Hi,

I hope this is the right placa to ask. If not then I apologize!

I have a webasto thermo top v, where the polarity was mixed so + got -.
This resulted in a big spark when connected, and after a few seconds wires was hot hot! It was disconnected after only a few seconds. After this the polarity was corrected and I wasn't able to connect to it with the computer.

So, I opened it up and took out the circuit board and had a look at it. Everything looks fine except a diode (that's what I found out it was) with the name 5KP30A.
Its fried.

After reading about diodes, I found out there are several types with different functions. But I don't know what type mine is though.

But I haven't succeeded in finding out what makes this thing blow up.. Is it because the reversed polarity?

Some of these where supposed to protect against this.. But will that mean it protects by blowing up?


As you probably understand, this isn't things I know much about, I work with concrete at a daily basis.
But I like to investigate and possibly repair things like this. If I'm able to do it myself.

So I have ordered a new one with the same name, and I hope that this might fix it. I just have to try.

But I still wondering why it blew up, if it's supposed to protect.. It doesn't make sense to me..

Hope somebody can help!

Regards
Fredrik

 

That wasn't just a diode, it was a transient voltage suppressor. It would normally be reverse oriented and not conducting. I suppose reverse polarity hookup caused a high current that cooked it. There may be a fuse blown also, or other damage.

 

Oh..
Couldn't see any fuses, but I will try to change it and see if it works..

What is the purpose of this thing anyways?
Just curious..

 

It's a big beefy zener diode. Zener diodes are oriented in reverse polarity so they are not conducting unless their breakdown voltage is exceeded. It does nothing whatsoever unless a large over-voltage spike comes along. Then it breaks down and conducts a LOT of current in the reverse direction. If the "spike' is more than a brief event, the diode would eventually overheat and be destroyed. But that would be a rare event.

If power is applied in reverse polarity, now that zener is oriented to conduct current all the time. Poof.

I'm speculating a bit since we don't have a schematic.

 

Thanks alot for a good explanation!

I don't have a schematic for this. But I will change the diode and see if it works again.

But, is there several types of diodes with the same name, so I could potentially order the wrong type?

I would assume that the number on it, Represent's the exact one I need..

 

You should be fine with the identical part number as long as it's the same type of component, e.g. a diode.