I have installed electric trim on my experimental project (auto type electric window lift motor turning the jackscrew). I would like to use two small limit switches but I can't figure out how to wire them. Thanks, any help would be greatly appreciated.
You're going to have to show how your trim switch is wired but I suspect you use the NC contact and place it in series with the switch on the yoke for the direction in question.
I am not sure what a NC contact is. I am using a reverse polarity switch (by the throttle) which runs the motor fine. I will try to put up my wiring diagram sketch.
NC stands for "Normally Closed" and NO stands for "Normally open". Normally marked on the switch or the diagram with the unit. One Caveat...NC can also stand for "No Connection". One problem with placing a stop limit switch in series is if the switch actually cuts power to the motor and you are using a polarity swap switch (DC motor, with voltage applied in "reverse" will cause the motor to run "backwards," which accomplishes the desired movement...but if you break the motor lead, now you can't reverse it as the switch would open to stop the motor acting as a limit switch, but it would prevent reverse polarity from being applied to it, as the switch (Now open) would prevent current flow.) A common solution for this is to use a limit indicator. A couple of ohm resistor in line with the motor drive will drop voltage a couple of tenths of a volt, but at motor stall at max limit will dissipate a bit more...enough to illuminate a LED to full brightness. it can also actuate a relay to give an indication of limit and to stop the motor if an indicator is not sufficient. One such design is often used in ham radio "Screwdriver" antennas.
One such design is here: http://www.ad5x.com/images/Articles/LimitSWRevA.pdf Works well, cheap and easily built. AS soon as the motor hits stall condition, the dissipation on the resistor will jump, illuminating the LED.
Derswede
Last edited by Derswede; 02-24-2016 at 01:10 PM. Reason: I can't type...!
You can solve this problem by putting the limit switches in the right place in the circuit. The trim actuator switch is a DPDT, center off, spring return, right? So you wire the limit switches onto the positive side of the actuator switch before you join with the negative wires to go to the motor, like this:Originally Posted by Derswede
That way, each limit switch only interrupts the circuit for one direction of travel, and not the other.
- Andy
My bird has a 6 terminal DPDT switch for the Flap Switch mounted on the center console. I want to add a second flap switch making it easier for the left seat pilot to operate the flaps without removing the their hand from the stick.
it appears to me the only way to do this without modifying the basic flap system electrical design (this is a certificated airplane) would be to lead all six wires from all of the terminals of the existing switch to a second DPDT Switch.
Or is there an easier method with fewer wires? Alternatively, how would one wire a two button (up button, down button) solution that could be put on a stick grip?
thanks,
ted
Hmmm... if this is a certificated airplane you need to be talking to an A&P or a licensed avionics guy. I'm not entirely clear on the rules but this is potentially a significant change to the aircraft that involves a Form 337 and FAA approval. That will dwarf actual electrical design issues.
That said, speaking as an EE (and emphatically NOT as an A&P), to do this safely you'll need more than 6 wires and switch contacts. Problem is, if you simply wire a second DPDT switch in parallel with the first, there's a possibility that pilot and passenger (or co-pilot) will each operate one of the switches at the same time. If they're operated simultaneously in opposite directions, you are now shorting battery to ground, which is going to be real hard on the wiring. If you're lucky and the circuit is appropriately protected all you'll do is pop a breaker. Still a nasty distraction on short final.
This is the same problem you have with power window switches in cars, where both driver and passenger have a switch controlling the same window. There's a lot of wires at the driver's switch, because it's hooked up to preempt control from the passenger when it's operated. It's doable with enough switch contacts, but basically you need to run the wiring from one switch through the other so the latter disconnects the former when it's operated.
Re your last question... Yes, you could use a pair of push buttons just as easily as a single switch. Ignoring the dual control issue, the push buttons would have to be DPST, normally open. You wire up the buttons to replace the DPDT switch so that corresponding connections on one side of the push buttons are wired together - these replace the center connections on the DPDT switch. Then the other sides of the push buttons replace the corresponding end contacts of the DPDT switch. Again, if you want both a pair of push buttons you need additional contacts so that operating the pushbutton disconnects the other switch, so that you can't accidentally try to drive the flaps in both directions at the same time.
- Andy
I suggest you visit the Aeroelectric list over at Matronics. You are unlikely to find more horsepower, wxperience, and savvy in this field anywhere else.
Thank you all for your help. I think I can finish wiring it now if I get heavier limit switches. I have a mechanical indicator similar to a PA-22 but an LED limit indicator is a good idea I had not thought of. Aeroelectric is also great. Again, thanks.
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