Bearded Occam said:
Yes, I would greatly enjoy seeing such a circuit. I may still wimp out
because of the safety issues, but at least I will have an idea of how
a charger should work. Also I actually have a timer that I plug my
soldering iron into, so I don't forget to turn it off.
Thanks!
View with courier or other fixed width font.
Here is what you need:
1 10k trimmer. A cheap one is fine, the setting is not too exact.
1 1k resistor
1 10k resistor
1 1.8k resistor
1 1 ohm 2W resistor (one of the big square ones)
2 2N3906 or pretty much any TO-92 PNP transistor
1 2N4401 NPN (or any TO-92 NPN transistor)
1 IRF1405 N channel MOSFET (or equivalent, see below)
1 heat sink for the MOSFET (TO-220 variety)
1 mounting screw and insulator for mosfet and heat sink
1 regulated 24V@1A power supply
Here is the circuit:
24V o-----o------------------o--------------------o +
| |
| .-.
| | | 1k
| | | Batt Here
| '-'
| |
| .---o---.
| | | .----o -
.-. |< >| ___ |
10k | |<----------| PNPs |--|___|--o
| | |\ /| 1.8k |
'-' | | |
| | | |
| | | ||-+ N-Channel
| | | ||<- Mosfet
| .---- o-------)--------||-+ Use Heat Sink
| | | | |
| | | | |
| | | | |
| .-. \| | |
| | | NPN |-----)-----------o
| 10k | | <| | |
| '-' | | |
| | | | .-.
| | | | | | 1.0 Ohm
| | | | | | 2W
| | | | '-'
| | | | |
| | | | |
GND o-----o--------o-----o-------o-----------'
Adjust by putting a 10k resistor between the outputs, and
setting the - output to 6V using the trimmer.
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
How it works:
The mosfet and NPN transistor form a fixed current source. They limit the
current through the battery to a value near 700mA. The two PNP transistors
compare the output voltage with the voltage on the wiper of the trimmer.
When the output voltage on the - terminal gets down to the voltage on the
wiper, the current source is turned off. As the battery keeps sucking
current, this differential pair will keep the voltage constant.
You adjust it by putting a 10k resistor across the output (instead of the
battery). Then, use the trimmer to set the - of the output (using your
multimeter) so that it is 18V below the + output.
You could probably get away with a 20V Power supply with these parts. With
an IRF1405, you might be able to skip the heatsink. However, you probably
won't be able to get one of those at radio shack. Get a couple of TO-220
case N channel mosfets and a heat sink (along with some heat sink paste) and
a mounting kit. Handle the mosfets carefully, because you can blow out the
gate pretty easily with a static shock.
Regarding NPN vs PNP (in case you don't know)
For NPN, the collector is the more positive one, and current flows from
collector to emitter.
For PNP, it is the other way around, and the emitter is the more positive
one, and current flows from emitter to collector.