Power supply filtering (for shoe box thread)

Dragon Tamer said:

it's the heat with the 2955 responsible for charging the battery that is concerning me a little.

Click to expand...

Guess why the battery charging power transistor gets hot? Because its input voltage is way too high.

As AG says, your transformer is FAR, FAR too high - and apart from that I'm totally amazed by the huge amount of totally unrequired circuitry you're building - almost nothing on those three circuits is needed.

Get the correct transformer and start again.

If you think that I should get a new transformer, I will. But I don't really see how that affects the circuit. It has been working properly, but heat was just a side affect of all of the current that was going to the various parts. Granted that Q2 is dissipating a little more power than was initially anticipated.

Heat is caused by voltage times current. You cannot change the current but since your input voltage is way too high then by reducing it then the heating will also be reduced.

Q1 in the circuit is not the major heat producer, it's Q2. I can only drop the voltage on Q2 by 1.5V. If I go any lower the battery will not charge.

I tried switching out the transformer with a transformer with a lower output, the first on (24V) got extreemly hot and now has an output voltage of 18V. My highest current transformer (15V) was unable to supply the voltage needed by the circuit. I went back to the 33V transformer, and everything works just fine again.

Your transistors do not have part numbers so I guess that the hot Q2 is the BJE2955 power transistor in the MAX712 charger circuit?
It has an input of 17.3V and its output is about 7.9V when the battery is very low. Its current is 0.5A so it dissipates 0.5A x (17.3V - 7.9V)= 4.7W.
The input needs to be about 14V minimum to charge 8 cells. You are wasting 1.65W.



A few of your capacitors are listed as mF. What is that? 4.7mF means nothing. Maybe it is 4700uF.

Yes that is correct, Q2 is the 2955 charging the battery (it's actualy an MJE2955T, my mistake). The cappacitors that are labeled in mF are actually XXXXuF.