sizing a freewheeling diode for a coil

[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?

 

[Tam/WB2TT]

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?

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

Tam

 

[Phil Allison]

"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.

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

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

I'm not sure of the issues in specifying the diode.

** 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

 

[Eeyore]

"Matt"


** 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.

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

Graham

 

[John Popelish]

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?

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.

 

[John Popelish]

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

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.

 

[Tony Williams]

Eeyore wrote:

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.

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

 

[whit3rd]

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.

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.

 

[John Devereux]

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

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

 

[John Devereux]

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.

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).

 

[Tony Williams]

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

[/QUOTE]

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.

 

[Fred Bloggs]

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

If the switch transistor recovers very quickly and the current is large
enough, it may be possible to fail these ss diodes with hot spot
development. I have observed over 2V forward voltage development lasting
for upwards of 75ns at moderate currents this way.

 

[Jamie]

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

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

 

[Jamie]

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?

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.

 

[John Popelish]

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.

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:
http://ronja.twibright.com/datasheets/1N4148_1N4448_5.pdf

 

[John Popelish]

Jamie wrote:
(snip)

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

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.

 

[Fred Bartoli]

John Popelish a écrit :

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.

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.

 

[John Popelish]

John Popelish a écrit :

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.

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

 

[ekrubmeg]

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 1N4007http://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?

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.

 

[ian field]

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).

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.