# Charger - Supply

Discussion in 'Electronic Basics' started by Randy Gross, Nov 3, 2004.

1. ### Randy GrossGuest

Greetings,

I am wondering if someone has or knows of anyone who has used a 12vdc
charger as the foundation of a power supply? I have a 12v, 2 or 6 amp
charger that I would like to use for this purpose.

My Idea is to build a module that I could run the output from the
charger into and vary the voltage and amperage from 0 - 12vdc and, 0 -
6adc respectively. Simplicity is my goal, precision is a concern but
not a priority. Can someone help me with the circuit?

Randy

2. ### Randy GrossGuest

After some thought and a lot of math,

I am looking at a new approach. Dividing the voltage would have been
simple, if we put the power dissipated aside ( 72 watts ). The problem
comes with stepping the voltage up incrementally. Each step in voltage
would require a current division circuit ( dedicated ) to sustain the
0 - 6A output in that range. Thats 12 circuits with resistors ranging
from a few watts up to 60 watts, we're digging deep in the pockets
now! Unrealistic!

Instead, I've settled to keep the 12vdc constant and vary the current:
| | | | |
12vdc R1 R2 R3 R4 R5
6A 12 ohm 6 ohm 4 ohm 3 ohm 2.4 ohm
72W | | | | |
<----------------------------------------

Even still, the wattages range from 12 to 60 watts. I realize this is
old school and inefficient but, simple.

What I am trying to do is find the optimum DC control current for a
Saturable Core Reactor I constructed. I tried 700ma at 6v from a cube
and was encouraged by the response I got with less than an amp. My
goal is to push the core to the saturation threshold to determine the
maximun range of AC I can control with the reactor.

Randy

3. ### Randy GrossGuest

OK! Back to the future.

I think I've found a regulator that might fit the application. It is
the LM338. There is only one concern I have with applying it to my
purpose, the 6amp input. According to the LM338 datasheet, the
regulator has an output range "in excess" of 5A but, I could'nt
determine how much, sustained. The LM338 can handle surges up to 12A
but I'm thinking this is momentary. Could I parallel two LM338s' to
split the current and adjust them with one var. res. as in the
following circuit:

----------
| LM338 |
----|I O|------
| | A | |
| ---------- |
| | | | | |
12 vdc | | ---------- | | R R1 |
| | | LM338 | | | R 120 |
6A | ----|I O|------ R |
| | A | | | +|
CAP ---------- | | CAP
.01mf |------- | 1mf
| |-------------- |
| | +| |
| R2 R 5K CAP |
| --R-> 10mf |
| R | |
| | | |
--GND----------------------------------------------

or, can I increase the value of the current set resistor ( R1 ) to
lower the maximum current through the regulator.

Randy

4. ### Robert MonsenGuest

I don't know whether parallel LM338s will work. It might work properly.

However, here is a 5A adjustable voltage/current regulator that might
work for you. Look on pg 16.

http://cache.national.com/ds/LM/LM117.pdf

--
Regards,
Robert Monsen

"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.