slightly OT, need some electronic circuit help

[Michael Torrie]

On 10/08/2012 01:30 PM, Shane Hathaway wrote:
> The large current requirement (10A) and the need for a middle position 
> make the circuit interesting and difficult to achieve using low cost 
> analog components.  An analog H bridge would work well if you only 
> needed 2 positions.  You can't dump that much current through an 
> ordinary op amp.  (Audio amplifiers use big, expensive op amps that 
> easily surpass the cost of an Arduino.)  PWM is the usual solution for 
> handling that much current.

> However, can you achieve the middle position using a simple mechanical 
> spring?  That would allow you to use a very simple H bridge.
> 
> It would help a lot to understand the application better.  What kind of 
> resting position do you want?  When the power is lost, do you want it to 
> revert somewhere, or hold its position firmly/loosely?

Okay, so here's what I have.  I have a linear actuator, which is
essentially a screw driven by a motor.  And the actuator has a built-in
potentiometer to give you position.  The motor uses very little or lots
of amps depending on the load.  I won't be having much load, so the amp
draw is very low, like under 1 amp at 12 VDC, most likely.  The
actuator, being a screw, cannot move when power is not on.  So I don't
need to hold it actively.  It is also very slow (compared to a servo),
so PWM is not required.  Full on voltage or no voltage is sufficient.
As I said before, I don't need super-accurate positioning, so I don't
need the actuator to seek back and forth around the target point.  When
it gets there it can just stop, even if it's too far or too close by a
small amount.

This actuator is controlling a hydraulic valve assembly so it only needs
three positions: extended, middle, and retracted.  It will always be in
one of these positions; there's no resting position.

Nick's idea of using limit switches is a good idea.  I am confident a
simple circuit can do it all without that kind of extra mechanical stuff
though.

Really I'm not worried about amp draw because I can use the result of
the op amps to drive a relay to switch the higher amps if I need to.