A Simple Motor Control Circuit FWD RVS OFF

thatoneguy

Joined Feb 19, 2009
6,359
No. You are close, but look at the color colored lines in the one I did with the H-bridge drivers (so they could all be N-Channel).

Keep looking at the one I drew, and dump the MOSFET Drivers and MOSFETs. Make the top two transistors PNP, and the bottom Two NPN, then it would work. Note that the diagonals always get an equal signal, while those horizontally directly across from each other or in one leg do not get the same signal (Green and Pink are the complimentary inputs).

The engaged Transitors need to be diagonal from each other. So the top half of A through motor and out bottom half of B, OR, top half of B, through motor and out bottom half of A

Your first version would have worked, with the PNP transistors at the top, if you didn't tie the bases of the transistors together, see Wookies' explanation. This version won't work since PNP won't work as a low side driver unless you can get -0.6V on the base, which 0 isn't. For the NPN to conduct, you need to get 0.6V above the emitter, which is only possible if the PNP is turned on.

Does that make sense? Diagonals conduct for each direction, if one side or the other side is on, then it is straight to ground on that side.

Could be how I think, but each Transistor in H-Bridge has own control to prevent shoot through and to allow brake or coast.
 
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T.Jackson

Joined Nov 22, 2011
328
No, you're wrong friend.

What I have done on both sides is called a complementary symmetry pair. BJT audio amplifiers use the exact same schema.


*Complementary symmetry output
 

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thatoneguy

Joined Feb 19, 2009
6,359
I agree that push-pull works or "totem pole" or complimentary outputs on audio (linear) amplifiers, but not as switches all options on an H-Bridge. Since Brake and coast aren't possible.

--ETA: I will grant that it will work, given your truth table of disallowed states.

Hows that work?
 
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thatoneguy

Joined Feb 19, 2009
6,359
It would be ok. It would work.

I absolutely guarantee it.

The first would have worked, yes, if the bases were not tied together.
We agree on this, as long as both 0 and both 1 are disallowed.

--ETA: For the rest of the circuit, the SCR seemed the logical solution to start, For reversal, flip flop seems obvious to me, as it is complimentary outputs. Move the "end" switch to be in parallel with the start switch, on the V+ line to the anode of the SCR so power is removed when it is bumped. A DPDT "stop/start" switch. We do not know the voltage/current of motor other than 6-9V, so it's been 12V low current CMOS so far.
 
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thatoneguy

Joined Feb 19, 2009
6,359
Now how do we fulfil the requirements with only 6 more cents allowed in parts?
See --ETA above your post.

Not sure where 6 cents came from, we are on lower cost than counter at this point, as only the dual half H-Bridge has been decided on for motor.

The T-FF could be realized as a 2 transistor monostable multivibrator.
 

T.Jackson

Joined Nov 22, 2011
328
Could use a cheap 12F PIC to do the rest.

Or ...

  • Discrete TTL / CMOS ICs
  • More BJTs
Only one flip-flop is required, and it doesn't need to be elegant, so this could be done with more BJTs.
 
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thatoneguy

Joined Feb 19, 2009
6,359
That would be my preferred method, but OP doesn't have PICKit 2

May as well finsih this as we started. SCR has a 5mA holding current, so the LED will keep it on during motor stops and reversals.


Just need the reversal switch to toggle a transistor flip flip/multi(single, actually) vibrator, and the design is essentially finished.

Provided there is no need to stop the motor in a hurry. :p
 

T.Jackson

Joined Nov 22, 2011
328
That would be my preferred method, but OP doesn't have PICKit 2

May as well finsih this as we started. SCR has a 5mA holding current, so the LED will keep it on during motor stops and reversals.


Just need the reversal switch to toggle a transistor flip flip/multi(single, actually) vibrator, and the design is essentially finished.

Provided there is no need to stop the motor in a hurry. :p
OP would have a heart attack at the price and the complexity of it.

Keep it simple. ;)
 

thatoneguy

Joined Feb 19, 2009
6,359
add 2 transistors, 2 low value resistors, and a momentary switch, that's all the parts needed, connecting them is pretty straightforward for anybody that's made a dual LED flasher from NPN transistors, the multivibrator will provide only complimentary outputs, so it would work with your dual half bridge fine.

At this point, relays seem simpler. :D
 
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