I have a 12v brushed DC gear motor that I would like to have stop quickly after the mosfet shuts the power off. The motor has a rated current of 1/3 amp. I tried putting a diode and 2 ohm resistor across the motor terminals as shown on the schematic. The motor didn't seem to stop any quicker. I thought shorting across the terminals like this would produce a resistive braking. Am I doing this wrong?

 

It can be problem of switch.

 

It can be problem of switch.

What kind of problem that it could be?

 

Take out the 2 ohm and the diode short it with zero ohms ... you can do no damage . you will have to have a switch which disconnects the battery on breaking .

 

Read this thread as it covers the same topic.

Les.

 

If shorting the leads of a spinning DC motor does not stop the motor in near zero time then the motor is not capable of dynamic braking. A DC motor without permanent magnets will not dynamically brake. In essence, the DC motor must become a generator. Shorting the leads of a generator will stall the generator. Especially if it's just free-spinning.

My chop saw has dynamic braking. Let go of the trigger and the blade comes to a complete stop within less than 1 second. But if I pull the plug while spinning and do NOT let go of the trigger the motor can spin for several seconds. Maybe even 10's of seconds.

 

The motor didn't seem to stop any quicker. I thought shorting across the terminals like this would produce a resistive braking. Am I doing this wrong?

You can soon check the braking capability by just shorting the leads together and spin at various speeds to see the braking effect.
Even try to spin by hand the effect can be seen right away, if the motor has a strong field.
Max.

 

The link in post #5 in the Youtube video there were quite helpful. I will try adding a p-channel mosfet to disconnect the power supply when the motor stops. It sounds like that will solve my problem. Thanks!

 

I used a relay that shorts the windings. The control circuits for power door locks and motors do the same thing, With the FWD or REV relays off, the motor is shorted. A 12 V automotive relay is good for 30-40 amps.

 

You don't say how big the motor is and what the load is on it. Gear motors also don't stop as fast as a motor without gears, the inertial of the gear train still turns the motor, especially if there is a load attached, it back drives through the gearing.

 

Also if worm and pinion gearing, normally impossible to back feed.
Max.

 

It can be problem of switch.

It is definitely a problem with the switch. Sort circuiting the terminals of a spinning motor will usually cause instant hard braking. Several of my power tools have that feature. Also my electric lawnmower, which has a series motor. But it does put a hard load on the motor. Dynamic braking burned out my previous electric mower, so now I just pull the plug and let it coast. Only a REAL FOOL would stick their hand into a spinning mower. I would remove the switch but it is a shape that I can't find with a normal off/on function.