h-bridge will not stop motor

Hi James There are people on the forum that will tell us for sure but it looks to me like you are at least missing resistors between the collectors of Q-6 and Q-7 to ground. I do not know what values will be needed but this will at least turn the Q-3 and Q-4 off when Q6 and Q7 turn off. Also note your flywheel diodes need to be tied to the wires going to the motor. If this circuit is required in your design it is ok but a better way, particularly if you are controlling with an arduino, is to use a dedicated H-Bridge chip like a SN 754410. If you need more power for your motor you can use the chip to drive mosfets. Jeremy Blum in his new book "Exploring Arduino" covers this subject very nicely in chapter 4.

John

John is quite right.

You need some resistors on the base of Q1, 2, 3 and 4 to stop any leakage current from holding them on.

A 1K or 10K should enough.

Q1, Q2 should go to +12v, while Q3, Q4 should go to ground.

BUT

The bigger problem with this circuit though is the current flows from +ve down via the emitter of Q1, and out the base, then via R1 and R2 (both 100Ohm) then into the base of Q3 and out the emitter, thereby turning it ON.

The same on the other side with Q2 and Q4.

The transistors you've chosen are fine, but you cannot have a voltage greater than the supply (5v) on any IC pin .... hence the voltage at the base of Q1, Q2 or Q6,7 is going to be 12v -0.6 = 11.4.

Apart from using a HBridge IC, you could add an optocoupler

(sorry element14 seemed to have some issues saving the image.)

I have not added the base resistors, and Dx and Dy represent the two Arduino outputs you wish to use.

You should set the outputs either both HIGH or both LOW during setup.

This will act as a brake because effectively the motor will be shorted.

Mark

edit

I have just thought of another method, using 2 x NPN transistors to control the TIP125.

Please excuse the crude drawing .. its very late.

Refer to your drawing.....

Basically when Dx goes HIGH it makes Q3 and the extra NPN conduct. The extra NPN then makes Q2 conduct via the 100 ohm to ground.

The same thing for Dy which makes Q4 and Q1 conduct using the extra NPN.

The danger with this is you can make smoke if you end up with both Dx and Dy HIGH at the same time.

In the opto coupler version this cannot happen.

Very nice suggestion, I too would prefer option one due to the safety factor of not making magic smoke, even if your software is perfect, during transition times and also on power up there can be significant surges while the system stabilizes or as the transistors are changing state. This is minimized in the first option

Again using discrete components is a great learning experience but all the work is done if you use an HBRIDGE control chip, you can get these at a very low cost from EBAY as ready built modules ready to attached to an Arduino. so if your intention is to learn software and not hardware, I would suggest the module

if you want the whole experience, at least for this time around, and it will teach you allot then keep on trucking and good luck, looks to be a fun project.

Oh, one last thing, you should still include diodes across each transistor to protect against fly-back voltages from the motor, I'm sure mcb1 was trying to keep the circuit focused on the transistor driving and I know he knows this.

Thanks for your help.

I had to connect Vcc (12v) to the emitter of the tip125, then the collector down to the motor.  I did this to get the tip125 to act how it should.  Is that typical for that type of transistor, or could that be what you are talking about causing the tip 120 s to stay on?

Thanks for your help.

I had to connect Vcc (12v) to the emitter of the tip125, then the collector down to the motor.  I did this to get the tip125 to act how it should.  Is that typical for that type of transistor, or could that be what you are talking about causing the tip 120 s to stay on?