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Stuart Br
11-29-2011, 03:13 AM
I have an elderly clay pigeon trap in our club that is driven by a 12V DC motor. We are having an amount of intermittent problems and I am looking to apply an indicator to help the less technical in the club to carry out an initial diagnosis without the need for tools. What I would like is that when the "fire" button is pressed, to be able to show whether power is being applied to the motor or not. The theory being that when they press the fire button, and nothing happens, if the indicator comes on, they know that power is being applied to the motor and that the fault is on the motor side. If the indicator doesn't come on then the fault is in the control system.

My initial thoughts were to just connect a 12V LED module across the motor terminals, but I am a worried about back EMF from the motor destroying the LED. The nature of the beast is that the motor is stop/start, it runs for about 10 seconds than has about a minute idle time. It is also in a very harsh environment with a lot of vibration, which would preclude the use of a traditional filament lamp. The motor is rated at 27A FLC

I assume that I need some form of diode across the LED to dump the back EMF and protect the LED, but what type and rating?

Thanks in advance for your advice

Stuart

Evan
11-29-2011, 05:08 AM
Just use a regular incandescent marker light instead. Or buy a tester for a few bucks that looks like a screwdriver with a light in it with an extra lead for testing 12 volt circuits.

MaxHeadRoom
11-29-2011, 10:27 AM
In a DC motor the BEMF or generated voltage is in the same polarity as the applied voltage, and the only time the generated voltage is higher than the applied is if the motor is driven at a higher rpm due to overrun or back feeding.
You should not have a problem with a LED, especially if the load is permanently connected to the motor.
Max.

Stuart Br
11-29-2011, 11:11 AM
Well the load is definitely not constant, but the motor is connected to a pretty big reduction gearbox, so I don't believe that there is any overrun. The The trap arm moves through 360 degrees and the throwing action is via a powerful spring on a crank. The trap arm is not directly connected to the motor, a crank on the motor shaft picks up the trap arm for the cocking part of the cycle. The motor cocks the trap by winding the arm against the spring until just before top dead centre, when the motor is disengaged via a roller switch. Firing the trap bypasses the roller switch which engages the motor which pushes the arm over TDC, at which time the mainspring launches the clay. the motor then continues to drive around (under minimal load) until the cam picks up the arm again, driving it back to the cocked position.

Based on MaxHeadroom's advice. I will give it a go. I have a box of spare LEDs available, so no issue if it does go pop.

Thanks

Stuart

MaxHeadRoom
11-29-2011, 11:28 AM
Just noticed, My home county!

Remember if they are plain LED's they will require a resistor, if 12v automotive etc, then they should not require one.
Max.

Stuart Br
11-29-2011, 11:42 AM
Yep, resistor already dug out of the scrap box and tested on a 12v supply

Black_Moons
11-29-2011, 12:12 PM
In a DC motor the BEMF or generated voltage is in the same polarity as the applied voltage, and the only time the generated voltage is higher than the applied is if the motor is driven at a higher rpm due to overrun or back feeding.
You should not have a problem with a LED, especially if the load is permanently connected to the motor.
Max.

Actualy, when you stop applying power, The initial inductive spike is in the reverse polarity and may be up to several 100 volts, a diode in antiparallel with the motor or led would be recommended.

Evan
11-29-2011, 12:23 PM
Especially if the motor is intermittent. An incandescent bulb doesn't care about all this and also doesn't care about polarity. Check the brushes on the motor. They are probably worn out.

Stuart Br
11-29-2011, 12:31 PM
Evan,

I have had brushes sticking in the holders before on these motors

Stuart

Stuart Br
11-29-2011, 12:37 PM
Black Moons, can you please clarify "anti parallel" for the diode connection?
There are already a number of pretty hefty diodes in the control box, I believe across contactors. I haven't mapped the circuit to see whether there is one across the motor connections.

MaxHeadRoom
11-29-2011, 01:22 PM
Actualy, when you stop applying power, The initial inductive spike is in the reverse polarity and may be up to several 100 volts, a diode in antiparallel with the motor or led would be recommended.

But if a DC motor is turning at speed when the power is removed winding down it is still a generator, as it is when powered, and the generated output will oppose reverse polarity to a great degree.
Max.

Evan
11-29-2011, 03:34 PM
Yeahbut, it's intermittent. That can create good sized spikes.

Anti parallel means that the diode is connected so that it is reversed polarity in respect of the other diode (LED). Cathode to anode at each end. I still say use a light bulb. New tech isn't always better.

Gary Paine
11-29-2011, 04:06 PM
To accomplish what you want, you should let the indicator measure right across the motor; ie, hook to both motor leads. An LED with a current limiting resistor can be protected fine with just a cheap diode in series to help handle the voltage from the inductive spikes. Most any diode will have a sufficiently high PIV rating for the job. Size the resistor for only half the rated current to let it more easily handle an intermittancy issue.
I would not try to put a reverse polarity suppression diode in your circuit. Your object is not to try to suppress inductive kick and such a diode or MOV would need to be robust if it were added.

MaxHeadRoom
11-29-2011, 05:11 PM
As a point of interest, I do the service if required on my automobile electronics, and like many now, instead of a large harness all over the car it has a DataLink system, which means local electronic modules fed by a data link from the computer.
I don't see any reverse diodes across any of the many motors, which in light of the local electronics one might have expected if this were a problem?
Max.

Evan
11-29-2011, 05:44 PM
It's built in to the chip that controls the motor. The mosfet output will have clamp diodes right on the chip and if it's an H bridge the clamping is automatic by turning on the lower mosfets at turn off.