Helpo with an electronic brake for a DC motor

Help with an electronic brake for a DC motor

I have a small 12VDC motor (250 mA max.) that I'd like to add an electronic brake to. I control the motor (on/off) from a micro driving a mosfet on the motor's low side (see Brake.jpg for schematic). To brake the motor, I'd like to short the motor's contacts (after turning it off first, of course), which I'm trying to do via a 2N2222. This has no effect, though I've confirmed that physically shorting the contacts immediately after killing the power to the motor has the desired effect. I've attached a shot of the traces from my scope; the points where the probes connect to the circuit are indicated on the schematic by color and number ("T1" is trace 1).

You can see that when the motor is switched off (T4 goes low), the brake switches on (T1 goes high) for 250 ms, but there's no change in the voltage across the motor contacts (T2 and T3). I assume the ramp on T3 after the motor is turned off is the dissipating inductance.

Any ideas what I'm doing wrong? I'm a newbie, so it's probably something dumb and obvious. Thanks for the help.

PS - Sorry if this is the wrong forum. It seemed like the best match, if not perfect. Feel free to point me to the right one.

HI, I must up-front admit I am not an expert in this area. But from what I can imagine, a DC motor would have some inertia even when you stop supplying power to it. Shorting its supply might not have much effect. On the other hand, after you switch off its supply, a negative voltage applied momentarily would provide active braking. The duration of the -ve pulse required would, I suspect, depend on its speed when stopped.

Regards,

Anand Dhuru

Anand:

There are two principles at work. One is, when the power is switched off, the collapse of the electromagnetic field induces a current, and therefore a voltage, in the coil of the motor. This manifests as a negative voltage on the contacts (i.e., the previously negative contact becomes more positive than the previously positive contact). In addition, any momentum in the rotor enhances this current in the coil as the motor briefly becomes a generator. Shorting the contacts allows this current to recirculate through the motor, but since it's going in the opposite direction from when the motor was being powered, it attempts to drive the motor in the opposite direction, bringing it to a quick stop. This is the principle on which regenerative braking in electric cars is based. It is also why all inductive loads (motors, solenoids, relays, etc.) are supposed to have a reversed-biased diode across their contacts; according to the first principle, the diode will prevent the potentially dangerous voltage (which can be several times the operating voltage) from entering the circuit and feed it harmlessly back into the load.

I see; thanks for the lucid explanation. Looking at your schematic, isnt 1k too large a reistance for this app? I would reduce it to 10 ohms, since the transistor is conducting only for 250 ms. Also, in this case, I think the resistor is best put in series with the collector, rather than the emitter. AND, when T1 is shorting the motor, T4 is off, right? In which case, T1 cannot conduct, since its emitter is floating! Try grounding the emitter of T1 instead of connecting it to motor -ve.

Regards,

Anand