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Battery Charger from Aldi - Repair
chris-g - 1/10/13 at 12:05 PM

Hello All.
Last year I bought one of those battery charger/conditioners from Aldi. A couple of months ago it failed, completely dead but it had a 3 year guarantee, unfortunately there were no more in stock so I was given a refund and told to throw the faulty one away. I have taken it apart and discovered a blown fuse, resistor and possibly a FET in the power supply circuit. This is stretching my electronics knowledge so I am hoping some of you clever people can answer a few questions for me.
The resistor is a reasonable size so I assume its of a low resistance and high wattage. The colours appear to be black, blue, violate (but could be grey as the resistor is damaged) silver and brown. I make this to be 0.67 ohms, is that correct as it seems a very low value?
The resistor is connected to the source pin of a FET, part no TSF8N60M. I have tested the FET by connecting -ve lead of a multimeter set on diode mode to the source pin and then measuring with the +ve connected to gate followed by drain. I get a very low reading (1 to 4) and putting a finger across the pins has no effect on the readings. Is the FET dead? The testing was done with the FET out of circuit and on the bench. Also if it is dead can someone recommend an equivalent or alternative as this FET is not stocked by RS or Farnell.

Thank you for any help.

coyoteboy - 1/10/13 at 12:13 PM

Resistor is probably a current sense resistor (low resistance, high accuracy used to identify current flow by small voltage drop across it).

FET - not sure without sitting down and thinkin about it but they're normally uber-cheap to replace and may have been stressed so I'd personally as it's the output stage I'd replace them anyway.

FWIW I found my cheap aldi charger is incorrectly identifying battery voltage - it's reading 13.6v at "float" when the considerably better multimeter is reading 12.2.

MikeRJ - 1/10/13 at 02:08 PM

If the current sense resistor is burnt, then it's probably because the FET has shorted Drain to Source. However, with switching regulators the actual cause of this failure can be hard to find, FET's don't generally just fail for no reason at all. Very often it's due to electrolytic capacitors drying out internally, which lowers their capacitance and increases their series resistance. This can upset the dynamics of the control loop, causing the FET to be either switched on for too long and causing the transformer to saturate, or not being switched fully on which massively increases power dissipation and causes thermal failure.

britishtrident - 1/10/13 at 03:29 PM

Lidl have chargers on sale next week.

40inches - 1/10/13 at 04:53 PM

quote:
Originally posted by britishtrident
Lidl have chargers on sale next week.


I think I may get one, the Bike battery charger doesn't seem to like car batteries. Lidl

chris-g - 2/10/13 at 10:48 AM

Thanks for the info. So when I replace the resistor its important that its as close to the original value as possible? I am happy to replace the FET but that exact FET doesn't seem to be available here, an internet search brings up data sheets but nowhere that I can purchase one. Any suggestions for an alternative?
There is a big electrolytic cap but I assumed that was for smoothing. It doesn't seem to have the typical bulging end normally associated with a duffer. I will replace anyway as they are cheap.
Might have to buy a new charger, I am expecting Aldi to have these in again this month as its nearly a year since I bought the first one and they were available about the same time as the ones from Lidl.