Resistor in parallel with inductor on mains input to SMPS.

[Robotnik]

Hi
My first post here, hello all.
I have a burnt out resistor on the mains input to a small low power switch mode supply which powers the microprocessor board on a dehumidifier.

The resistor it totally burnt out, open circuit, value unknown.

I've searched online for the circuit, but nothing found, it is a 'Silvercrest SLE420-A1 Dehumidifier', resistor R5.

The resistor in question is right on the mains input of the circuit between the mains fuse and the bridge rectifier and in parallel with an 1100uH inductor, the resistor is in line with the live mains and all current used by the SMPS must pass through this component.

I've looked at some smps circuits online to get an idea of what sort of value to try but all the circuits I have found don't have anything except a fuse and a thyristor before the mains rectification.

I feel it must be part some sort of filter along with the inductor and is probably fairly low value, about 10 ohm or less but I don't know much about switching power supplies, any clues folks?
Thanks
Neil

 

[MikeMl]

The resistor is obviously placed there to act as a safety fuse. I predict that if you replace it, the fault that burned the original resistor is still there, and you will likely blow up the new one, too...

Small switch-mode power supplies at various DC voltage outputs and current ratings are less than the price of a cup of coffee at most thrift stores (they are called wall-warts). I'd just replace the dead supply with a wall-wart...

Ps, this post is BS. The real answer is in post #3....

 

[ronsimpson]

Is the inductor good? Measure the resistance. (open?)

 

[MikeMl]

ronsimpson is onto something. If the resistor is in parallel with a 1mH inductor, all the 60Hz line current would flow through the inductor, and almost nothing would flow through the resistor. The resistor is there to lower the Q of the inductor, to dissipate high-frequency noise that results from the smps switching frequency (likely hundreds of kHz), and all its harmonics. If the resistor opens, nothing much would happen except that the smps power supply would couple RFI hash back into the power line. If the inductor opens, the resistor will soon fail...

 

[ronsimpson]

I have put resistors across inductors in power line filters because the high Q inductors wanted to resonate at 2khz.
(dampening)
It seems wrong reduce the Q but I can't have the ring.

 

[Robotnik]

Is the inductor good? Measure the resistance. (open?)

Hi
Thanks for all the replys.
Yes the inductor is open circuit, I should have mentioned that.

Would it be possible that the resistor could be a 1.3megohm?
Er embarrassed because it's not open circuit, it is very blackened, the numbers have burned off but it measures 1.3m.
My apologies, I need to learn to use a multimeter, or stop being so lax.
Neil

 

[ronsimpson]

So you need the inductor and the resistor.
Can you send a picture? Is he inductor small? (0.2" in diameter)
I have an idea but I must know if the inductor is in the filter section.

 

[Robotnik]

Hi
The inductor is just like these green axial types
**broken link removed**

It has Brown Brown Red Silver rings 1.1mH
The resistor is a 1206 surface mount type.
Hope that helps.
Neil

 

[ronsimpson]

Inductor 1.1mH, 100mA, 14 ohm.
So the power level must be low.
I would try a 1k resistor. Jest a guess. Not critical.

So the real question is why did it burn. There may be more problems.
If you want to give it a try. Just short out the resistor/inductor and see what happens. A 10 to 14 ohm 1/4 resistor could be used for more safety. The coil resistance probably helps with inruch current.