Dehumidifier repair - SMPS question

Thread Starter

tjfs

Joined Dec 4, 2020
5
I am repairing a dehumidifier which won't power on, and I've isolated the problem to missing 5v to the CPU. I found the diode which rectifies the 5v rail before the 7805 rectifier was short. Surprisingly, when I remove the bad diode and power up, I have 7.5Vac at the transformer output, so it would seem the shorted diode didn't damage anything else. With no power supplied to the unit and a 9v battery connected to the input of the 7805 via a 48 ohm series resistor the CPU powers up and displays information on the LCD, so that seems to prove the circuitry downstream.

The SMPS is a simple design with a LNK625 driving a small E16 transfomer with a feedback winding and two secondaries. Both secondaries produce 7.5Vac. One output is rectified, smoothed and feeds the 7805, the other output is rectifed, smoothed and goes .. nowhere. I'm tempted to steal the diode from this output as it appears the same type. Probably best not to just bridge the output across as the second output winding might be lower current. The transformer is marked HS20411/E16 CQC however I can't find any information about it.

The dehumidifier is Chinese manufactured and was sold in the UK by B&Q as WDH-930DAH. The power board is labelled D2514-790.

Any thoughts appreciated.

Thanks
 

Dodgydave

Joined Jun 22, 2012
10,035
Post pictures of the PCB , if you want to swap diodes go ahead, but your meter won't read the AC voltage correctly as it's in the kHz region .
 

Dodgydave

Joined Jun 22, 2012
10,035
Looks like a UF4007 type.. this is the circuit diagram from the datasheet..LNK625pg

Yours looks like it has a diode from the middle winding to ground in front of the Transformer, ??


unnamed.jpg
 
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Thread Starter

tjfs

Joined Dec 4, 2020
5
There are two secondaries, no centre tap. The PCB layout is strange!

I have the dehumidifier working for now with a 9v battery providing power on the input side of the 7805 and the mains powering all the usual components. Replacement Schottky diodes are in the post, should arrive next week.

I calculate one 9v alkaline battery will power the unit for about a day of continuous operation, which is reasonable.

Tim
 

Thread Starter

tjfs

Joined Dec 4, 2020
5
The problem with that is the design uses the voltage at the input of the 7805 to power the relays.

I could certainly use a 9v mains supply instead of the battery, but I'm hoping the diode will fix it.
 

Thread Starter

tjfs

Joined Dec 4, 2020
5
New SR2100 diode arrived today, unit is working again. Thank goodness the SMPS control IC saved other parts from failing.
 
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