Resistor Blowing

RustyH

Senior Member
Good Day to All,

I have a general Elec Question.

What would cause a resistor to blow?

I have a makita battery charger, and it stopped charging. I opened it up to find one of the resistors were blow, with burnt residue on the PCB. I tried to decipher what the resistor value and think I determined it being a 0.1 ohm resistor. (It was blue in color with 4 bands or Brown, Black, Silver and Gold)

I therefore brought some 0.1 Ohm +/-5% resistors yesterday, they arrived today and so I soldered one in to position. Plugged it in and bang, it went again.

Any ideas what I should be looking for that would cause this?

Thanks


20140416_133821.jpg
 

Goeytex

Senior Member
I can guess.

It could be shorted cap somewhere in the circuit is causing the switching supply not to switch placing a constant high current through the resistor.

Test the caps for shorts. A shorted cap may appear to be swollen and may even have some leakage residue nearby. A diode might also be shorted but they tend to open when they fail , but not always.

Can you tell where the resistor connects ? Is it from ground to the source of a FET or transistor? A valse of .1 ohm suggest that it may be a current sense resistor.
 

John West

Senior Member
Basically, you'll need to do full troubleshooting of the circuit, because there isn't one particular device that is always the cause of a blown resistor. Typically, desoldering and measuring of high current diodes and transistors is the nature of the job.
 

Buzby

Senior Member
If you search Google with the part number of your charger there is a good chance you will find the circuit diagram somewhere. This will help considerably in diagnosing the fault.

Good luck,

Buzby
 

RustyH

Senior Member
Thanks for the replies guys,

From what I can see, the resistor links between the transformer (I believe its a transformer!), and the large Cap in the picture. A few other bits are connected to the tracks going to the resistor as well, These include the large white ceramic resistor (10w), the two small blue disc shaped caps, and the two blue components that have "56K(ohm symbol)J" on them?

Ive tried to show the routes in the below pictures

20140416_133843.jpg
 

Ravenous

Member
Be very careful! That supply has a 400V capacitor in it! Don't run it with the case open unless you know exactly what you're doing!

And please send us a post again tomorrow so we know you're still alive. (Only half joking)
 

RustyH

Senior Member
Im still alive!

Tried to do a capacitance test with my multimeter (Clarke CDM 50) but it only measures up to 360 UF, so I cant test that Big capacitor
 

Morganl

Senior Member
Most probably it is the switching mosfet that is shorted inside.
Have repaired a few, even designed for production, but it was in the millenium before this...

If you are lucky it will work after changing the fet and that resistor. Of course first ohm the fet. Do not forget to test the rectifier - it have apparently cunducted pretty much.

Very seldom modern electrolytes get shorted, especially without visual signs, like top blown or bent.

If you stil want to test the cap, disconnect at least one end from rest of cirquit. charge it to say 10V, then let it charge another cap. If the other cap is same valye voltage will be halved. For other values do the math. Capacitance value is OK to vary much. What happens with aging is that internal resistance increase more than the capacitance decrease, and resistance is more problematic bot in operation, and to measure. I use a signal generator, resistor and a scope. Set op square wave, and look at how much the cap voltage steps at the square wave transistions. Select a reasonable resistor to get in this case at least 100 mA if the signal generator can do it. As this is a picaxe forum: use PICAXE as signal generator :) parallel all pins on port A, and do something like
do
porta=0
pause 10
porta=$FF
pause 10
loop
 

Morganl

Senior Member
Another way to measure the cap when you have the signal generator is to connect a known cap in series with the cap to be tested and the resistor to the signal generator, and compare how much thy charge, angain by oscilloscope but now do measure the step change (that is resistance) but the amplitude of the slopes only.

There are many ways.
 

John West

Senior Member
The simplest test for that cap is a resistance test. If it isn't shorted, it isn't blowing up the resistor. Also, if you start with a fully discharged cap, you can watch the resistance change as the cap charges, indicating it's at least acting as a large reservoir capacitor, even though you don't know the exact value it has. Typically, you'd want to do that test out of circuit, but it often works in-circuit.

Again, the things most likely to blow up the resistor are shorted power semiconductors.
 

john2051

New Member
Have you tried it without the resistor? Just wondered because some smpsus have a bleed resistor across the main cap usually around 1Mohm.
regards john
 
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