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Ideas for an Auto ON-OFF switch controlling a motor?

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blackhope121

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I have designed a circuit that will open an exhaust cutout based upon rpm of the cars engine.

Once the frequency (tach signal ) reaches a designated point, the circuit will open a relay.

My problem is that valve is controlled by a momentary switch. Pretty much you have to hold the button to close it or hold it to open it.

I built my circuit hoping that it was a simple on and off switch. The manufacturer said that they did not use an on/off switch because eventually the motor will blow out since its trying to continually open or close the motor past a physical boundary.

My question is does anyone know about a device that will control the motor (open, close) but then stop it once its reached a physical barrier

Notes
- the valve motor operates between 11- 16v
- the valve motor uses 22mA to open or close
- once the valve has reached its physical stopper (when i hold the button down too long) the current greatly increases and the voltage drops to about 8v
- Im using a frequency to voltage converter lm2917n connected to a relay to operate the circuit.


here is the the valve that I am using
**broken link removed**
 
You can add a current sensor to detect the high current and shut off the voltage. You will need a latch or flip-flop circuit to control the power. The RPM change will turn the flip-flop/switch on and the over-current circuit will turn the flip-flop/switch off.

Do you reverse the voltage to the motor to reverse the valve? How is that accomplished?
 
You can add a current sensor to detect the high current and shut off the voltage. You will need a latch or flip-flop circuit to control the power. The RPM change will turn the flip-flop/switch on and the over-current circuit will turn the flip-flop/switch off.

Do you reverse the voltage to the motor to reverse the valve? How is that accomplished?

the polarity is reversed to operate the valve backwards.

I was thinking of a latch flip flop device but im not sure how i would implement it. Also specifically do you know a device that senses high current? like specifically what chip?
 
If you place a small resistor in the ground side of the load current, then it's easy to use an op amp (**broken link removed**). A 10 ohm resistor would give you a 220mV signal across the resistor at 22mA (Are you sure the motor only takes 22mA?). The op amp output goes to a comparator that is set to trip at the desired current. The comparator then triggers the flip-flop.

The complete function takes a reasonable amount of circuitry to do. I would suggest doing a block diagram (state diagram) showing the sequence of events for both opening and closing the valve. From that you can generate a functional block diagram of the circuit blocks (amp, FF, comparator, etc.) to perform the events. Then convert the blocks to a circuit schematic. The logic functions could, of course, be done by a microprocessor.
 
OK, then a 1 ohm resistor will give 0.22V at nominal current.

I see how the low side current sensing could work and the resulting voltage could be amplified by the op amp. Im assuming once the output voltage of the op amp is greater than the ref voltage of the comparator, the output of the comparator will switch to the new higher voltage.

How would i use that to trigger a latch.?

Im also thinking of how i could incorporate a polarity change in order close the circuit ?
 
The over-current comparator output could go the Set or Reset inputs of a flip-flop which controls the power.

Two ways to do the polarity change is with a transistor bridge circuit or a DPDT relay. Depends upon whether you prefer a solid-state or mechanical approach. The solid-state is more reliable, but the relay is simpler.
 
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