Can't find the resistor value.

BobaMosfet

Joined Jul 1, 2009
2,110
Actually i have tried by connecting simple dc bulb to it... The 7.21 volts that is on output it drops to zero after loading. So i thought there must be something wrong with the feedback so i replaced tl431 after that alteration this ticking thing happened.
If your voltage drops, it's because the load is drawing too much current (or another way to look at it is that the supply isn't able to supply enough current). Current and Voltage NEVER operate independently of one another, it is a reciprocal relationship, because voltage is not an indicator of what's there, it is an indicator of what has been CONSUMED in moving current through the 'gap' (anything between the poles of any power-supply is a 'gap' a form of resistance or impedance (or both)).

So what's preventing it from supply enough current?
 

MisterBill2

Joined Jan 23, 2018
18,167
https://m.aliexpress.com/item/32818...y8O6Wke-fzAE4DK8o0PTsdiZ6GWddLF31561046070653

What you guys think of this power supply... Its on sale... Should I buy?
With the cut off wires it is obviously removed from something. and the rating is 5 volts at 2 amps, a big space from 12 volts at 10 amps/ At 39 cents it is a bargain if it is any good, though. But I have no clue about it being able to put them in series or parallel. Ten of them in series-parallel would give you 10 volts at ten amps, all for $3.90 + shipping. So it depends on what voltage/current you are looking for. AND if the products actually work.
 

MisterBill2

Joined Jan 23, 2018
18,167
If you meant this as a joke I am not sure, But I have had several cheap supplies simular to those and they served me well for the applications I used them on.
It does seem that there are often a lot of 5 volt supplies offered at cheap prices. If they could be adjusted down to 4.6 volts then three of them in series would be 13.8 volts which is the nominal car battery voltage with the engine running. Or set them down to 4.5 volts and have 13.5, a very useful voltage as well. So there is a way to use those 5 volt supplies.
 

MisterBill2

Joined Jan 23, 2018
18,167
Anyone there? I've read so many documents and articles about the topic in 2 to 3 days regarding smps repair. In almost every article it was written that there must be a shorted component on secondary side which is making controller ic kick start again and again but it cant because of the over current. But i cant find any shorted component on power supply board despite of that resistor, which i replaced with 10k first as Dodgydave suggested but later replaced 10k with 47k still there isn't any good news.

I'm feeling embarrassed and ashamed of myself, im a electronics student and will be getting degree in about a year and I'm not even able to repair a simple power supply.
Don't feel bad about a switching power supply! They are about the most complex analog circuit you will face, given that they are highly stressed and optimized for minimum parts and cost. There is a whole lot of complex stuff going on in that small but not simple circuit.
AND, as I look at that circuit, it seems that the zener diode at the gate of Q1 may not be quite right any more. I don't think that you have mentioned the cause for that one to have failed. And parts seldom fail for no reason.
 

Thread Starter

rahul411

Joined Feb 19, 2018
260
I replicated the board, used almost every component from old one but replaced all resistors, caps (not the big ones) and everything on secondary and control circuitry.
And voila! It works.
The reverse engineered schematic was almost correct despite of one node. And the board design is similar. I home etched it so there was some limitations on spacing and traces width, so the overall board size is 15mm longer and about 5mm wider than original one.

All n all it works but i don't know how long.
For test i have connected small loads like 12v dc motor, voltage stays exactly at 12.2v after 5mins of motor running. Everything was nice and cool after that.

Tinning of power traces are left, i ran out of solder wire that's why.
 

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MisterBill2

Joined Jan 23, 2018
18,167
I replicated the board, used almost every component from old one but replaced all resistors, caps (not the big ones) and everything on secondary and control circuitry.
And voila! It works.
The reverse engineered schematic was almost correct despite of one node. And the board design is similar. I home etched it so there was some limitations on spacing and traces width, so the overall board size is 15mm longer and about 5mm wider than original one.

All n all it works but i don't know how long.
For test i have connected small loads like 12v dc motor, voltage stays exactly at 12.2v after 5mins of motor running. Everything was nice and cool after that.

Tinning of power traces are left, i ran out of solder wire that's why.
Quite a good job of home-making a circuit board, and the wider traces will allow better heat dissipation. And I am really impressed that you were able to reverse engineer the whole thing.THAT is a major achievement indeed!
 
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