Re: 12V to 7.2V DC converter circuit?

g20zoom@xxxxxxxxx wrote:
The problem that I and others I know use the 7.2V battery pack
infrequently...say every few weeks, at which time the nicad battery is
dead. This involves waiting to charge it backup and also some
maintenance when using the batteries to prevent memory problems. It
turns out that most of the time, we are near a car, so I thought it
would be a cool addition to have some kind of 12v to 7.2v converter,
something I could just plug into the cigarette adapter and go. On the
surface, in my hand I have a small battery back and in my real vehicle,
I see a huge battery with high capacity that I want to tap into. At
first glance, seems like an easy feat to drive a tiny electrical motor
with a 12v car battery, but the electronics are proving a bit more
complicated to get the stall amps(25A) to start the electrical motor.
I didn't realize this was so complicated to build because of the
current to start/drive an electrical motor. So it's looking more like
the battery pack is indeed the way to go.



I saw this posted on the other group, and posted an answer
there. If all you want to do is run the motor, and you don't
care if the speed varies, you don't need a regulated supply.
Based on the figures posted in the other group, I think
you can do this really easily. Put a 3 ohm, 50 watt resistor
in series with the motor and measure the voltage across
the motor. If what I read is correct, the motor is rated
for 6 to 9 volts. If the measurement across the motor is
within that range, you're good to go. (You can use three
1 ohm 15 watt resistors in series in place of the 3 ohm
resistor.)

I *seriously* doubt that tiny motor has a 25 amp stall current.
But let's say it does, and there is difficulty starting it.
Use the following circuit:

+ ---+---[3ohm]---+---[Motor]---+
| | |
| | <----+ |
| | Ry-1 |
+-----o |
| |
| C1 |
+------+-----||----[Rly]---+
| 100uF |
[10K] |
| |
Gnd --------+-------------------+


When you plug it in to the cigarette lighter jack,
the cap draws heavy current through the relay, which
causes it to energize and close the contact Ry-1,
providing a start pulse to the motor at as much
current as it draws. The cap charges fairly
fast, which reduces the current through the relay,
so it de-energizes and the motor now gets all of
its voltage through the 3 ohm resistor. The 10K
resistor discharges the cap when the thing is
unplugged.

Use a heavy duty automotive relay. You may need
to experiment with the size of C1 - the larger
it is, the longer the start pulse.

Ed
.

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