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Re-capping a 12v PSU - mystery component

stuartb3502

Jan 12, 2023
5
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Jan 12, 2023
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5
Hi,

I'm taking tentative early steps in mending stuff and hoping to go from there to making things. For practice, I am trying to replace a couple of capacitors on the 12v output of a car battery charger. This was mine from new, a few years old and was working until I plugged it in one day and it died. Visual inspection showed that a couple of the caps at the output (12v) had literally gone pop spewing their contents around. I am confident I can replace the two caps.

But there is another component which appears missing nearby (see picture). This is marked K1 which I understand suggests it may have been a relay or contactor of some sort. You can also see the spaces for the two missing caps (C10 and C11).

broken-charger.jpg

Although this is a cheap charger and I could just replace, I want to fix it for the challenge and to learn.

I have gone to the trouble of obtaining a similar charger (essentially the same but with a different brand name). That one does indeed have a small switch as shown below which is marked as being in the OFF position. The switch is not accessible from outside the case (i.e. it would seem to have no purpose for a user).

new-charger.jpg

The switch is connected between the two right most pins (as seen above) of a connector block. The LCD display plugs into that connector block on the other side of this board. It appears as though the switch, if closed, would connect a pin on the display to a chip on the reverse side of the board.

Looking again at the broken charger it doesn't look like there was a similar switch in that spot although something was clearly there.

So I'm a bit stumped as to what this could have been and wondered whether better brains and those with some actual knowledge of circuits :) might have some answers or at least suggestions for things to look at. It's a bit of a fun mystery at this stage with an element of being stubborn about wanting to fix something/not landfill something given it just needs a couple of caps and bit of solder plus whatever this thing is.

Given that the new charger's switch is open, my next step will be just to replace the caps and see what happens, but I was hoping someone might have some ideas.

Thanks.
 

Neal

Dec 23, 2009
32
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Dec 23, 2009
Messages
32
It was just a jumper wire which was cut, same as switch being open on other charger
 

stuartb3502

Jan 12, 2023
5
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Jan 12, 2023
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5
Thanks very much. Makes sense. Presumably they produce different versions of this item and the only difference is whatever that switch does. Will have a go at repairing.
 

Delta Prime

Jul 29, 2020
2,118
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Jul 29, 2020
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2,118
Hello there.
The switch is not accessible from outside the case (i.e. it would seem to have no purpose for a user)
Caution!The switch along with the potentiometer or variable resistor,white and blue component, IMO is used for calibration of the unit. Those capacitors died of violent death.Total Carnage. looks like fun.
 

stuartb3502

Jan 12, 2023
5
Joined
Jan 12, 2023
Messages
5
Ah, good spot. I knew what the pot was for but that makes sense that the switch or jumper plays a role in calibration.

The new charger has wax or whatever on the pot, but I won't touch the old one and will check volts on output if it works again.

Fun pic of what it was like when I opened it below. Fault finding was not too difficult! I guess I'll find out whether it took anything else with it or something else caused this.

IMG_20220722_174012.jpg
 

bertus

Moderator
Nov 8, 2019
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Nov 8, 2019
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3,381
Hello

Be carefull with the jumper
It could well be the selection between 120 and 240 volts input voltage
Wrong setting will create a lot of damage

SMPS input circuit.png

Bertus
 

stuartb3502

Jan 12, 2023
5
Joined
Jan 12, 2023
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Thanks for the warning - always good to be careful with mains voltage! I'm confident that this is at the 12v end of the supply. I know there are 110V and 220V versions of this charger. I can't see anything obvious at the mains end to allow switching of input voltage, but I'll try to figure out what the circuit and components are (for learning if nothing else).
 

stuartb3502

Jan 12, 2023
5
Joined
Jan 12, 2023
Messages
5
Just wanted to wrap this up by saying thanks again. I recapped and the thing did power up, but a couple of issues. I must have caused some damage to a pin on the LCD display during disassembly. The backlight flickers and the contrast is out. I may be able fix,but it looks quite delicate.

I tried it on a battery and I’m not sure yet whether it’s charging correctly. The display only shows about 5V when a working charger shows 12-13V. Need to test some more.

Thanks for the tips anyway - I have learnt a fair amount which was the point and I have other broken stuff to practice on :)
 
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