H-Bridge Only In One Direction

Hi,

I'm currently making an H-Bridge to control the direction of this motor. I'm using the schematic for an H-bridge found on this site:
http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html

Instead of using the transistors used on their schematic, I'm using 2n3904/2n3905 BJT transistors and 1N5818 Shottkey diodes. Everything else is identical.

When applying a 12VDC voltage and giving putting the enable signal low, forward signal high and reverse signal low, the motor turns clockwise as expected. However when enable is low, forward is low and reverse is high, the motor doesn't turn and I noticed that the top left and bottom left transistors heat up. I'm assuming that somehow theres a short circuit happening in my H-bridge when reverse is high through the two left side transistors, but I don't know why. I've double and triple checked my circuit and it all seems fine.

Any ideas would be great! I'm using an Arduino Fio to control the signals. Could this be because of a faulty opto-isolator?

Sounds to me like the top left transistor in the schematic is always on, which works fine in forward, but when the bottom transistor fires on for reverse mode, it creates a short since the top transistor never turns off.

Recheck the wiring around the optoisolator and make sure the opto LED is connected correctly.

is your circuit on a breadboard , perf or pcb. do you have all the pullup and down resistors. i had to change my resistors abit. i kept poping my 2n2907s and ended up
making a bigger pcb for it. the first one the size of a dime.

You have a Opto-coupler backward. I bet you wired both sides the same it' not real clear in the drawing but there crossed.

I think this is a better set up

I'm working with a breadboard. I used your circuit be80be and I'm still having the same problem... I've checked my optoisolator wiring and it all looks good.
I'm using a PS2501-4 and my pins are wired like so:
1,3,5,7 are wired to the 470 ohm resistors. 1 and 7 go to fwd, 3 and 5 go to rev.
2,8 are wired straight to rev
4,6 are wired straight to fwd
10,12 are wired to 12V
13,15 are wired to ground
9 goes to bottom right base
11 to bottom left base
14 to top right base
16 to top left base

I'm stumped =(

That optocoupler LEDs are wired incorrectly.

Pins 6 and 8 should go to "Enable".

2 goes to "Reverse".

4 goes to "Forward".

Jon Wilder said:

That optocoupler LEDs are wired incorrectly.

Pins 6 and 8 should go to "Enable".

2 goes to "Reverse".

4 goes to "Forward".

Click to expand...

I initially had that. My wiring is for be80be's circuit suggestion where there is no enable. I wasn't going to use the enable anyway, just simply need the motor to turn in one direction, stop for a long period of time, then switch directions.

And are you using the correct transistors in the correct locations?

Hm... I just changed some transistors around without touching the circuit (I at least don't think I touched the circuit). The motor spins in either direction and works great! I notice though now the top right transistor gets hot on one of the directions still. I also only get about 8.5-8.7V to the motor, but I expect that's normal in an H-Bridge like this one?

EDIT: Ok so after a bit more testing, I found out that of the three PNP transistors that I have, one of them does not work in the H-Bridge at all, one works perfectly, and the last one works, but heats up a lot! Is this common with these transistors?

EDIT2: Changed the 2n3905 PNP transistors to 2n3906. I'm getting motor voltage to be about 12V now and no transistors are heating up. Thanks guys!

I'm not sure what the current draw of your motor is, but those transistors you're using are only rated for 200mA continuous collector current. You might wanna consider using the transistors that were originally spec'ed to be used with that H-bridge as they are a power darlington transistor and are capable of handling a bit more (8 amps continuous) if your motor draws more than 200mA.

Have a look at this baby **broken link removed**

I have used it more then once works fine in little mouse bots