Re: 12V to 7.2V DC converter circuit?

g20zoom@xxxxxxxxx wrote:
Thanks for moving the post. Sorry, I didn't know.

I think I need about 3-4 amps. It's a motor to start a small nitro RC
engine (.15 size)
I guess the motor is about a mabuchi 380 in size.
The starting requires a bit of pulse starting(ie on for 15-20 seconds
off again, on again) so I imagine the initial start loads maybe a
little higher?

I took this from some other posting on the web:
checked the specs for the 380/400 motor too :Specification
Nominal voltage 7,2 V
Operating voltage range 3,6 ... 8,4 V
No-load rpm 16400 min1
No-load current drain 0,5 A
Current drain at max. efficiency 3,3 A
Current drain when stalled 21 A
Max. efficiency without gearbox 72 %
Length of case, excl. shaft 37,8 mm
Diameter 27,7 mm
Free shaft length 13,8 mm Shaft diameter 2,3 mm Weight 73 g


Your best approach is to use NiCds and charge them from
the car or the AC mains if available. Why force yourself
to start the RC engine near the car? The charge circuit
using the car or a 12V supply is real simple: one resistor,
a 1N400x diode and an LM317.

-----
+12 ------Vin|LM317|Vout---+
----- |
Adj [R] 5 ohm 1/2 watt
| |
+----------+--->|---> To NiCd +
1N400x

Gnd --------------------------------> To NiCd -

This will provide a charge current of 250 mA, which
will take 14 hours to charge a fully discharged 2.5 aH
C cell. You can get more sophisticated, but it is
probably not necessary.

But if you must, you can run a comparator across the
NiCd pack. When the voltage rises to full charge, the
comparator switches and drops out a relay, which
disconnects the charge circuit from the source. That
prevents draining the car battery and terminates the
charge to the pack so it can't overcharge. (You can
leave a NiCd on a 14 hour charger for well in excess
of 14 hours with no problems, but not forever.)
If you elect to go with the comparator/relay disconnect
circuit, you could change the value of the resistor to
1.5 ohms, 2 watt. The charge rate would than be .83
amps, and charging a fully discharged pack would take
about 3 1/2 hours.

Ed
.

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