Dehumidifer - Does not Power On. Faulty Power Supply Board.

Welcome to electronicspoint!

You say in your post you are a complete beginner to electronics, you do however appear to know what you are looking for. If you are a beginner be very very very careful.

Has the appliance been in use for a while? If so the relays could have suffered mechanical wear damage. As the capacitors appear to be fine, the IC has already been replaced and the fuse checked this really only leaves the relays I would recommend you replace these. Should IC 3 have only 7 legs?

Are there any components on the reverse of the board? If so could you provide a picture.

I hope this helps you,

 

The chip you have "replaced"....for want of a better word...... is a control chip for initial power to the board.
IC4 (above it to the right) is a feedback opto-isolator chip from the transformer (yellow Square device beside IC3) to IC4.

Shorting capacitors is a bad idea...instead, use a resistor...plenty of places to read about this on the web.
Similarly having one hand behind your back ( I hope this is an attempt at humor but many will take you literally)

If you are certain you are getting main supply in, then there is no longer a need (for you at least) to do any other live work so be absolutely certain everything is disconnected from high voltage.

Test the 4 diodes (D1,D2.D3,D4) these make up a bridge rectifier which supply IC3 and hence control to the transformer.
Again you can look up how to test these with supply disconnected and using any basic multimeter.
Check fuse with multimeter also. (not just visual) in circuit would be best as I have seen dicky connections there.

It is possible (probable) that the transformer has a fault which would explain IC3 blow up in which case a new board would be almost 100% required.

 

1. Capacitors look ok.Keep in mind that i have encountered shorted capacitors that looked ok.
The only way to know for sure is to check with capacitometer. But 90% of the times if they look ok they are ok.

2. Yes ic3 can only have 7 pins because pin 6 is not used.
(Correct me if i am wrong) This chip is the one that feeds the high freq transformer and defines the output voltage by switching on and off.

If IC3 was damaged it would blow the fuse for sure and it would also damage the smoothing capacitors and the the rectifier bridge.

I agree with post #3. Only you do not need to discharge the caps because they have small capacitance and drain very fast.

You can check the voltage on the leads with a voltometer and just wait untill it gets low enough (below 20volts i would say). they sould drop down to zero very fast though.

 

Checking the diodes requires that you remove them from the PCB. My opinion is replace theb with a bridge rectifier just to be sure. (costs like 1$)

The transformer almost never fails. IC3 fail very often for a number of reasons.

My reccomendation.

Carefully plug the circuit into mains power and measure voltage across the pins of the Blue capacitors next to the transformer. If you get voltage there then you know that high voltage section si ok.

If not then replace ic3, replace the diodes with a bridge rectifier (easyer and faster) change E1,E2 caps and the fuse and you should be ok.

If you suspect a transformer failure then Google its code to find the diagram,remove it from the PCB and measure resistance among its terminals as shown on the diagram with a multimeter.

 

Tests I recommended were with power disconnected , hence check the diodes, I meant with diode test on a multimeter without power.
The reason I suggested you discharge the capacitors with a resistor is that one day, if you persist with bad habits, you will short out a large one and it may well blow up in your face.
The "cylinders" marked "222" and "330" appear to be chokes and should have near zero resistance.
The secondary of the transformer provides low voltage to the rest of your circuit so if faulty then nothing works so it cannot be "switched out" as you put it..
I doubt you will source the transformer as a part if this is the case which is why I suggested a replacement board if it proves to be at fault.
It is of course possible that you may have inadvertently damaged your new IC3 on installation with excessive heat but only further testing will determine this. It does contain some parts that could be damaged by static charge also.
This can come about by using soldering irons without any static protection.
Contrary to the suggestion above, transformers do fail and mostly on the primary side where windings are very fine gauge and flashovers between windings can occur causing open circuit.
If the electrolytics around the incoming section were faulty, it would lead to some instability but I doubt this is the case as they do look ok (no bulges in the tops and no leaking) you would still get some signs of life.
The 2 connections on the rear side of the board immediately above the surface resistor R31 should be your transformer primary and have a high resistance compared with the secondary which should be the 2 connections to the left of the yellow line on the painted white strip.

 

The black patch next to the "D5" smt component (bottom right of back of PCB), is this a shadow or is the component damaged? If this is damage then that is a/the problem.

 

I wonder if the optocoupler works...

Have you considered replaceing the High voltage section with an other circuit ? This is what i mean:

The blue caps next to the transformer are smoothing caps. They provide a steady DC voltage for the rest circuits. (because the output of the high freq transformer is not steady dc voltage) Many times if i can not find what is wrong with a psu i just replace it with an other psu circuit that meets the requirements (same voltage and amps). (note : i do not replace the whole board). In other words just change the DC source of the board. (note that on the back side the high voltage section is marked and separated from the low voltage section)

Do you understand what i mean ?

One other thing i do to test that the rest of the circuit is not damaged is that without connecting it to Mains supply i remove the transformer and feed the rest of the circuit with the DC voltage it needs from a variable lab psu i have.

 

Are you familiar with a multimeter as it seems not from some of you answers.
I originally said many measurements can be made without power connected and especially for a safety reason.
If you are poking around the P1N1 region and the FUSE with power on these are high voltage areas.

As for P4-N, P1N1, P2N2, P3N3, they are all the same connection (neutral of the mains supply).
There will be zero voltage across the fuse if it is ok but again do not test these things with the power on, absolutely crazy thing to do.

Edit:- From your question about ......."(i just put my red probe on each pin of the capacitor)...do I need to put the black probe anywhere?" .....I'd say you have no idea about starting to test and using a multimeter.
Best advice I can give you is to get someone with electrical qualifications to test this for you before you do yourself or someone else an injury.

 

Hi there wills07417

Are you still interested in fixing this board?

If so, have a look at the vertically mounted grey resistor RF1, between the fuse and the four diodes. What are the colour markings on it? Measure its resistance with your multimeter on the 200Ω range, and one probe to each end. You don't need to take it out of circuit for this.

Also, tell us the make and model of your multimeter.

 

A multimeter on resistance (Ω) range can indicate short circuit, open circuit, or something in between.

Set your multimeter to the resistance range, and with the probes held apart, look at the display. This is the "open circuit" indication. Your multimeter will show this when there is no electrical path between the probes, i.e no way for current to flow. This corresponds to infinite resistance.

Now hold the probes together firmly. The meter should display some low number. This is the "short circuit" indication. Your multimeter will show this when there is a perfect, conductive electrical path between the probes, and current can flow easily. This corresponds to zero resistance, although the display may indicate a small number (this number corresponds to the resistance of the multimeter leads, which is never exactly 0Ω).

Now re-measure that resistor. If you get an open circuit reading, then yes, the resistor is toast. But the colours you gave correspond to 8.5Ω which is not a normal resistance. Can you upload a photo of it please, in natural light if possible.