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OK to use 1N4148 100V-max diode to suppress relay coil transients?

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JDW:
I've always used 1N4004 or 1N4007 diodes to suppress relay coil transients in my engineering designs without problems, despite their switching speed being slower than a 4ns 1N4148.  Faster switch speed is typically preferred for something like this, but as you all know, a 1N4148 can only handle 100V max (some SMD types are only 75V max) whereas a 1N4004 can handle up to 400V.  Why does that matter?  Well, for most automotive relays that I use, I've measured the transient peaks to be well over 150V in most cases.  So for that reason I feel it best to go with a slower yet safer 1N4004 or similar due to its max voltage spec.  And while there are high voltage Schottky diodes out there, they are more expensive and therefore less desirable for coil spike suppression.

The reason I ask this question about the 1N4148 is because I have been asked by a client to price-quote an existing design where a large number of relays are used and external 1N4148W diodes used to suppressing relay coil spikes.  I am thinking it is best for me to advise the client AGAINST use of those 1N4148 diodes, but before I do that I want to hear the thoughts of other EE's out there.  Specifically, I would appreciate hearing some sound engineering advice on this point that goes beyond the mere, "well, I've used 1N4148's to suppress coil spikes for years and haven't noticed them fail."  Datasheets aren't everything, but the specs really do matter.  The fact remains a 1N4148 is rated at 100V max and a 150V+ transient from a coil is over-voting that diode.  In my mind, that is a recipe for failure at some point down the line, especially if the relay switches quite often.  Therefore, is it really a good idea to use 1N4148W diodes to suppress relay coil transients, especially in a design that requires long term reliability? 

The relay coils in my client's application are rated for 12V, 24V and 48V. The application where this circuit board will be used is in a 48V autonomous vehicle used in factories. Power on the circuit board which is feed to the relays is supplied by the following 4 DC-DC converters, ensuring a stable power source regardless of vehicle battery voltage dips:


* CHS4004848
* CHS1204824
* CHS604812
* CHS3004824
I look forward to hearing your thoughts.

Thank you.

jmelson:

--- Quote from: JDW on April 17, 2020, 01:12:37 am ---I've always used 1N4004 or 1N4007 diodes to suppress relay coil transients in my engineering designs without problems, despite their switching speed being slower than a 4ns 1N4148.  Faster switch speed is typically preferred for something like this, but as you all know, a 1N4148 can only handle 100V max

--- End quote ---
The purpose of the diodes is to recirculate the current in the coil, thereby PREVENTING the voltage spike.  So, as long as the diode is working, there will be only a very slight voltage transient when the coil is turned off.  So, the 1N4148 should work just fine.

Jon

JDW:

--- Quote from: jmelson on April 17, 2020, 01:36:39 am ---
--- Quote from: JDW on April 17, 2020, 01:12:37 am ---I've always used 1N4004 or 1N4007 diodes to suppress relay coil transients in my engineering designs without problems, despite their switching speed being slower than a 4ns 1N4148.  Faster switch speed is typically preferred for something like this, but as you all know, a 1N4148 can only handle 100V max

--- End quote ---
The purpose of the diodes is to recirculate the current in the coil, thereby PREVENTING the voltage spike.  So, as long as the diode is working, there will be only a very slight voltage transient when the coil is turned off.  So, the 1N4148 should work just fine.
Jon

--- End quote ---

Jon, I certainly appreciate your speedy reply and your advice; however, I still have two concerns:

1. The phrase "SHOULD work" which implies "hope" that it will work rather than guaranteed success.  I cannot leave the door open to chance.
2. Based on what you said, what then about a 50v-max Schottky diode across the coil?  Surely at some point, the voltage spec of the suppression diode really does matter, and in that case, what is the voltage cap we need to consider here?  So long as the max voltage spec of the suppression diode is higher than the steady state voltage of the relay coil it's safe to use?

Please also consider that some relays have some pretty massive coils.  That is why I specified that some relays will be 12V across the coil, others 24V and yet other 48V across the coil.  Some of the relays have 4 switches in them.  The transients I am talking about are therefore different from the average relay you may have dealt with, hence the concern expressed in my original post. 

You seem to basically be saying that "voltage spec of the diode doesn't matter because if the diode is working, even if the diode is a 50V-max part (a diode voltage rating higher than the steady-state coil voltage) it should be okay because it surpasses the transient."  I am just trying to confirm if that is really so, regardless of relay coil voltage and coil current.

Thank you!

uer166:
Something is wrong with the assertion that a diode-clamped relay coil can generate 150V, that is not how the circuit works. The diode needs to be rated to the relay coil working voltage + some overhead. So in case of a 48V coil, you can't use a 20V-rated diode, it will simply break down when the relay turns on.

I've used a 75V diode, as well as the 1n4148 for a massive 48V coil relay with no problems, and verified that no spec of the diode is exceeded at all temperatures. It turned out that the 1n4148 had very large margins.

When you say "150V spikes", you might be talking about classic resistor-based snubbers, I've seen that in automotive. That doesn't apply here at all since there is no resistive snubber, just a recirculating/clamping/snubbing diode whatever you want to call it.

This is a huge topic, did you know that using a zener clamp instead of a recirculating diode can massively improve relay contact life? That is because the relay opens much quicker, since the coil energy is dissipated in the zener, and the contact arcing lasts for less time.

Someone:

--- Quote from: uer166 on April 17, 2020, 02:00:46 am ---This is a huge topic, did you know that using a zener clamp instead of a recirculating diode can massively improve relay contact life? That is because the relay opens much quicker, since the coil energy is dissipated in the zener, and the contact arcing lasts for less time.
--- End quote ---
Zener for the win...  however when the OP is agonising over pricing of 1N4148 vs 1N4007 it is possibly outside their constraints. If cost is the driver I'd be flipping the problem on its head and going for an SMD load of as many parts as possible, but thats getting off topic.

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