Unknown diode. Need help

Thread Starter

jijo55

Joined Feb 18, 2020
4
Hello everyone,

Currently I am trying to repair my air conditioner's DC motor.
the aircon is 12LGC and the motor model is MFE-40 VVL.
but this is not important.

I have opened the DC motor and looked on the PCB inside. There is 2 ICs and several resistors, capacitors and... one diode.

Outside the diode looks good, there is no physical damage, but has over 4MOhm resistance in both directions -> it should be replaced.
The diode is marked only as "20" and I cannot find what is it.
Is it a zener or shottky?

I have made a picture of it and attached the scheme. May be someone familiar with electronics can guess and suggests what kind diode I should use.
 

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wayneh

Joined Sep 9, 2010
16,275
My guess, and that's all it is, is a 20V zener to suppress transient spikes on that 15V line.

The bad news of that, if true, is that the diode doesn't normally do anything. A failure would not cause symptoms. But it might indicate that a serious transient occurred and may have damaged other things.
 

Alec_t

Joined Sep 17, 2013
10,918
Outside the diode looks good, there is no physical damage, but has over 4MOhm resistance in both directions -> it should be replaced.
If the 'diode' is in fact a bidirectional TVS device, you would expect a very high resistance in both directions if the device is ok.
 

Thread Starter

jijo55

Joined Feb 18, 2020
4
@wayneh
Thank you for your reply.
Yes, the IC behind (SMA6822) is also damaged. The resistance between vcc and vb1, vb2, vb3 is different which is a sign, that it should be replaced.

@Alec_t
Good suggestion! May be it could be such thing. The "diode" has no cathode marking.
 
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Thread Starter

jijo55

Joined Feb 18, 2020
4
Reversed the diode. Here are the readings:
20.2 / 20.07 / 0.025mA
21.04 / 20.03 / 0.16mA
22 / 20.01 / 0.3mA
I think the diode is OK, and I'm gonna re-solder it.

Thank you! :)
 

Alec_t

Joined Sep 17, 2013
10,918
From those and the previous results it looks like a diode with a reverse breakdown voltage around 21V. So I agree with Wayneh that it's probably a (nominally) 20V zener or Scottky diode.
 
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