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The circuit I am using is attached.  I know the circuit works because it will move the motor forward and reverse on 9V.

When I apply 9V DC to it, things work as they should.  I can use my transmitter to forward and reverse the motor.

When I apply 12V DC from a lawn and garden tractor battery, the TIP 127s begin to smoke.  I have already burned one out.

After replacing the burned TIP 127 again it works on 9V but again apply 12V DC and more smoke.

Any suggestions what might be the problem?  I really need to control this 12V motor (the motor is a DC brush type, drawing approx 120mA without a load)

Thank you

Can you use a multimeter to

Can you use a multimeter to verify the outputs you are getting from the servo controller? Are Output1 and Output2 from the h-bridge ever high at the same time? If so, you will turn on a TIP127 and TIP120 on the same side of the h-bridge. This will essentially ground the 12V battery across those two transistors, which would cause a short and LOTS of current.

The 9V battery is low current, so even if you are grounding it, you won’t get too much current. With the 12V you have enough to smoke your transisors.

Whether this is your problem or not, your causing too much current to flow through your transistor. There’s no other reason for it to burn like that.

h-bridge test results

The first thing I did was re-connect the servo motor via clip leads and set the pot so the servo motor neither ran in either direction.

Then putting my meter neg lead to ground, I checked the outputs of the servo board.

Having the plus lead connected to output 1, I turned on the receiver switch.  My meter read .08V.  When I turned off the receiver switch, my meter spiked up to 1V and gradually dropped to 0 in approx. 12 sec.

Following the same procedure, when I turned on the receiver,  output 2 would spike up to 4.6V and gradually drop to .09V in approx 40 sec.  When I turned the receiver switch off it would spike up to .77V and gradually drop to 0 in approx. 12 sec.

Then with the receiver switch still on, I put the plus lead to out put 1 and turned on the transmitter.  Output 1 would go to 4.44V and when moving the trigger forward would go to 4.53V.  Moving the trigger to reverse output 1 would go to .02V

Following the same procedure, but going to output 2.  My meter would read 4.43. Moving the trigger forward would go to .02V and reverse would be 4.51V.


Testing the ouptut of the H-Bridge:

With the transmitter and receiver off each output read .02V

On output 1, turning on the receiver, my meter would spike to .47V or some different amounts up to 1.2V (it was different all the time) then it would drop to .02V in approx. 2 sec.

On output 2, turning on the receiver, my meter would spike to 9.4 and drop to .02 in approx. 2 sec.

With the receiver on, then turning on the transmitter, output 1 would  seem to go to 1.2V, then moving the trigger forward it would go to 7.5V.  Moving the trigger to reverse it would go to .47V

For output 2 would seem to go to .86V, then moving the trigger forward it would go to .47V and reverse would read 7.71V

Since I have limited electronics experience, I am not sure what this all means, but my objective is to use my remote control unit to control a 12V motor forward and reverse.

Thank you.

Thanks for the detailed

Thanks for the detailed testing steps. That is helpful.

BTW, you should always turn on your transmitter BEFORE your receiver. Otherwise your receiver my interpret weak signals and interference as valid commands and cause wierd behavior. This may be why your outputs varied a bit when you were taking measurements with the receiver on but the transmitter off.

It sounds like with the transmitter on, and without touchnig the trigger, both output1 and output2 of the servo board are a logical high. This turns on BOTH sides of the h-bridge at once, causing the short I wrote about. The TIP127 and TIP120 on each side of the h-bridge are all on at once. This doesn’t turn your motors, but it does allow a ton of current to flow down each side of the h-bridge.

As I mentioned, the 9V battery can’t supply enough current to blow out your transistors, but the 12V can. This is why your h-bridge “works” with 9V, but  not with 12V.

You need to correct this situation before moving forward. Both sides of the h-bridge should not be on at the same time. I’ll try to investigate with my own setup later this week and see what I am seeing.


I found a similar H-Bridge at www.bobblick.com/techref/projects/hbridge/hbridge.html

He gave a recommendation of adding a 1K resistor to the base of each TIP for higher frequency applications.

It requires 2 additional pn2222s and 8 additional resistors, but it works on my 12V lawn & tractor battery with no smoke and the TIPs are cool.  I removed the additional diodes, since he said they were built into the darlington transistor.

Now, I’m happy.

**h bridge **

How about you use opto-isolators to replace the transistors (pn2222A) in your schematic. They really simplify a lot of things when working with H-bridges as well as keeping your control electronics and H-bridge totally separate. Here is a schematic I found that can handle DC voltages between 3 and 24 volts, and motors that when stalled do not consume more than 5 or 6 amps of current. If you decide to us it, you can connect your PN2222A transistors to the reverse and forward lines. One on the top left and bottom right corner (forward) and top right and bottom left corner (reverse), this applies only as the schematic is facing you know with configured current flow. If you wish to force a stop on the motor then you will need the ENA*line. http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html . the shematic is under building the H bridge. i oppologize for this but it appears i cant attach any file to my text.  From my understanding, you need to place a PNP and NPN at opposite corners. One as source and one as sink. The pn2222A has to be PNP and NPN and I would say that you need 4. Not 2. The tip127 is most probably burned out because in the schematic both your pn2222A transistors are NPN. So when turning on the PNP from a NPN you internally short out the tip127 that is PNP. And I would say your schematic is double grounded and the tip2222A is not grounded properly but through your power supply. This could cause some unexpected behavior by your hardware.  I am no expert with electronics and my own knowledge is rather limited, so i am only speculating. I hope that this might help you in some way. I would advise reading the entire booklet that has the schematic I mentioned.


h bridge

Thanks T 88


It does look like a good  h bridge

the cheapest solution for

the cheapest solution for me is to use Transistors or Relays.