Failed resistor value

Thread Starter

CircuitSurfer

Joined Sep 19, 2009
25
Is it possible to figure out what the resistor value was for the burnt out resistor in this NiCd battery charger for a 12V drill.

The circuit board has 1N4001 markings on it but the board is populated with 1N4002. Would that have caused the failure?
 

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Alec_t

Joined Sep 17, 2013
13,219
Drill brand/model?
What is the recommended charging current for the battery?
Battery capacity (mAh)?
 
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dl324

Joined Mar 30, 2015
15,405
Is it possible to figure out what the resistor value was for the burnt out resistor in this NiCd battery charger for a 12V drill.
If you have access to an undamaged unit or you can trace the circuit and make an educated guess.
The circuit board has 1N4001 markings on it but the board is populated with 1N4002.
1N4002 has a higher breakdown voltage than 1N4001 and there's usually no price difference, so the substitution is okay.
Would that have caused the failure?
Too much power dissipation.
 

sagor

Joined Mar 10, 2019
786
That resistor looks to be possibly a current limniting resistor. Cause of failure could have been a short circuit on the output leads or a bad battery pack it was charging (aka taking too much current due to shorted cell)
 

Alec_t

Joined Sep 17, 2013
13,219
IF the resistor power rating is 1W (seems possible given the resistor size) and IF the cells of the battery each have a rated capacity of 2400mAh and IF the charger is designed to charge at a 1/10C rate then the resistor value could be in the ball-park of 1/(0.25^2) = 16Ω. But Dodgydave's guess could be right.
If one or more of the battery cells has shorted internally then the charging current would be higher than the designed value; hence the burnt resistor. You need to replace any shorted cell, otherwise any replacement resistor will die the same death.
 

Thread Starter

CircuitSurfer

Joined Sep 19, 2009
25
Drill brand/model?
What is the recommended charging current for the battery?
Battery capacity (mAh)?
Black & Decker Fire Storm CD431
The battery is rated at 12V. There is no marking for capacity.
I don't know what the recommended charging current is but the output from the wall charger is 14.5V at 200ma.
 

MisterBill2

Joined Jan 23, 2018
13,673
Resistors usually fail because of excess current, often caused by excess voltage, with the resulting excess power causing more heating than the resistor is designed for.
And certainly with a board that small, and no components on the other side, it is a minimum cost charger.
The failure may also have been caused by using the wrong power module with the charger, assuming that the charger is not wired to the module directly. Some folks do not understand that not all wall-warts are the same. To charge a 1 volt NiCad battery pack the transformer would be marked between 8 and 12 volts AC. (actual voltage may differ with load). So if somebody connected a wall wart for charging a 12 volt gell cell pack the resistor would smoke.
 

Thread Starter

CircuitSurfer

Joined Sep 19, 2009
25
Resistors usually fail because of excess current, often caused by excess voltage, with the resulting excess power causing more heating than the resistor is designed for.
And certainly with a board that small, and no components on the other side, it is a minimum cost charger.
The failure may also have been caused by using the wrong power module with the charger, assuming that the charger is not wired to the module directly. Some folks do not understand that not all wall-warts are the same. To charge a 1 volt NiCad battery pack the transformer would be marked between 8 and 12 volts AC. (actual voltage may differ with load). So if somebody connected a wall wart for charging a 12 volt gell cell pack the resistor would smoke.
A correct wall wart was used for charging because it is wired to the charger. The minimum cost charger was probably designed to fail. As was pointed out by other users the declining battery health probably led to the demise of the charge circuit.

After a replacement part search it became apparent from retailers warnings that the charger will fail once the battery fails. Planned obsolescence.
 

Audioguru again

Joined Oct 21, 2019
5,404
Many years ago when I used Ni-Cad batteries they frequently shorted and needed a high current pulse from a charged capacitor to zap away the short.
I think your battery has a cell or two that are shorted and caused too much current to burn the resistor.
 
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