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  #1  
Old 01-18-2012, 10:55 PM
threec threec is offline
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Default DC reverse polarity automatic switch

Hello all, I have gone through a bunch of posts hoping to find the solution to my problem. But everyone else is doing something far more complicated it seems (which makes me wonder if my project is that simple and I am just missing some very mundane knowledge).

I have a linear actuator, it extends when 24VDC is applied and retracts when the polarity is reversed. I have a remote switch that can be latched, released, or momentarily latched. This is for a remote site powered by solar so I don't want to draw amps for no reason.

Ideally I want to be able to latch the control, extending the actuator, then release it, then at some point in the future I will latch the control again and the actuator should retract.

I am guessing there is a nifty easily bought switch that simply reverses the DC polarity to the output when it is activated but I don't know what that would be called. If anyone knows of something that I can purchase that would be great. I can also build something if that is the only option but it seems to me this should be something that gets called for a lot.

Thanks in advance, if I just need to look at a link that I am so far missing that would be fine too.
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Old 01-18-2012, 11:05 PM
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tracecom tracecom is offline
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Quote:
Originally Posted by threec View Post
Ideally I want to be able to latch the control, extending the actuator, then release it, then at some point in the future I will latch the control again and the actuator should retract.
What exactly do you mean by "latch the control?"

How much current does the actuator draw? Do you have a link to the actuator specifications?
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Old 01-19-2012, 12:36 AM
BJT_user BJT_user is offline
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Quote:
Originally Posted by threec View Post
I am guessing there is a nifty easily bought switch that simply reverses the DC polarity to the output when it is activated but I don't know what that would be called.
Double-pole, double-throw switch or relay will do the job of reversing polarity nicely. But it would be easier to advise you if we knew more detail about what you are trying to set up.
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Old 01-19-2012, 03:25 AM
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BillB3857 BillB3857 is offline
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Are there limit switches at each end of travel position? Will it need to be stopped at positions other at the two extreme positions? SgtWookie has a couple of really neat reversing systems that he has posted. One of the versions is shown on post #2 here....http://forum.allaboutcircuits.com/sh...=motor+reverse
You may need to make minor mods to that circuit to meet your needs. .
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Old 01-19-2012, 02:35 PM
threec threec is offline
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So some clarification - we have a remote location that is running mainly on solar power. The shelter as a louvre in it that we want to be able to manually control (open/close). In the shelter we have a management box that monitors for alarms and also has two control relays. These relays can be latched, disengaged (if latched) or momentarily latched remotely by logging into the box.
I have an actuator to control the louvre and I don't need it to stop half way or anything - going to the ends is fine.
The actuator is 24 VDC with max draw of 1.8 amps.
Applying power is the easy part, but reversing the polarity is where I get confused.
Thanks
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Old 01-19-2012, 02:50 PM
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BillB3857 BillB3857 is offline
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If you aren't wanting to have continuous current draw, you will need a way to know when the actuator has moved to full stroke. Limit switches would be the most reliable but high current draw or time could be used as an alternative. If I were tying to do this, I would use two 555 timers. These timers would drive relays. one for open and one for close. Timers would be set for a period slightly longer than the time required for the actuator to move full stroke. The timers would be individually triggered by your control box. Do you think this would work for you?
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Old 01-19-2012, 04:41 PM
threec threec is offline
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Timers could definitely work - the actuator fully extends in about 2 seconds (5 cm stroke) so it doesn't need to apply power long. The box that we have has a built in timer so it can apply power for a variable amount of time (like 2 seconds) and then shut off. It is the reversing of the polarity that is stopping me.
Really I just need the same circuitry as a garage door opener - hit the switch and it just does the reverse of where it is at.
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Old 01-19-2012, 05:04 PM
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Most garage door openers use internal limit switches to control when to stop and then set up for the other direction. Since your control box has two outputs that can be timed, use one output to fire the OPEN relay and the other timed output to fire a CLOSE relay. Did you look at SgtWookie's drawing? Your control box outputs would replace the individual push buttons. You could use, at your option, the S3 and S4 limit switches.
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Old 01-19-2012, 05:52 PM
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Unfortunately we have a use for the second control relay - it is wired to a heater that will be used in the event that the propane catadyne heater was to malfunction. So I really only have the one control relay to work with. I looked at the schematics but see that it does look like it would need two switches. I am looking for something that would only use one switch - I am guessing (obviously know very little about this) that there would be a relay that would trip into three states as voltage is applied and removed and that the tripping of it would provider either no power, positive 24 DC or negative 24 DC.
Sorry to make this so complicated as I am probably just not explaining it quite clearly.
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Old 01-19-2012, 06:55 PM
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Do you have four spare wires going to the shed that can carry 2 amps each? If so, a momentary ON/Off/ON DPT toggle switch will do what you want..,
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