# Making a controllable H-bridge

UPDATE I found out the issue. Specifically, the PNP transistor I got. I used a BC557 - for some reason its collector-emitter current is very very low. It does open when the base-emitter junction goes right. But there's practically no current going through it. I tested it for all 20 of them, even unused ones. Same issue. Is this a problem with my transistor choice or a bad batch?

simulate this circuit – Schematic created using CircuitLab

This is a circuit i'm using to make a controllable H-Bridge.

Since the diagram's a little messy, I'll explain a bit. SW1 (which is actually a bunch of transistors, but it's on a separate circuit so I've simplified it to a switch) should change which direction the motor goes in.

If we consider the "up" position only, Q1 and Q2 should switch on due to the higher potential - Q2 should connect Q3's base to the positive rail, and Q1 should connect Q4's base to ground. This will allow current to flow from the positive rail through the PNP Q4 into the motor's "OUT" terminal, run the motor, then flow out into Q3 and towards ground. Mirror process for the opposite side.

The problem is that this doesn't work. There's a minute voltage across the motor (~0.02V). The batteries are definitely running since voltage across them drops to about 2.6V. The only other evidence of it running is that Q3 heats up a lot after a while. I think other transistors might be, but I'm afraid I only noticed Q3 doing so.

Personally I think it's something to do with some transistors not switching on, or some transistors switching on when they shouldn't. Any ideas?

• No base resistors? – Passerby Jul 20 '13 at 22:14
• Oh that's a good point, forgot to put those in (the actual circuit, not just the schematic). But would that make the difference between working and not working? – Alex Freeman Jul 20 '13 at 22:26
• Totally. You might have fried a transistor or two as is. Depending on the motor and current it uses, you might need feedback diodes as well. And pull-down resistors after the switch, because otherwise the inputs to q1/q2 or q5/q6 are left floating, instead of solidly pulled down. – Passerby Jul 20 '13 at 22:34
• Your schematic is "upside down" with the power rail at the bottom and a floating ground icon. – pjc50 Jul 22 '13 at 8:36
• Yeah that was my bad. Still, the schematic works. I found out the issue btw. Specifically, the PNP transistor I got. I used a BC557 - for some reason its collector-emitter current is very very low. It does open when the base-emitter junction goes right. But there's practically no current going through it. I tested it for all 20 of them, even unused ones. Same issue. Is this a problem with my transistor choice or a bad batch? – Alex Freeman Jul 22 '13 at 16:20

1. L293, L293D; QUADRUPLE HALF-H DRIVERS, Accessed 2014-06-19, <http://www.ti.com/lit/ds/symlink/l293d.pdf>