"Alec S." wrote in message
ups.com...
Sam Goldwasser wrote:
"Alec S." writes:

On Oct 9, 3:40 pm, Meat Plow wrote:
On Mon, 09 Oct 2006 12:03:33 -0700, Alec S. Has Frothed:

(Oh and I made the drawing earlier, and it's not quite accurate-the
ground transistors are backwards. The one I used while hooking it up
(because it was clearer) is this
one:http://www.markallen.com/teaching/im...cs/hbridge.gif
but without the resistors.)

I started to reply to your original post and then read this one.

The circuit using the TIP120s is fine and your other transistors should
work as well. The 510 ohm resistors may not allow enough current into
the bases of the transistors to run the motor.

Actually, I just used that schematic for the layout, I'm using the
transistors I described in the original post and no resistors.


And as someone else noted, the base drive voltage has to be higher than
the emitter voltage on all the transistors by enough to provide sufficient
current through the bases of both the top and bottom transistors.

Well, I need 9V out but I'm getting only 1/1000 of what's going in.


How much current does your motor require?

Don't have a clue. It's a toy motor that runs on batteries.


What are you using for a power supply?

A universal power adapter with polarity and voltage control.



The first thing that comes to mind in a situation such as this (a novice
constructing his first project) is that the circuit is not wired correctly.
Actually, I don't think that you understand all the 'basic' transistor theory
that you think you do. As a starter, you should understand that a transistor's
base should never be driven with a hard voltage source. Doing that could result
in destruction of the transistor(s). If you don't have the money to buy small,
low-cost items such as resistors and transistors, maybe you should find another
hobby. I have salvaged parts from scrapped equipment for many years, too, but
getting the right parts at the time you need them is haphazard at best.
Perhaps to satisfy the group members that you truly understand the theory of the
circuit, why don't you explain to us how you think this circuit should work?
Include current flow and voltages that should be present at all the transistor
nodes. Include the input voltages and voltages at each of the motor terminals.

Then we'll see how your thinking might be in error, or what else could be the
problem.

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.