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Old 14-09-2020, 07:46   #1
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Diode freewheeling

I wonder is anyone can help. To switch off my motor, a switch puts 12v down a wire to a solenoid and it stops. There is a freewheeling Diode that runs back to the power source from the switch when I choose the engine off position. This Diode broke and started switching off my motor when it felt like it, sometimes after 15 mins running , sometimes straight away. I took it out now and all runs fine. However, itís part of the Perkins sabre engine panel and I would like to replace it. I think itís there to avoid spikes.

Could anyone tell me what sort to use, I canít read the writing on it and itís no longer available.
Thanks John
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Old 14-09-2020, 16:08   #2
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Re: Diode freewheeling

If itís really a freewheeling diode, also called a snubbing diode, itís usually placed, with reverse polarity, across the power leads of an inductive circuit. When normal polarity power is applied, the diode is reverse-biased and does nothing. When the power switch (or relay, push button or whatever, opens, the magnetic energy in the coil can generate a reverse polarity spike. With a snubber diode, this reverse spike is shorted out and hopefully dissipated as heat. The diode is used to protect the switch contacts.

With a STOP solenoid as you described, the diode normally connect the diode across the two wires at the solenoid. It will still work if itís connected anywhere from the solenoid +12 to ground. You can buy 200-volt, 6amp axial lead diodes for about 50 cents.

Being cautious, and not having the wiring diagram for YOUR motor handy, I would verify that itís really supposed to be a snubber diode. If itís really wired back to some voltage source other than the Stop switch, then itís being used for some other purpose.
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Old 14-09-2020, 19:16   #3
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Re: Diode freewheeling

Quote:
Originally Posted by Fuss View Post
Could anyone tell me what sort to use, I canít read the writing on it and itís no longer available.
Thanks John

Many will work OK, a 1N5400 would be a common choice, readily available and under $1 each. Buy a box and you'll have spares



https://www.mouser.com/ProductDetail...tHzWFhYg%3D%3D
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Old 14-09-2020, 23:37   #4
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Re: Diode freewheeling

We use 12Vdc 1.2Amp solenoids and we can see 500V+ on the back EMF spike, reality is it is probably higher than this but to fast for us to capture. It will kill switches, relays, PCB's etc. in time.

Definitely replace the diode straight away.

I would recommend you use a 1N5408 diode not knowing how much current you draw, this is capable of handling 1000v spikes repetitively.
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Old 15-09-2020, 00:10   #5
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Re: Diode freewheeling

The OP never said where he was or what kind of access he has to electronics parts. So, rather than advise a specific diode, which might be hard to find, I’d just recommend ANY diode with a rating of greater than 400 volts PIV and at least 6 Amps. The 5400 series might be too small for a kill solenoid and costs about the same as the larger diodes.
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Old 15-09-2020, 00:29   #6
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Re: Diode freewheeling

One US supplier that I quickly found is www dot MPJA dot com.
1000 PIV @ 6 A. Stock number 25224DI. $0.76/each. The minimum shipping charge buys 10 of them. Try to find something equivalent locally.
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Old 21-09-2020, 00:27   #7
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Re: Diode freewheeling

Thanks very much for your replies, I appreciate them. I am in Europe in the Greek Islands, so when I am back, I will go to an electronics shop and get a replacement diode. I hope the new one is reliable as it is a bit of a safety issue when the motor can be stopped by such a small part...Iím thinking of installing a switch in the stop wire so that I can control when the motor can be stopped ... this way if the fault occurs again, 12v cannot flow down the stop wire.
However, Iím not sure I like adding this complication. I have seen that in the latest instrument panels, the stop circuit is a separate push button with no diode.
John
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Old 21-09-2020, 04:20   #8
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Re: Diode freewheeling

Electronics shops, Iím afraid, are a thing of the past. Nowadays, while ordering on-line, I can still remember how they smelled and the names of the people who would get your discrete parts from a million little drawers behind the counter
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