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.

 

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

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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.

 

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 ..

 

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.

 

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.