H-Bridge Troubles

Let's Make Robots
1

I was able to remove the chip from the RC car receiver board, but I didn't have the dexterity and soldering skills to solder leads into the tiny board to use the ready made h-bridge.  My next step was to try and build my own h-bridge, but that isn't working out very well.

I tried to follow this design for an h-bridge.  First, I built the whole circuit and tested it out.  The motor didn't even hiccup.  Next, I tried testing one section at a time.  I started out with the PNP.  I am using a 2n4403.  Here is the Multisim schematic with simulation results for PNP circuit as well as a picture of the actual circuit on my breadboard.

multisim_pnp_switch_for_motor.jpg

PNP Schematic with Simulation Results

PNP_switch_real_circuit.jpg

Real PNP Circuit

Once again, the motor didn't move.  After pulling my hair out for a while and reading my electronics textbook I gave it a rest and moved onto the NPN.  I am using a 2n2222a.  Here is the Multisim schematic with simulation results for NPN circuit as well as a picture of the actual circuit on my breadboard.

multisim_npn_switch_for_motor.jpg

NPN Schematic with Simulation Results

NPN_switch_real_circuit.jpg

Real NPN Circuit

The single NPN transistor circuit worked great.  The motor was running like it was connected straight to the power source.  So, I decided to use only NPN transistors and see if that would work.  I made half the bridge to test it out.  Again, here is simulation and real.

multisim_npn_double_switch_for_motor.jpg

NPN Double Switch Schematic with Simulation Results

NPN_double_switch_real_circuit.jpg

NPN Double Switch Real Circuit

The NPN double switch didn't work.  I know I am doing something wrong.  I just don't know what it is.  I ordered this h-bridge in case I can't figure this out and also because it seems that it would take up much less space compared to the components I would be using to make my own.  I also have my eye on the kit that OddBot mentioned.  If anyone has some advice I would really appreciate it.

EDIT 9/22/2012: Well I got the half bridge to work using the design mentioned in the beggining.  Expanding to a full h-bridge now to see how that works.

EDIT 9/22/2012 again:  Yeah, the whole problem was my BJT's.  Some of them are broken.  Full h-bridge according to the design mentioned above is working now.  Next step, obstacle detection.

2

Looking at the schematic of the 754410,

it looks like you constructed your half h-bridge wrong. The 754410 and the L293 are similar ICs. They are both quad half h-bridge chips. A half h-bridge is the vertical part of the robotroom circuit you used originally. What you finally tried using is jus the top/bottom half of the h-bridge from robotroom. Realize, a half h-bridge will only turn a motor on and off. You will get no bi-directional control from said circuit.

3

Testing it out

Well, although the final goal is to get a working full h-bridge, right now I am trying to get my half h-bridge to work.  When I tried using a single PNP as a switch it didn’t work.  Any idea as to why this might be happening?

I was only able to get the NPN to work as a switch and that’s why I tried to use only NPN transistors for the half h-bridge, but that didn’t work either.

EDIT: My PNP was broken.  I must have burnt it out with the testing.  I felt it get really hot at one point.  Continuing my testing now.

EDIT 2: I got the half bridge to work…all this from a faulty BJT.  I should really make sure my components are working when every possible resource online says that the design should work.