AC adapter broken after repairing cable [solved]

Thread Starter

Jorne

Joined Feb 28, 2020
22
Hello, I had an old AC adapter (230V AC to 14V DC). There was a small part where the insulation of the cable was damaged. You could see the copper coming through. So, I cut the damaged part away, soldered the wires back together, and insulated them with heat shrinks.
However, when I then tested the the adapter, it seemed broken. I measured only 3V DC at the output instead of 14V DC. I consider the possibility that the adapter was already broken before I repaired the cable, because the positive and negative wires could have been touching at the part where the insulation was damaged, so shorting the output. But, don't such adapters have built-in short circuit protection? It was from a cheap electric drill set though...
Another possibility is that the adapter got destroyed by the heat of soldering the cables, but I doubt this.

I would like to hear what you guys think of this. Was it most likely destroyed by a short circuit due to the broken cable insulation, or maybe the soldering or some other reason?
 

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LowQCab

Joined Nov 6, 2012
1,110
Getting ~3V would seem to indicate that the Cable is shorted,
and the Power-Supply is in Current-Limit Mode.

Trying to just "patch-up" a damaged section of Cable does not allow you to do a thorough
inspection of the Wires.

You need to completely cut-out the damaged section,
then carefully strip back the insulation,
then separate the conductors and neatly twist the strands of wire together,
then test for voltage before soldering the Plug-End of the cable back together.

If you still get 3-Volts, throw it away, and go to your nearest Good-Will Thrift Store,
they will have a big plastic tub full of Power-Supplies and Wall-Worts for ~$1.oo each,
take your pick.
.
.
.
 

Thread Starter

Jorne

Joined Feb 28, 2020
22
Getting ~3V would seem to indicate that the Cable is shorted,
and the Power-Supply is in Current-Limit Mode.
I understand your reasoning, but I checked for a short circuit by measuring with a multimeter on resistance setting between the positive and negative of the DC plug and it indicated OL or megaohms, so the cable seemed fine.
 

Thread Starter

Jorne

Joined Feb 28, 2020
22
I have found the issue by taking the power supply apart and measuring the transformer winding resistance.
- Primary winding -> OL (not OK!)
- Secondary winding -> 4.5 Ω (OK)

After some research on the internet, I found out that these small transformers inside wall warts are often protected by a thermal fuse that is placed in series with the primary winding and hidden behind the plastic that covers the windings.

I assume that the power supply has been shorted due to the damaged cable insulation before I tried to fix the cable. The heat generated during the short-circuit probably triggered the thermal fuse inside the transformer. This explains why I measured the primary winding as open circuit.
 
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