# 12VDC control circuit??

Discussion in 'Electronic Basics' started by Steve, Oct 27, 2003.

1. ### SteveGuest

I'm upgrading a boat autopilot system. I will be retaining the control had
which outputs + or - 12vdc to a linear drive.. The current is limited to

The new system will also have a 12 vdc motor but the current will be as high
as 15 amps. The control would be the + /- output from and must deliver power
to forward and reverse the larger motor.

I know how to build a controller using relays, but the MTBF would be too
low, for a long passage. (20-30 + or - pulses/ min.)

I'm not very good at designing transistor things, but I can build just about
anything that someone else can scketch for me.

(I do have a very basic understanding of transistors and digital logic. I've
built many complex electronic and electrical projects (Heathkit,etc) but
never really learned too much.) ;o(

Anyone have a basic transistor circuit idea that will do this or a good
reference with example circuits.

Steve
s/v Good Intentions

2. ### MichaelGuest

I'm assuming from your description that your old controller uses a
bridged output to pulse the motor one way or the other. If thats the
case, the simplest solution is to find some chunky NPN & PNP
transistors that will handle 15A-20A, and buffer the original lines
with emitter followers as shown.
Note that this will method will drop 1.2V less that you had accross
your old motor due to the transistors dropping .6V each. Note also
that at 15A the on devices will be dissapating 9W each, so spit on
them now and then, and if the spit sizzles then these devices may need
a heatsink. It depends on long and how many pulses.

+12V
|
|
|
|
+----------+----------+
|c |
OLD + b |/ \| OLD -
-----+-| |-+----
| |> MOTOR <| |
| |e .---. | |
| +--------| X |--------+ |
| |e '---' | |
| |< >| |
+-| |-+
b |\ /|
|c |
+----------+----------+
|
|
|
|
===
GND

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