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Old 12th July 2010, 02:05 AM   #1
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Default Diode Question

Im working on this amp and came across 4 diodes.

They are marked P6KE350A

3 of them measure 0.390 in diode check.

The other one mesaures 1.028 in diode check and out of the board.

Does it apper to be defective??
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Old 12th July 2010, 02:28 AM   #2
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Could be it has soaked up a spike and died.

It appears to be a voltage suppressor diode.
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Old 12th July 2010, 04:47 AM   #3
gootee is offline gootee  United States
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It appears to be three the same and one different, with all supposed to be the same. So I'd say yes it appears to be defective.

What I can't understand is why someone would have used TVS (Transient Voltage Suppressor) Zener diodes with a breakdown voltage of 350 Volts (332-368 V min/max) and a peak pulse current of 1.3 Amps and a peak pulse power dissipation rating of "600 Watts at 1 ms" !

What are you protecting if you don't clamp anything under about 350 volts??!! (Or am I missing something?!)

If this is for a car audio system, then I would have used something like the 5KP28A. They clamp at around 31 Volts, and can handle hundreds of amps of current and can dissipate 5000 Watts, for a very short time. See the pdf at the link below. You would probably still want to use up to three 5KP28A TVS diodes in parallel (for a 12V system; or up to eight for a 24V system, depending on your "source resistance"), right across the +/-V input to the system you want to protect, to be able to safely (i.e. safe for the TVS diodes) handle a large-sized load dump event. (And I would probably either use bi-directional devices or use double the number of uni-directional devices and put them in both directions.)

Here is a very good paper about the entire concept of using TVS diodes (and other means) to protect electronic systems from over-voltage transients in automotive environments:

http://www.radiocad.com/_downloads/L...aper-final.pdf

One major "automotive transient voltage" problem is an event called "load dump". You might think that it's not very likely to occur, since it basically only happens when the alternator is running at high output and the battery or some other significant load is suddenly disconnected, giving the alternator current "nowhere to go", inducing a very large voltage spike. (The ISO standard says to be able to withstand 87 Volts in a 12 Volt system, or 174 Volts in a 24 Volt system, with the voltage staying over 10% above nominal for up to 400 ms! [See pdf at link.])

But it can happen VERY easily if you get a "jumpstart" or "boost" for your dead battery, and do it the usual (wrong) way. Imagine your car's battery is inconveniently dead. We've probably all done it like this, before: You connect jumper cables to your battery from another vehicle's battery. That vehicle's engine is running and your battery gets charged enough and you start your car. You then disconnect the jumper cables. Bingo! Remember, your battery is still basically dead. So when you disconnect the other vehicle's battery, it is almost as bad as "disconnecting the battery when the alternator is running". "Load dump"! LARGE voltage pulse!

The best way is probably to not have either vehicle running, and just let one battery charge the other for a while. But if you "must" do it the other way, it will help at least a little if you turn on some loads, in your jumped vehicle, e.g. headlights, before the cables are disconnected from the battery, so the other loads can absorb some of the spike's energy.

Back to preventing damage to your vehicle's electronic systems from the load dump's large voltage pulse: Even better than TVS diodes is the last circuit shown in the PDF at the link I gave. It uses a p-channel MOSFET to simply DISCONNECT the +V line from your equipment, before a spike can get through. (And if you connect an identical mosfet just upstream of that circuit, with its gate tied to ground, then it should also protect against "reverse voltage", e.g. backwards-connected jumper/booster cables or battery.) There are also chip manufacturers who make ICs that take the place of the simple bipolar-transistor trigger circuit in the schematic, such as Linear Tech Corp and Maxim, which give some added features compared to the simple circuit shown.

Tom

Last edited by gootee; 12th July 2010 at 05:02 AM.
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Old 12th July 2010, 04:59 AM   #4
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I don't have a 350 volt TVS but a lower voltage TVS reads ~0.58v forward and there is no reverse leakage. If the ones that read 0.39 have no reverse leakage (meter on ohms, diode out of the board), they're likely OK and the one that reads 1+v is likely defective.
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Old 12th July 2010, 05:08 AM   #5
gootee is offline gootee  United States
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Quote:
Originally Posted by Perry Babin View Post
I don't have a 350 volt TVS but a lower voltage TVS reads ~0.58v forward and there is no reverse leakage. If the ones that read 0.39 have no reverse leakage (meter on ohms, diode out of the board), they're likely OK and the one that reads 1+v is likely defective.
Are they marked "P6KE350A"?

Last edited by gootee; 12th July 2010 at 05:11 AM.
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Old 12th July 2010, 05:46 AM   #6
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I replaced the diode and now it measures the same.

Im probably gonna have to start a new thread for the amp im working on since I wanna make sure i dont miss anything before i power it up.

The guy that owns this amp has had alot of problems with it so i told him i would take a look and see if i can get it up and running.
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Old 12th July 2010, 05:54 AM   #7
gootee is offline gootee  United States
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I think it will be difficult to test a TVS diode with the Diode Test function of a multimeter. The voltage-clamping is done using their Reverse Breakdown Voltage, not their Forward Voltage. They are installed with their anode toward the +V voltage. When Vbr (Reverse Breakdown Voltage) is exceeded, between +V and ground (or -V), they suddenly conduct, and the voltage doesn't rise too much more.

I agree that you have found one that is different than the others, and that it is likely that that one is bad. But to really test their clamping level, you would need to apply an increasing reverse voltage until they started to conduct. And if they are P6KE350A, it should take at least 338 Volts before any significant current flows through them. Until then, the reverse leakage current will probably be about 5 microamperes.

Cheers,

Tom
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Old 28th June 2013, 02:32 PM   #8
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Default TVS Diode

Quote:
Originally Posted by mike49504 View Post
Im working on this amp and came across 4 diodes.

They are marked P6KE350A

3 of them measure 0.390 in diode check.

The other one mesaures 1.028 in diode check and out of the board.

Does it apper to be defective??
You should measure all of them out of the board if you are using the diode check..

Last edited by AudioEngineer; 28th June 2013 at 02:35 PM.
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