I am attempting to build a transistor h-bridge to replace the DPDT relay
I'm using at the moment to switch the direction of a motor. (this is for
a project run by a 16f84).

I've seen this circuit (see schematic below) in several places.
Switching on a pair of transistors (transistor 1 and 4) should feed the
load ("M"). Switching off the first pair and switching on the second
pair (2 and 3) should do the same with reverse polarity.

This seems straightforward, but it simply won't work for me. At first I
was using lightweight transistors and I was concerned the current for
the motor toasted them, so I rebuilt using TIP122s. I've tried replacing
the motor with various other loads, including a lightbulb, leds, and a
huge 10 watt resistor. Still no luck.

What happens is... nothing. The motor won't turn, lights won't light,
etc.

According to the power supply meter there seems to be an awful lot of
current going somewhere, but not through the load (enough so that I
don't want to leave it on long enough to take readings).

However, if I put a meter across the motor connections I get a very
slight reading that is consistent with what I expect to see (the
polarity switches when I switch base pairs). This reading is really tiny
and, again, the load doesn't seem to be receiving any power.

I'm probably missing something very basic but it's been a few days and I
can't figure this out... any suggestions?

Beautiful Ascii schematic
(without the current limiting resistors for the base):

M)otor
C)ollector
B)ase
E)mitter

       +5v
 C1-----|------C2
B1              B2
 E1-|       |- E2
    |---M---|
 C3-|       |- C4
B3              B4
 E3-----|------E4
        |
       gnd

---------
Andrew Sempere
andrew@evolve.net