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12v current sensing stop control


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patro46 
Member - Posts: 1
Member spacespace
Joined: August 05, 2012
Location: Oklahoma, United States
Posted: August 05, 2012 at 4:02 AM / IP Logged  
I am needing a solution to a rather expensive wireless 12v electric tonneau cover. Pace Edwards sells the one I want...for a paultry 3.5 c notes. I can get a simple 30 amp two channel wireless relay for around 40 bucks. What it doesnt have that the high dollar pace edwards unit does is the ability to "sense" when the cover is fully retracted or closed to prevent burning up of the motor or breaking some expensive hardware. I realize I could use a physical switch on both ends with more wiring to run and hide but Pace Edwards has something integrated into their control that senses an increase in amps when the units hits full open or close, senses this and shuts power off to motor to prevent damage in the event someone doesnt let off of the remote open or close button. How do they do this, and can I incorporate this into my 40 dollar 12v wireless control solution? I have also pondered a 12 volt wireless wench control for this task, however I think I'd have the same issue with a current sensing shutoff device. BTW the motor is a 12 volt high torque w/electric brake and is rated around 6 to 7 amps.
Thanks!
oldspark 
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: August 05, 2012 at 8:05 AM / IP Logged  
Current sensing can be done by measuring the voltage across an in-line resistor (V-IR).
A series 0.01 Ohm resistor will be 0.01 x 7 = 0.07V at 7A. It should be rated at 1W to tolerate 10A of current (P=IIR = 10x10x0.01)
Similarly a 0.1 Ohm gives 0.7V at 10A (10x10x.1 => 10W rating for 10A)
The latter is enough to turn on a transistor (ie, ~0.7V), but usually Comparators are used.
A timer may be another solution.
Or even self-resetting circuit breakers eg - 5A for a 7A motor assuming only tens of seconds are required to open & close - provided the power is turn off after some delay.
hotwaterwizard 
Silver - Posts: 1,350
Silver spacespace
Joined: December 11, 2003
Location: California, United States
Posted: August 05, 2012 at 10:00 AM / IP Logged  

. My first thought is one of those Resetting circuit breakers.

12v current sensing stop control -- posted image.12v current sensing stop control -- posted image.

Here are a couple of Ideas

http://www.powersystemsdesign.com/circuit-protection-considerations-?a=1&c=1401

http://www.electronics-lab.com/blog/?tag=overcurrent

John DeRosa (Hotwaterwizard)
Stockton California
When in doubt, try it out !
howie ll 
Pot Metal - Posts: 16,466
Pot Metal spacespace
Joined: January 09, 2007
Location: United Kingdom
Posted: August 05, 2012 at 11:57 AM / IP Logged  
Plan A use a DEI 429t with built in thermal sensors triggered by your remote's lock unlock wires, this is an analogy after all to close/open a window.
Plan B measure the time taken in each direction, add 1-2 seconds then incorporate these times into a pair of DEI 528T timer relays.
I've used the second method many times on folding door mirrors.
P.S. 6-7 amps is less than an electric window motor, about the same as a door lock actuator.
denton92 
Member - Posts: 2
Member spacespace
Joined: October 29, 2012
Location: Ontario, Canada
Posted: October 30, 2012 at 1:14 AM / IP Logged  
Why didn't you build your own current sensing circuit it will at lease avoid the expenses and can really help you solve the problem regarding the breaking up of the expensive hardware and burning of motor. A relay will help you too to avoid this problem. You don't have to buy it, a little research will do the job and also you will gain the knowledge on how to build this kind of thing.

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