Charging cicuit repair

[fraxinusg]

Hi guys, Brand new member, I hope you can help.
I bought a 7.2v lithium power drill which uses a 10.8v mains adapeter hoping to be able to charge it from a car/boat battery. I worked first time, but now appears to have blown the input stage on the charging circuit (in the drill - it's not a removeable battery). The enclosed pic seems to show the offending item Q4 with the marking CF ST6. I've searched manufacturers web sites and various catalogues without much success. An ohmmeter measures 360ohms either way between the tag (common to centre pin) and the right pin and open circuit either way to the left pin. Clearly dead therefore! Any ideas what it is so I can try and find a replacement?

Although R8 looks like it might be damaged a reading on that gives 0.5 ohms which matches the R500 marking, so looks like it just got some cosmetic collateral damage!

 

[Hero999]

R8 is a 0.5Ω resistor.

Q4 appears to be bad but I'd test it to confirm this.

Is it still within the warranty?

If so, I recommend you take it back to the shop for a replacement, rather than attempting to repair it yourself which will void any warranty.

 

[MrAl]

Hi,

Yes it appears that the device that looks like a transistor has overheated and burnt out.
This can happen when the input voltage goes above the 10.8v it is meant to wrok with
normally. You might trace out the circuit to see how it may be fixed, or get a new one.
If you get a new one and you want it to work with a 12v battery that might be being
charged too, you have to drop the voltage down to 10.8 or something like that.
Remember though a 12v battery being charged can be as high as 14v or more.
Also, unless that number leads you to a specific device you would have to determine
if you need NPN, PNP, NMOSFET, or PMOSFET, or something like that.

You can also build your own charger that can work off of 12v.

 

[fraxinusg]

Thanks for the responses guys. It probably would qualify for warranty, but only if they didn't work out how it got blown, and only if I knew where the receipt was! Looks like it's going to be a private fix.

The markings on SMT devices seems a routine problem. Is there no reference table to help identify such things? The nearest I can get is from the package, which I think looks like SOT-89 and I've found an ST microelectronics BJT device 2STF1360, which says it's used as an LED driver, but the data sheet says the marking is completely different

In respect of trying to work out what it is; the pin nearest the Q4 marking leads to the +ve side of an LED via a resistor labelled 561 (560Ω ?). If I plug in the standard charger this LED does not light, even though the battery is almost flat. This would be consistent with my open circuit reading to this pin.

 

[MrAl]

Hi again,

Sorry i cant help you figure out what kind of device it is, and you should know that that may not be
the only thing that blew out.

If all else fails you can build your own circuit that will work on 10 to 15v approximately for around
5 to 10 dollars.
I always recommend charging Li-ion cells individually though, so it would be better if you can find
out if your pack has more than two leads. If it only has two leads, then you would be charging
two at a time with the same charger current. I dont like doing it that way myself but if your
device already does that then perhaps we can design the charger to work at the higher voltage.
Lets see what else turns up in this thread first.

 

[fraxinusg]

Hi Guys,

Realised a mate had an identical drill, and I persuaded him to let me take it apart. The readings from Q4, in circuit, are from two DMMs as follows:

DMM1:
Hfe 803
B-C 261mV
C-E 539mV
B-E 538mV

DMM2:
Hfe 613
B-C 341mV
C-E 554mV
B-E 538mV

The identification of BCE are taken from an ST Micro NPN SOT-89 bipolar, but seem to be borne out by polarity, since above voltages are listed +ve first and other polarities produce open circuit.

The B-E is presumably caused by some other part of the circuit, and thus casts considerable doubt on the Hfe readings (quite apart from the difficulties in holding three probes on such a small component giving potential inaccuracies - though the readings were all taken several times)

Do these values prove anything, apart from the polarity?