slightly OT, need some electronic circuit help
torriem at gmail.com
Mon Oct 8 14:03:21 MDT 2012
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
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
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.
More information about the PLUG