Shorted Diode...need info

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Just wondering if anyone knows the answer to this one
1990 Escort GT, I tested the diode for the AC Clutch coil and it comes up shorted. Also checked resistance and the same both ways @1.7 ohms.

Would this be a 1 amp, 400v or maybe a 6 amp 400v diode? Can't find much in the way of information googling.
Thanks
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ohms.

As long as the diode is still in the circuit, you'll be reading the resistance of the coil itself. One end needs to be isolated, or the diode removed, in order to get a proper reading. Once removed, it should read open in one direction and short in the other (or, at least, a high resistance in one direction and little or no resistance in the other).
SC Tom
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Forgot to mention, it is removed, I have the harness out of the vehicle.
Thanks
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snipped-for-privacy@optonline.net wrote:

What marking is on the diode?
This is just a snubber, I think.
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Not sure of the term "snubber" unless we're talking oil patch.... usually called a "clamping" diode oin the trade, it's job is to provide a path for the inductive current produced when the AC clutch coil is de-energized. Same deal???
ohms.

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Yes it just prevents spikes to the rest of the car when the coil field collapses when the clutch is de-energized.
Thanks
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Jim Warman wrote:

Yes, same thing. <http://en.wikipedia.org/wiki/Snubber
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On Oct 15, 1:58 pm, snipped-for-privacy@gmail.com wrote:

I tried taking it apart, but it nothing left of the component inside, just bits. Looking for the volts/amps spec for this diode so I can replace it with a similar harness or just the diode. Anybody???
Ford part on the wiring diagram is EOVB-14A604-AA
Thanks
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Why not just hard wire a diode across the harness near the connector?


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Sure I'd love to, I can pick one up at Radio Shack in five minutes... but which one??? They're not all the same, what's the forward voltage need to be, what's the amperage limit need to be... if I knew I'd be on my way.
Thanks!
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Sure I'd love to hardwire a diode across the harness near the connector, but which one? They're not all the same. What's the forward voltage need to be, what's the amperage limit need to be... if I knew I'd be on my way. __________________________________________________
Forward voltage is fixed by the diode itself at about 0.6 volts.
Reverse voltage capability should be greater than 12 volts. 40V is okay.
Forward current capability should be at least the coil current. Measure the coil resistance and calculate the current at 12 volts. Current going through the operating coil is what the diode has to bleed off.
Connect the diode cathode (minus) lead to the coil +12V wire. Connect the diode anode (plus) lead to the coil ground wire.
Good luck.
Rodan.
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Rodan wrote:

I forgot about the forward voltage, I had my terms screwed up in my frantic quest for knowledge.
Ok, coil resistance is ~4 ohms, voltage is about ~12, so the coil needs to bleed off about ~3 amps. Reading the diagnostic literature it says the coil should draw ~3.5 amps, so figure 4 amps to be on the safe side.
So I need at least a 4 amp diode to snub the coil field collapsing. Now a standard 6 amp diode that can handle up to 400 volts should do the job? Would that theoretically or practically "snub" the collapsing field to protect the rest of the car's circuitry? Assuming more is not necessarily better in this case, but at least not detrimental.
Thanks for the electronics lesson!
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snipped-for-privacy@optonline.net wrote:

But you only got 50% on the test!
1. YOu dont get more power out than you put in!!!! 2. Remember the coil 'engaged' current does NOT pass through the diode
3. You know that the nominal (theoretical) collapse voltage is 10 x input... so that is 150 volts (rounded) - In FACT the rev voltage at 50 would work fine, because it will only see 15 VRev for all intents... but to be safe, always buy for instantaneous application which would be 150, rounded up to 200 v
AND given the 10x voltage.. 4. Doesnt it make sense that nominal collapse current is one-tenth the input? 5. And when you look at the spec sheets, the nominal current rating is for CONSTANT!!! PEAK current .. for the 1 amp 1N40xx series is THIRTY AMPS - NO current passes through the diode when clutch is engaged, and only for a millisecond or several while voltage is removed.
So if you see a handy diode that's rated at 2 or 3 amps at 200v, and it's got a brand on it... it should be more than enough. Certainly dont need a 6 amper.
--
Yeh, I'm a Krusty old Geezer, putting up with my 'smartass' is the price
you pay..DEAL with it!
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Backyard Mechanic wrote:

<snip>
I'm really not getting all this long answer stuff. I'm always up for learning something new but this is really taxing my brain.

Ok, thanks!
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Ok, coil resistance is ~4 ohms, voltage is about ~12, so the coil needs to bleed off about ~3 amps. The diagnostic literature says the coil should draw ~3.5 amps, figure at least 4 amps to be safe. So should a 6 amp 400 volt diode do the job?
Certainly, with a margin of safety. As "Backyard Mechanic" points out: Diode current ratings are for CONSTANT current. The 1 amp 1N40xx series can handle 30 amps of PEAK current. The diode operates milliseconds each time the coil is shut off. So if you find diode rated at 2 or 3 amps at 200v, it should be more than enough. Certainly don't need a 6 amper.
Would that "snub" the collapsing field to protect the rest of the car's circuitry? I am assuming that more is not necessarily better but at least not detrimental.
Certainly, with a margin of safety.
NOTE: The voltage spike from the disconnected coil will try to rise infinitely to maintain the coil current, until some current path is found. It could be through a suppression diode, a resistor, a capacitor, an ionized spark gap between the relay points, or a breakdown of insulation in the coil, wire harness or controller circuit board. Without protection the usual path is the relay point gap. Controller boards are built with protection against voltage spikes, but sometimes one gets through.
Actually, because your existing diode is shorted and not open, it is still providing voltage suppression by acting as a resistor. The problem is that it draws unwanted extra current during coil engagement, and it may eventually open up.
Good luck.
Rodan.
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On 15 Oct 2006 14:59:05 -0700, snipped-for-privacy@optonline.net wrote:

1N4006 or 1N4007 is the workhorse of diodes. They work as general purpose rectifiers and snubbers, they're rugged (rated at 1A but will handle 30A peaks) and they're made by the millions so they're dirt cheap. When all else fails, that's what I grab.
The 1N4006 is 800V PRV, and the 1N4007 is 1000V PRV, and either one will handle the kick from all but the biggest relays and solenoids.
--<< Bruce >>--
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snipped-for-privacy@optonline.net wrote:

If the diode is only 1/8" diameter, it could be rated for up to 2.5A. If it's more like 1/4", it could be up to 6A. The worst-case I know of is for a CRT TV horizontal output damper diode, which is rated for around 1500V (curent rating is typically 3A). Ignition coil primaries can peak at up to 600V. I'd want at least 3-4A. If you're paranoid, wire a relay between the clutch and A/C control, remembering to include a diode across the relay's own coil (1A, 500V is OK). Don't take chances with the clutch coil since the car's computer may drive it directly.
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snipped-for-privacy@optonline.net wrote:

The usual design procedure for high current damping diodes is
greater than 10x primary voltage, so 200 Volts
and current rating = 15 to 20 % of primary current
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try a P600M heres a link http://www.rfparts.com/diode.html#rectifier_diodes
I would put a 0.1 UFD 500 volt ceramic capacitor across the dide to pass the first part of the surge. http://www.rfparts.com/caps_ceramicdisk.html
http://www.web-tronics.com/21fa010.html
On Mon, 16 Oct 2006 02:10:27 GMT, Backyard Mechanic

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http://www.datasheetcatalog.com/datasheets_pdf/1/N/4/0/1N4002.shtml
Dont overcomplicate it...Note the peak non-repetitive current for a 1A 1N40xx is 30A.
Typical 'flyback' coil current shouldnt be more than 1.5A for a very short time.
The .1ufd is a good idea, all things perfect, you should never see 50v let alone 200, but 200 volt rating on either should be fine
As to why your diode failed in the first place, a poor contact or connection resutling in chatter MIGHT fry a typical damping diode.
unusual one of those 'short', long term... usually they short, then open due to high current.
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