H-Bridge with Braking

[fantasymick]

I have an H-Bridge driven with PWM from a 555 and some simple AND and NOT gates configuration to control 2 npn's and pnp's (TIP120,142). But now I want to add braking to my circuit because it turns out the DC motor for a robot I'm designing doesn't stop where it's suppose to be after power is turned off (it would roll a bit).

I don't want to buy a new IC for this purpose so I'd like some suggestion in how to desgin some logic (possibly using OR,AND,NOT) so that I can have braking control. I'd have three logic inputs, for ON/OFF, FORWARD/REV, and BRAKE ENABLE/DISABLE.

I've checked H-Bridge driver IC's with braking datasheets but none have the actual logic used (they all have some big block that says "logic" or something).

Thanks.

 

[TheOne]

If you switch both top or bottom devices on it will effectively brake the motor by shorting the windings.

 

[fantasymick]

That I know. I wanted to see if people can help me out with the logic part of the circuit that allows me to perform the operations using the inputs mentioned above.

Thanks.

 

[williB]

It seems to me that a few pulses of very short duration in the opposite direction , from the one it was going , should stop it from rolling a bit..
it sounds like a software solution ..

 

[Tom_Braider]

Long before the electronic circuits, people used to 'brake' motors by a heavy power resister across the DC terminals. Depending on the motor size, you'd probably want to add a power capacitor in to handle surge and arcing.

The theory is that most perm-mag DC motors can also generate current. (Just as pre-alternator car generators could serve as motors with a DC source across them.) If the motor has to produce current, that takes 'work' (in the physics sense) and therefore sets up a field current that rapidly brakes the armature.

 

[Oznog]

It seems to me that a few pulses of very short duration in the opposite direction , from the one it was going , should stop it from rolling a bit..
it sounds like a software solution ..

That would create high current surges on the power supply. It would be cleaner to just short out the motor.

I don't think turning on both high transistors or both low will work in a bipolar circuit. MOSFET, yes, but the bipolars won't conduct current in the other direction. You'd need a pair of bipolar braking transistors or better yet a single MOSFET.

fantasymick, your question is strange. You say you're controlling a robot with PWM but don't mention a microcontroller, which would manage these signals. The basic H-bridge has 4 inputs at the hardware level, which is where you must implement the brake concept. The "ON/OFF, FORWARD/REVERSE" you mention has to be going through some sort of controller logic. Unless we know what this is (schematics) and why you need to use it, I don't see how we could tell you to modify that logic to accomodate a brake.