Thx Al,
yes I saw your email address but was hesitant to bug you with a direct email - easier to NOT respond to a post which you might not have read than on a direct email though
Actually my problem is that the smart charger circuit intended for the follow up to the MR16 CPF project light isn´t supplying the juice needed.
Its a pretty basic circuit using the Philips TEA1101 and is intended to smartly charge upto 10 Nixx cells in series. Its a DIY-kit I modified to charge 12 cells and to modify the charging currrent I switched from the resistor you normally calculate to your desired current to a 10K potentiometer - this worked pretty much perfect so far.
When I got my new 6A/24V Power supply to replace the originaly used 1A/20V units I more or less just throttled up the current to see at what point which components would fail (Bad idea). To make a long story short - it worked upto 5A but not for long
After some nasty fiddling to bring things back to work (outch) the circuit (actually a second one) is working again - quite some parts beefed up - the bad thing is whatever I do its not supplying more than 1A anymore - throttling up the resistor throttles up the current - but only to max 1A - going up further doesn´t change anything.
I will list the changed parts but my "guess" is that the exchanged transistors while being in the same "series" just do different - my hope is that with some of the calculations/simulations you seem to be so familiar with which you used for the Zetek circuit or maybe just out of your memory you might be able to point me to the trouble-spot.
One area changed is the supply voltage to the IC where I used a BC639 (as well as a BC547 earlier) instead of the original BC548 just to be on the safe side as the 24V AC I transform to DC gives 30V under light load - the IC just gets 8V as befor and I don´t expect this to be the problem.
Inside the charging path there only is the main power transistor, a Diode to prevent cell discharge when the power is disconnected and a 0.1 Ohm resistor (shunt) which voltage drop is used to measure the actual charging current.
I swapped the original BD436 against an BD442 and in between also tried a BD912 - the original BD436 was the one which blew but gave me 3-4-5A befor it did so, the 912 gave 1,5A max - the 442 1A max - the 442 as far as I understand is a direct pin-to-pin compatible and functionally equivalent type to the 436 just able to handle higher voltages - I more or less suspect the problem here but don´t know better.
The transistor which gives the input to the power transistor is a BC337 which I also replaced against a BC639 and thinking the problem is there again replaced with a BC337-40 but this didn´t changed anything. This transistor is connected to the analog output of the TEA1101.
The original 1N4001 diode was initially replaced in the first place against a 3,5A 1N5404 or something and now a 6A type is used of which I don´t know the number but guess this is more or less not an issue anyway.
The shunt / resistor was originally a 1W or 1/2W type and now is a heavy 5W one as I didn´t had any 2W type available which would have been enough I think.
As far as my limited knowledge goes the beefier diode or resistor shouldn´t do any harm - the replacement of the transistor(s) was done with some external advice and the guy still thinks it should work this way.
If the whole thing wouldn´t have worked just perfect and great for a while before blowing I would have given up already - but I saw it working fine with the (almost) original parts at the needed current (3 to 3,5A would be nice) but when I wanted to test the limits and ran it too long too high it just went south with some smoke and such - actually it didn´t blew in that meaning - it more or less heated up more and more until something went bust - I was just too stupid to stop early enough.
I would very much appreciate your advice
TIA
Klaus
