Making a Circuit Perform an Action Upon Losing Power

Thread Starter

Kriegbaum

Joined Aug 18, 2017
3
Hey Folks,
Most of my work is confined to lighting, so bear with me if I'm asking something crazy here. I'm looking to build a circuit that runs a motor up to a limit when it receives power, and then runs the motor back out when it loses power. I would prefer to go with a solution that does not involve continuous power consumption while it is active (I think I have a solution that involves gravity which I'm shying away from for the above reason.) The load being moved is between .5 and 2 pounds, and the run distance is probably below 18".

I have some vague idea involving a bank of capacitors, but I am not terribly familiar with the tools out there that will help me accomplish the footwork of supplying forward and backward polarity.
 

AnalogKid

Joined Aug 1, 2013
8,947
What are the motor requirements for voltage and current?

Depending on that, a DPDT relay might be very low power. Its coil would be the only thing powered continuously. Hard to beat it for simplicity:

2 limit switches
1 DPDT relay
Capacitor bank

EDIT: two reversing rectifier diodes are not needed.

ak
 
Last edited:

Thread Starter

Kriegbaum

Joined Aug 18, 2017
3
Max,
Any situation involving gravity would require the device to expend energy resisting gravity while on I believe, and I would like to have little to no power consumption once the initial motor run is complete. If you have a solution in mind where that is not the case, I would love to hear it.

AK,
I just looked into the DPDT relay, and that looks like a good candidate. Still working out the motor specs because I don't know the size limitations yet. I think this is the piece of the puzzle I was missing. I have to do some research on capacitors, I assume the diodes are there to protect the power supply?
 

MaxHeadRoom

Joined Jul 18, 2013
22,386
You have not really stated the type of motor. A DC motor will self generate when mechanically back fed or ran under gravity.
Which can be stored and assist the storage time.
Max.
 

Thread Starter

Kriegbaum

Joined Aug 18, 2017
3
I'm looking for a low voltage DC application. The power cycle would be the device turning on, running for a matter of seconds until it is in its completed on state, staying on for several hours, and then being switched off and the motor will run it back to its off state over a similar period of time. Am I wrong about the power requirements for a device that has to hold back gravity while its on? I've looked at a worm drive motor, which from what I understand will hold its place once its power has been cut, but that loses the ability of gravity to return the system to its off state without power. I don't think I would be able to take advantage of the power regained by backfeeding the DC motor.

You had mentioned capacitor storage being imprecise, is that an issue that would be mitigated by a pair of limit switches? Or was your concern more about the ability of a capacitor array to deliver enough power?
 

WBahn

Joined Mar 31, 2012
26,398
Max,
Any situation involving gravity would require the device to expend energy resisting gravity while on I believe, and I would like to have little to no power consumption once the initial motor run is complete. If you have a solution in mind where that is not the case, I would love to hear it.
Not at all. Imagine pulling a weight up via a pulley and then engaging a catch that holds it in place. It will stay there until doomsday without any power consumption. When your system loses power, all you need is just enough stored energy to release the catch, and that can be made very small indeed.

There are lots of alternatives here. You have gravity, spring tension, hydraulic accumulator, etc..

The latter is how an F-15 fighter aircraft is started. There is no battery on the aircraft. So to give it self-start capability, when the system is running two self-displacing hydraulic accumulators each store something around a gallon or so of hydraulic fluid under 3000 psi. With a simple tug of a cable in the cockpit, a valve is opened that lets this fluid turn a hydraulic motor that is used to start a 700 shp turboshaft engine, called the Jet Fuel Starter, that is used to then start the two main turbofan engines. The JFS accumulators also provide limited emergency hydraulic power for other things, including brakes (though no pilot uses that feature if they can avoid it by taking a cable instead since there's no anti-skid control) and emergency gear extension. In fact, the emergency gear extension is another example of storing energy and then mechanically trapping it and using a small amount of energy to release it. In the event of a hydraulic system failure, the JFS fluid is used to open the wheel well doors via a very short-stroke actuator which also mechanically releases the gear lock allowing the landing gear to drop under gravity down into the slip stream, which thing snaps them back into the down-and-locked position.
 

AnalogKid

Joined Aug 1, 2013
8,947
The diodes are part of a standard circuit for reversing a DC motor in a circuit with limit switches. BUT, because your reverse direction has an independent power source (the capacitor bank), the diodes are not needed. I edited my previous response.

ak
 

MaxHeadRoom

Joined Jul 18, 2013
22,386
You are correct, a worm and pinion drive system will not back feed at power off.
If just a question of saving power but still available, then I agree with the idea of running up to the desired height and then operate a solenoid or some method of latching such as double acting magnet solenoid, only requires power at latch/unlatch.
Then power on to go down, then a worm & pinion could be used as in a linear actuator etc.
A little more of the mechanics of the operation would be helpful.
Max.
 
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