I am working on a machine to dispense paper coupons. It uses a 12 volt DC motor to dispense the coupons off a roll. I need the motor to stop quickly each time a coupon is dispensed so I am using a dynamic braking to stop the motor. The motor cycles on and off according to a signal from a 555 timer. A typical cycle would be run motor for 2 seconds - stop for 5 seconds. This machine could run continuously for hours. I have found two different ways to set up a circuit to brake the motor. One uses a relay and the other a pair of mosfets. They are shown on my sketch. I would like to eventually make these machines and sell them. I am wondering if there is a good reason to use one circuit over the other. There isn't much difference in cost between the two. Is one circuit more reliable than the other or are there perhaps other reasons to pick one over the other.

 

If you get satisfactory results with the Mosfet brake, I would go with that.
Max.

 

Ok, I will go with the mosfet pair. Thanks.

 

You need to give us more details about the motor. Other options are:
https://forum.allaboutcircuits.com/threads/breaking-dc-motor.15066/

 

The motor is a small 12v DC gearmotor. It draws about 0.8amps and runs at 60rpms.

 

I agree with Max. A mosfet brake is the best option, mainly because you'll get much better repeatability than using a mechanical relay.

Also, relay's contacts tend to change their conductivity with use, and so are the less reliable of both options.

 

Looking down the road: Which is easier to repair? I would think a relay can be built onto a plug-in base whereas the FET's would require desoldering and re-soldering operations. While the FET's are more reliable, the relay option presents easier field repairs, should that be an option. If you're selling these without a service contract then go with the FET's. However, the relay option would require a FET or a BJT to drive the relay. You would also need a flyback diode on the relay coil to prevent BEMF. Cost Per Unit may go up slightly.

 

Looking down the road: Which is easier to repair? I would think a relay can be built onto a plug-in base whereas the FET's would require desoldering and re-soldering operations. While the FET's are more reliable, the relay option presents easier field repairs, should that be an option. If you're selling these without a service contract then go with the FET's. However, the relay option would require a FET or a BJT to drive the relay. You would also need a flyback diode on the relay coil to prevent BEMF. Cost Per Unit may go up slightly.

I always use terminal blocks (and not solder) to attach FETs to my PCBs, for the very same reason that you've just explained.

 

Sometimes you can get away with just a resistor across the motor terminals - 100 ohms or so for 12V. Burns more power and doesn't stop as fast but simple and reliable if it works for the application.

 

Must confess, I don't know why I would want to dispense two seconds worth of paper every five seconds.

@cmartinez Hadn't considered that as an option. Excellent point.

 

Must confess, I don't know why I would want to dispense two seconds worth of paper every five seconds.

There is a pause while the operator removes the coupon and then restarts the motor.
Also, it is my understanding that I don't need a flyback diode at the motor for BEMF with the mosfet braking. Is that correct?

 

it is my understanding that I don't need a flyback diode at the motor for BEMF with the mosfet braking. Is that correct?

You would need the flyback ONLY IF YOU USE A RELAY. The flyback would go across the relay. Not the motor.

 

If you are designing the machine from scratch maybe you could take a leaf out of the way similar to the way the snack food industry does it with the hot wire sealer machines.
In this idea you do not stop the motor, but leave the motor running and lift a drive wheel off the coupon strip, the drum or dispenser that has the coupon role could posses some kind of friction in order it stops when the wheel is lifted and does not overrun.
This also assumes the motor is not driving the coupon drum itself, just the non-lifted roller.
As to the original design for braking, normally a high ratio GB/small motor combo has inherent braking.
Just another view of it.
Max.

 

There is a pause while the operator removes the coupon and then restarts the motor.
Also, it is my understanding that I don't need a flyback diode at the motor for BEMF with the mosfet braking. Is that correct?

I'd place the diode there regardless. If only to protect the fet from spikes caused by brush commutation. Are you using pwm?

 

No pwm.

 

This also assumes the motor is not driving the coupon drum itself, just the non-lifted roller.

This raised a thought in my mind: (and no, I didn't hear an echo) When the spool is full, lets say 3 inches diameter. Running the motor for two seconds is needed to dispense a coupon. (somehow the paper is cut or perforated). Five seconds later it drives another coupon. With each revolution the roll gets smaller and smaller, thus precipitating the need for a longer run period because two seconds at 60 RPM will dispense more coupon at 3 inch diameter than it will when the roll is close to empty.

You MAY need to add a sensor to determine when the full length of the coupon has been dispensed. Then if a knife cuts the paper - or however you plan on separating the coupon. Otherwise some gorilla is going to come along and yank on the coupon and pull several coupons out at once before they realize they need to tear the coupon.

FURTHER: If every five seconds a coupon is dispensed, what happens when there's nobody there to collect the coupon? Does the roll reach the floor before the next person comes along?

AND: How long will a roll last when it is dispensing two seconds every five seconds before the roll is empty?

Sorry, I don't mean to rain on your parade. I have a propensity for pointing out peripheral issues. In other words, I'm finding problems to the solution when I should be helping you find solutions to the problem.

 

Sorry, I don't mean to rain on your parade. I have a propensity for pointing out peripheral issues. In other words, I'm finding problems to the solution when I should be helping you find solutions to the problem.

Not raining on mine, if the motor is constantly driving one of a pair of rollers as I mentioned and the top one is lifted for a preset time, the feed rate should always be equal.
Max.

 

if the motor is constantly driving one of a pair of rollers as I mentioned and the top one is lifted for a preset time, the feed rate should always be equal.

Yeah, I think I recall you saying that, now that you mention it. I had it in my mind that the roll would be mounted on a shaft and the motor turn the shaft. A feed roller is probably the solution to the issue I mentioned.

However, I still wonder about the other issues.

 

Sorry, I don't mean to rain on your parade

I just wish it was warm enough to rain! The 555 timer is part of a control circuit and the 555 stops the motor when a coupon has been dispensed. There is a light sensor that detects the end of a coupon on the roll. The motor is restarted by the operator when a coupon has been picked off.

 

The motor is restarted by the operator when a coupon has been picked off.

OK, glad you've considered that. But haven't I already seen these in grocery stores?

As for wishing it could rain - I hear you. They wanted me to stay in Wisconsin through the winter. I told them I wanted to spend some time with the wife and family; and to finish some plans I've had for a while. Glad I told them no. I can't handle -55˚ F wind chill. Expecting a daytime high close to 40 degrees. (not bragging)