Limit switch with diodes
David Calkins
I need a limit switch for a motor, which goes both directions off an
ESC. So a simple NO/NC switch won't work. I'm thinking of a SPDT
swicth with a diode on one side. So if it hits the upper limit of the
motor, current cannot continue to flow forward, but could flow backward
reverseing the motor. Likewize with a reversed diode on the bottom end.
image attached.
Thoughts? I'm certain it will work, but am suprized i can't find one
out there off-the shelf I can buy.
Also, the Amps on the motor will get pretty high, so I'll probably need
something in the 25A range, so a rectifier seems more likely than a diode.
--
David E. Calkins - http://www.robogames.net
-----------------------------------------------
Silicon shall replace Carbon.
The revolution will be automated.
Randy M. Dumse
> Thoughts? I'm certain it will work, ...
> probably need something in the 25A range
Two thoughts.
First Probably only need one diode. If the circuit is "broken"
by one diode in one direction, the second diode will only add to
the wasted voltage drop. In that sense, a SPST switch NC is all
you need. Short across the diode for normal operation. Remember
this switch is probably momentary, or once into the stops, you
couldn't re-reverse after clearing the stops.
Second, look at a store that sells towing and automotive
equipment, like Camping World or JC Whitney and the like. They
have diodes for insertion between cars, to isolate the towing
vehicle's electrical system from the other vehicles system, and
only operate (say) the brake lights. They're pretty high
current. Oh, more expensive than I remembered.
http://www.campingworld.com/shopping/item/hy-power-diodes-2-pk-/
14631
I found last time I looked it was easier to get what I needed
through those channels, than it was looking for high current
diodes in electronics stores.
Randy
anfederman@comcast
The amperage is limited by an auto type fuse - so I guess you could sneak a
25 amp fuse/circuit breaker in there as long as you weren't worried about
voiding the warranty.
Spike H-Bridge Relay ~$35.00
"Spike is a 20 Amp, H-Bridge Relay Module that is small enough to be
remotely mounted almost anywhere on your robot. Spike is designed for
driving small motors in forward, reverse, or stop (brake). Spike is
opto-isolated at the signal input to protect the Robot Controller against
motor noise and return currents. Requires a 3-wire cable for connection to
the Robot Controller.
Ideal for 2 independent solenoids, light, pumps.
Design will accept auto-resetting circuit breaker.
Compact design.
"
http://www.vexrobotics.com/217-0220.html
At that price it is cheap enough even for Dave;-)
--------------------------------------------------
Dave Curtis
On Oct 6, 2011, at 6:47 AM, Randy M. Dumse wrote:
> David Calkins said: Thursday, October 06, 2011 3:25 AM
>> Thoughts? I'm certain it will work, ...
>> probably need something in the 25A range
>
> Two thoughts.
>
> First Probably only need one diode. If the circuit is "broken"
> by one diode in one direction, the second diode will only add to
> the wasted voltage drop.
Agree on this point.
Actually, this is a pretty common arrangement in larger antenna positioners/rotators. Typically, a rotatable beam is set up to rotate 360 degrees, or perhaps with some over-travel, like 450 degrees. Anyway, there are limit switches set up to stop the travel at the end stops; these are wired with a single diode but otherwise just as you suggest. This stops the antenna at the end stop but allows you to rotate the antenna in the other direction away from the end stop.
I've never seen limit switches with diodes like this available as a combined unit. Every rotator I've seen does this with discrete diodes. If the rotator is well designed, the diodes are behind a nice little access hatch so that they are easy to change out while hanging off of a tower at 70 feet in the air. Passing T-storms can take out the diodes even without a directly lightning hit. I suspect your application would also benefit from the ability to easily change a toasty diode, so discrete diodes don't seem like such a bad idea. It probably goes without saying, but the voltage rating on the diodes should be 4X or so the motor voltage so that commutation noise doesn't take them out.
-Dave N6NZ
Here is the rotator I use for my 7 MHz, 3 element Yagi:
http://www.m2inc.com/products/rotors/or28002.html
David Calkins
Though it does need a diode in each direction, as it has a full forward
limit switch is differnt from the full back limit switch. Although in
the mid-range, neither diode would be in series. The diodes only switch
on at the far ends, thus keeping the circuit closed, but the current can
only flow one way. If an NO switch enters the circuit, that I can't
turn the motor either* way, since the circuit is now open.
Since the diodes only come into play at the far reach, it only does the
voltage drop as the arm begins its opposite sweep. Once started, the
limit switch is released, thus the circuit again has no diodes actually
in line. Until the other limit switch is pressed.
David E. Calkins - http://www.robogames.net
-----------------------------------------------
Silicon shall replace Carbon.
The revolution will be automated.
David Calkins
switch. I see how it operates, but it's too iffy for my needs (pun
intended)
David E. Calkins - http://www.robogames.net
-----------------------------------------------
Silicon shall replace Carbon.
The revolution will be automated.
> ------------------------------------------------------------------------
>
David Calkins
> On Oct 6, 2011, at 6:47 AM, Randy M. Dumse wrote:
>
>> David Calkins said: Thursday, October 06, 2011 3:25 AM
>>> Thoughts? I'm certain it will work, ...
>>> probably need something in the 25A range
>> Two thoughts.
>>
>> First Probably only need one diode. If the circuit is "broken"
>> by one diode in one direction, the second diode will only add to
>> the wasted voltage drop.
>
> Agree on this point.
The arm as designed has about a 70 deg arc. It has to stop completely
at either end, not just the top end. As my previous explation, the
diodes are only ever in the circuit if a given switch is pressed.
In my image, it may look as though it's a DPDT single action switch.
It's not. It's two separate DPST switches, separate from each other.
Dave Curtis
OK, yeah, I misunderstood your drawing. In any case, the general scheme works great, modulo component ratings. But I think you have to wire it up out of discrete components, I don't think you'll find it pre-packaged.
The wiring I've typically seen is a diode in parallel with a SPST/NC snap switch. The switch conducts DC either way until the limit switch is depressed, at which time the diode allows only one-way current. This allows the motor to reverse. Put one set at each end.
anfederman@comcast
motor leads to each other - that way they act as a brake. This is done for
worm gear motors that are handling a lot of weight - so that stop means
'locked' in position. If you float - and have a big heavy weight at the end
of a lever, you'll back drive the motor - putting back EMF into your main
power supply.
The reason I mention this is that it is a concern with solar tracker
controllers.
--------------------------------------------------
From: "David Calkins" <dcal...@robotics-society.org>
Sent: Thursday, October 06, 2011 9:52 AM
To: <hbrob...@googlegroups.com>
Subject: Re: [HBRobotics] Limit switch with diodes
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