Andrew Sempere wrote:
>
> 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


Keep in mind that the TIP122's are darlingtons with internal resistors
from base to emitter on each transistor of the darlington. They are 5k
for the input transistor and 200 ohm for the output transistor. We may
need to know what your base resistors values are because of this. The
resistor should allow at least 10ma to flow into the base.

Second, you do not need (or want) a base resistor for transistors 1 and
2 since this is an emitter follower configuration and the resistor will
decrease the drive by dropping voltage from any base current flowing
through it.

Third. Since the transistors are darlingtons, they will drop a lot of
voltage when on.

For transistors 3 and 4 the on voltage is about equal to the base to
emitter drop of the input transistor plus the saturation voltage for the
output transistor. This will be about 0.8 volts minimum increasing with
the load current.

For transistors 1 and 2 the on voltage will be equal to the sum of the
base to emitter drops of the input and output transistors of the
darlington. This will be about 1.3 volts minimum.

If you add things up you will find that the motor will see a maximum of
about 5 volts minus 0.8 volts minus 1.3 volts which is only about 2.9
volts. This voltage decreases with heavier loads.

Do you really intend to deliver 10 watts to the motor? If so the voltage
will be a lot lower. Maybe less than a volt.


-- Rich