a/c diode

What is the function of a diode in hot water shut off solenoid?
I am in a process of fitting a/c in to my E30 M10 316 (without factory a/c). I have just fitted complete box (with heater and evaporator).
While components testing I must have reversed the polarity, and burned the diode. I replaced it and shut off valve works. But it works without diode too. Only with burned diode will the fuse burn.
And is it some special diode? I fitted one that was probably from a broken old TV.
Another question. Looking at a wiring diagram I noticed "a/c comp oil temp switch". I do not have that. Do I miss this component, or I have a/c set from different year.
And there is also a diode at compressor clutch. Is this diode function the same as function of the one at a hot water shut off solenoid?
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/__/ / \ ** Registrovani korisnik Linuksa #291606 **
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Yvan wrote:

If these diodes are directly across the coils in question (not in series with them) - they are used to suppress voltage spikes that can be caused by the coils being de-energized. If they are in series with them - I have no idea of the function.
They may be zener diodes - which means - yes - they are special, or they could be plain diodes, but even plain diodes have a current and voltage specification, and if you used one that was meant for high voltage and low current, and it was in series with the coils, it wouldn't surprise me that it blew up. IF these are still available from BMW - I'd suggest getting the correct one.
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about something:

I would guess that it's a back emf shunt.
A wire wound electro magnetic component like a solenoid when switched off will produce a power spike as it's magnetic field collapses. This spike can hurt delicate electronics, so it's best to just short it out with a diode.
That's my guess to it's function anyway.
It could also be a zener diode, to prevent too high a voltage being presented across the solenoid... Only the number from it will tell.
Dodgy
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Nedavno Dodgy pise:
| A wire wound electro magnetic component like a solenoid when switched | off will produce a power spike as it's magnetic field collapses. This | spike can hurt delicate electronics, so it's best to just short it out | with a diode. | | That's my guess to it's function anyway. | | It could also be a zener diode, to prevent too high a voltage being | presented across the solenoid... Only the number from it will tell.
It broke to peaces when I tried to remove it, so I can not read numbers from it.
If I can find another hot water shut off valve that I can read diode numbers from, would it be a mistake to put it further up at connector (not as originally at the end of solenoid wires)? I replaced diode while valve was out of the car, but now it is hard to reach.
BTW I put a diode that is about the same size (4mm long and 1.5mm in diameter). How do I check if it is not burned (without removing it)? I thought that resistance should be different when I reverse polarity on ohm-meter (it should be one value for solenoid wire resistance, and less for diode - huuh I hope you understand what I am trying to say, English is not my native language). It is not. It's the same both ways.
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Yvan wrote:

Yvan,
Others have explained the diode and I agree with them, we call them "free wheeling diodes" in the electrical industry and they are meant to collapse the coil induction effect when de-energizing the coil. In DC Circuits they allow the use of smaller contacts on the interrupting relay.
The relative impedance of the coil to the diode means that there will be little difference in the measure with an ohm-meter in either direction.
If you have an old multi-meter (non digital) you can measure the voltage across the coil when it is energized and then de-energize it. The voltmeter will register a slightly negative voltage upon de-energization, the negative voltage is the forward bias of the diode as it allows the coil induced voltage to decay in the loop.
Just about any power diode larger than 500mA should work as I think they all will have a minimum 600V rating and I am sure the coil will not put out more than 1/2 amp but measuring the energizing current and sizing to that will guarantee it will work.
Good luck with it...
Bob
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Nedavno Bob pise:
| If you have an old multi-meter (non digital) you can measure the | voltage across the coil when it is energized and then de-energize | it. The voltmeter will register a slightly negative voltage upon | de-energization, the negative voltage is the forward bias of the | diode as it allows the coil induced voltage to decay in the loop.
I do have non digital voltmeter. So if needle does not go from 12V to negative when coil is de-energized diode that I fitted is OK?
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Yvan wrote:

If the diode is open circuit the voltage will go to higher than the forward bias of a diode (probably to -12V or so given the automotive supply) and decay slowly if the diode is working properly the voltage will go to a maximum of the forward bias (0.6V for Si and 0.2V for Ga) and decay quickly.
Try it on a coil without a diode and then on one with a diode. You will see the difference.
Cheers, Bob
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Nedavno Bob piše:
| If the diode is open circuit the voltage will go to higher than the | forward bias of a diode (probably to -12V or so given the automotive | supply) and decay slowly if the diode is working properly the voltage | will go to a maximum of the forward bias (0.6V for Si and 0.2V for | Ga) and decay quickly. | | Try it on a coil without a diode and then on one with a diode. You | will see the difference.
I will, thank you.
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Bob wrote:

But the decay will be too fast to see with an analog voltmeter. You would really need to scope it.

Zactly.
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That gives the clue. A diode (for this application) is a device with a very low resistance to DC in one polarity direction and very high in the other. When wired in series with a device it can be used to protect from incorrect polarity. When wired in parallel with an inductive load like a solenoid or relay coil it is used to limit the voltage 'spike' (back EMF) when the coil voltage is switched off. If you reverse the polarity in such an application the battery sees a short and blows the fuse and wrecks the diode, which becomes a short circuit. The solenoid itself will not be damaged.

There are many different types, but a 1 amp rectifier diode with a PIV of 200 volts or so should be fine for this sort of thing. They cost pennies.
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Nedavno Dave Plowman (News) pise:
| When wired in parallel with an inductive load like a solenoid or | relay coil it is used to limit the voltage 'spike' (back EMF) when | the coil voltage is switched off. If you reverse the polarity in such | an application the battery sees a short and blows the fuse and wrecks | the diode, which becomes a short circuit.
So how do I test if diode is not blown (when diode is connected to the solenoid)? I knew that it did not create short circuit, but how do I knew that it had not gone open circuit?
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The simplest way is with the diode test function on pretty well every DVM.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Nedavno Dave Plowman (News) pise:
| > So how do I test if diode is not blown (when diode is connected to | > the solenoid)? I knew that it did not create short circuit, but how | > do I knew that it had not gone open circuit? | | The simplest way is with the diode test function on pretty well every | DVM.
Will that work while diode is connected to the coil (and I knew diode was OK before I fitted it)?
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A DVM costs less than 9 Euros in a DiY store. It's able to deal with 200 mV to 300V (AC & DC) and from 100 micro Amps up to 10 Amps (DC & AC), it allows also to measure the resistances and checks diodes and transistors. The accuracy is around 1%, far more than you should need for usual operations. Worth to buy it, believe me. Probably on e-bay as well. To e-mail, change noise into sound.
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Dave Plowman (News) wrote:

...which would work if it is removed from the circuit. But if the low resistance of the solenoid coil is still across the diode (electrically) you can't check it that way.
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