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Damper Diode Question

 
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toozy2
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PostPosted: Sat Sep 18, 2010 6:25 am    Post subject: Damper Diode Question Reply with quote

I have a sharp microwave that cooks on high regardless of power setting. I've traced the problem to a shorted NPN bipolar junction transistor. This transistor switches a relay on and off and the relay controls power to the microwave generator. Of course there's a damper diode in parallel with the relay coil. I've desoldered one leg of the diode and tested it, and my meter says it's good. I didn't think it was normal for a transistor to just fail on its own. I always thought something else killed them like heat, excess voltage/excess current. I was wondering if I should just replace the damper diode to prevent the new transistor from being killed by counter EMF, or if I should just continue using the old diode. If I do replace it what should I replace it with? My meter has a diode test function that shows .7 in one direction and open circuit in the other, so it's a silicon diode. Can I just use any old silicon diode or should it be a special one?

I've taken the necessary safety steps like disconnecting the primary of the high voltage transformer so I can run it and do tests without cooking myself
repairfaq.org/sam/micfaq.htm#micpwiml and I know about not touching the big evil capacitor, magnetron, high voltage transformer, and big square diode.

If you're curious it's a Sharp model 308 JW.
The controller is FA 757DR

Thanks in advance.[/url]
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Sep 18, 2010 6:06 pm    Post subject: Reply with quote

Based on your description, the diode in question is perfectly fine. I have never heard of it being called a damper although it is in fact does that to the back EMF. If you are going to replace it, it is best to replace it with a similar diode of at least equal or higher value. --not just any silicon diode. Is there any number on it? It is always good practice to do a search for Datasheet and compare specs to whatever you are replacing it with. You mentioned the driver transistor is shorted. So why not just replace the faulty transistor instead of the good diode and are you sure the transistor is bad?

While the diode is meant to prevent the back EMF, transistors do & will fail in operation. There could be several reason(s). For one thing, in consumer applications, components are rarely selected to operate with higher head room. This is mainly due to cost factors involved.

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toozy2
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PostPosted: Sun Sep 19, 2010 6:10 am    Post subject: Reply with quote

I thought the transistor was bad, I was assuming it was a normal NPN transistor Embarassed . Here's what I get when I test it out of the circuit with my meter on two different settngs:

probe connection_____diode_____ohm setting
___________________setting

base -, Emitter +_____3.0329V____8.971K
base -, collector +____OL_________OL

base +, emitter -_____2.524V____8.97K
base +, collector -____2.524V____3.57Megohms

collector +, emitter -__OL_______OL
collector -, emitter +__.7V_______3.58Megohms

The numbers on it are:
G243
M431

OR

C243
M431

I can't tell if that's a C or a G. I've tried looking them up in google (of course) but can't find anything. The cicruit board is printed E X B (actually, only the E and the B are printed the X isn't marked as anything, but I figured it was collector). The E is connected to the - of an electrolytic capacitor, so I thought it was a bipolar junction NPN.

Is it possible that this transistor is some kind of special transistor? It's not behaving like a normal NPN. Why can't I get datasheets? Any ideas what to do next?

I found this microwave in the garbage so this is just a learning experience, if it can be fixed great, if not, oh well. I don't have datasheets or schematics, is there anything I can do?

Thanks again.
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Sun Sep 19, 2010 7:23 am    Post subject: Reply with quote

I've had many a triac short out causing full power operation at all power settings. Wink
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sun Sep 19, 2010 8:27 pm    Post subject: Reply with quote

toozy2 wrote:
I thought the transistor was bad, I was assuming it was a normal NPN transistor Embarassed . Here's what I get when I test it out of the circuit with my mete....


Can not be sure but I think what you have is actually an NPN digital transistor such as KRC243M? It is rather difficult to test them with meter due to built-in resistors and results are not reliable; May or may not be bad?
You may also have a Triac controlled magnetron that can give you the symptoms as Jts suggested. For the sake of learning & if you feel like spending the time, you can do a little search and even draw the control schematic. It is not that difficult. The key is controlling the Magnetron with HV dc voltage along with safety switches. Some use a triac for control and some don't. Just make sure to stay away from the charged cap and HV txformer as you are already aware. Here is some info;
http://www.gallawa.com/microtech/Ch8Pg2.html
Here is a schematic for LG brand that does not use a Triac;
http://tinyurl.com/2entjw7
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