Help with R value, please (Bosch battery charger)

Further circuit info:

A 22-ohm (? red/red/silver/gold) 1/2-watt resistor connects the FET's source to the primary of the chopper transformer. The now-destroyed 1/8 watt resistor, also connected to the source, connects to pin 3 (current sense) of the UC3843 SMPS controller.

The Fairchild data sheet .pdf file:

(Fig. 8, bottom of P.6) shows a 500-ohm resistor from the current sense resistor to pin (3) of the UC3843. Might this be an acceptable value to use?

The test circuit (same data sheet, Fig. 1, bottom of P.4) shows a 5K variable resistor. Maybe a maximum value for my application?

Can y'all help me calculate the risks of guessing at this value, and some limits I might use?

Thanks,

Fragment shows first band to be brown; all others obliterated.

Resistor corpse measures 185 ohms. Could this be at all reliable?

Thanks,

What did it smell like? Normally you can determine ohmic value by smelling the fumes within +/-20%. Don't let the smoke out. Harry

DaveC schrieb:

The value of this resistor dependes on the switching frequency (look for R/C at pin 4) and the capacitor at current sense. But you should look for the reason of burning up, in most cases the reason is the shunt resistor between source of switching MOSFET and primary ground, maybe bad soldering. Surely the UC3843 ist dead, too.

Regards, Dieter

Don't try to find that resistor's value experimentally. You might cook your batteries. Instead go here:

see if you can get the schematic. There is a phone number on that page: 1-877-Bosch99 (267-2499) Perhaps you can sweet talk them into giving you a schematic.

All that said, I strongly suspect the problem is not limited to the resistor. Something made that resistor blow by drawing too much current through it.

On Thu, 1 Jul 2004 16:27:27 -0700, snipped-for-privacy@bellatlantic.net wrote (in article ):

Ok, it's time for True Stories...

I shipped the charger in my luggage when I flew from California to Arizona. The rough treatment my luggage was given by an unnamed airline resulted in a huge G-force to the cordless tool's carrying case, in which the charger was stored.

I plugged in the charger soon after arriving, and... Flash! Pop!, no charge.

I opened up the charger and found that a large, heavy toroid choke had pulled itself out of its solder. Looks like a cold solder joint was a contributing factor. The choke had shorted something when it landed, and when I plugged it in... ZAP.

I soldered it back in its place and removed the dead resistor. And there you have it.

In retrospect, I should have left the toroid where it was and then taken the charger to Bosch for a warranty replacement -- clearly a cold solder issue from when it was manufactured.

But now that the evidence has been destroyed, I have to find the right resistor.

I'll try Bosch Tools web site, but that is limited to parts for the drill. The charger is listed as one part.

Other suggestions are welcome. Anyone work for Bosch that can get the charger's schematic for me?

Thanks,

contributing

Maybe someone has one which they could pop open and measure/examine?

jak

On Fri, 2 Jul 2004 10:04:07 -0700, jakdedert wrote (in article ):

Yes. Anyone who has one of these, here is the location of R10:

Thanks,

Assuming you do find the right value, have you worked out what made it go phut before you replace it?

d Pearce Consulting

On Sat, 3 Jul 2004 00:05:53 -0700, Don Pearce wrote (in article ):

Yes. Choke shook loose in shipment, shorted to something on PCB when I plugged it in.

See my posts earlier in this thread.

On Sat, 3 Jul 2004 10:42:23 -0700, DaveC wrote (in article ):

On Sun, 4 Jul 2004 00:27:02 -0700, H. Dziardziel wrote (in article ):

Thanks for pointing out the operating range of pin 3 input. Gives me value to shoot for.

A new (for me) discovery:

Resistor (my exploded component) seems to be R in this figure from the data sheet:

It looks like R might just be part of the RC filter to "suppress switch transients", and not part of the current sense circuit, per se. The C used on this PCB is labeled "681". Is it possible to calculate R based on data sheet specs and this value of C? (Oscillator in the SMPS IC is 50 KHz.) Or is it trial-and-error time?

Thanks,

On Sun, 4 Jul 2004 08:49:43 -0700, DaveC wrote (in article ):

Oops... that is the rate of the oscillator in the test circuit shown in the data sheet.

Oscillator f is determined by external components connected to SMPS controller pin 4. In the case of my PCB, the C is labeled "472", and the resistor is 5100 ohm? (Gn/Bn/Bk/Bn). (European color code looks strange to my eyes...). What f would calculate to?

Thanks,

DaveC schrieb:

This will give about 75kHz switching frequency, with a filter capacitor of 680pF you should use a resistor in the range 470R-1k, I would take

680R.

Regards, Dieter

On Sun, 4 Jul 2004 11:06:01 -0700, Dieter Wiedmann wrote (in article ):

Dieter, Would you mind describing how you determined the two values?

75kHz and 470 - 1k ohms

I'd like to be able to calculate these myself, in the future.

Thanks,

On Sun, 4 Jul 2004 11:06:01 -0700, Dieter Wiedmann wrote (in article ):

I chose 1K resistor. The remains of the resistor seem to show a small fragment of the casing on which appears to be part of a brown stripe. This "1" value can only be 1K. Also, several of the data sheets show 1K resistors in the RC filter.

Installed the resistor, fired up the charger... nothing. Next step: replace the UC3843 SMPS controller IC. It probably got fried when this resistor exploded.

I'll keep you posted...

Thanks to all,

DaveC schrieb:

Datasheet: f(sw)=1.8/RC

f(3db)=20 to 40 * f(sw), optimum value depends on secondary side rectifiers.

Regards, Dieter

DaveC schrieb:

Not probably, but surely.

Regards, Dieter

On Mon, 5 Jul 2004 11:09:14 -0700, Dieter Wiedmann wrote (in article ):

Checking around the circuit, I find that the current sense resistor is open. Need a 0.22-ohm, 5 percent, 1-watt replacement. Since today is Independence Day holiday, tomorrow at the earliest. Then probably the SMPS controller IC.

Thanks,

DaveC schrieb:

Did you read my first posting on your problem?

Regards, Dieter