1n4148s fail across small relays, the 'off hook' relay in
a modem. The cause? Surges in the switched circuit. Big ones. The
loop area of the relays' contacts and connections--as laid out on the
PCB--was sufficient to couple said surges into their coils & pop
1n4148s. A transformer, it was.

This modem was in my PC. After losing several diodes in succession,
I fixed the problem by 'improving' the layout to reduce the loop area
& proximity to the coil.

Designing industrial stuff, we picked relays wired such that the one-
turn switched loop wouldn't couple back to the coil, and laid out
traces carefully on both sides exactly to avoid this problem.
Otherwise, lightning-induced surges can make quite a mess of things as
they propagate backward from the relays and onto your board.

Cheers,
James Arthur

 

[Robert Adsett]

http://www.theregister.com/2007/05/04/aacs_crack/

That's further off track than I'd expect from you Paul. What has this to
do with suppression diodes?

Robert

 

[Jasen]

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.

last I checked 1N4148 was rated for 200 mA continuous.

Bye.
Jasen

 

[Marra]

Tony Williams wrote:

1N4148s have a maximum reverse recovery time of 4ns. 1N400x diodes are
around 1,000 times longer.


Do tell.. how would that work?

Best regards,
Spehro Pefhany

you get contact bounce on the relay.

 

[Jamie]

1N4148s have a maximum reverse recovery time of 4ns. 1N400x diodes are
around 1,000 times longer.




Do tell.. how would that work?


Best regards,
Spehro Pefhany

Was that suppose to be a joke?

 

[Spehro Pefhany]

Was that suppose to be a joke?

Not really, I really have no idea what s/he is talking about in terms
of potentially harmful heating. Do you?


Best regards,
Spehro Pefhany

 

[Robert Adsett]

you get contact bounce on the relay.

And?

Robert

 

[JackShephard]

How many times have I told you to stop talking to your
self? I'm glad you're finally coming to terms of what you are.

Are you now ready to join the human race?

The person you need to blame this on is "Lamey The Cable Guy" or
whatever fucking stupid nym he is going by now.

The posts is a forgery/clone or whatever you want to call it. I did not
write it. That goes for the next one you read as well. I was never in
this thread.

 

[ian field]

I think you mean this one:

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

Backspacing the URL to "appnotes/" presents a selection of pdf's.

 

[John Fields]

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

---
Really?

How about some facts?

Like which relays are you talking about?

Do you have some part numbers and some circuits or is this just some
more of your pretense at knowledge bullshit?

 

[Matt]

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
http://www.fairchildsemi.com/pf/1N/1N4007.html
(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?

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

http://relays.tycoelectronics.com/appnotes/
http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3264.pdf

 

[Adrian Tuddenham]

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.

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.

 

[John Popelish]

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.

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.

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.

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