Hi,

Several years ago I was trying to figure out a way to allow a motor to
be controlled by a computer interface (to allow it to go both forward
and backward by just applying a voltage on one of two lines). I
designed a device that could (theoretically) do it using transistors.
I later learned what the name of the device I had designed was.

Last night I finally managed to build an H-Bridge. I had previously
tried it, but there were problems (mostly materials wise) that
prevented it.

I made it last night and hooked it up to a DMM to test. I used a 9V
battery as the external power source and gave it a run. It worked
great, the DMM read about -9 when using one lead, and +9 when using the
other. The problem is that it wasn't ±9V but rather ±9mV. The
polarity was correct, but the output was about 1/1000 of the input.
This makes it useless since I don't have a 9,000V power supply to drive
the motor.

I'm guessing that the transistors I used are the problem since the rest
of the device is just wires; that they are somehow also acting as
resistors.


Here's the schematic I made of the H-Bridge I built:
http://img233.imageshack.us/img233/8966/hbridgeua9.gif

I'm using regular wires from various computer parts (the kind that
connect buttons and LEDs on the front panel to the motherboard), and I
got the transistors from the circuit board of a television. They are
marked "C815 Y312" (I checked them with the diode testing function of
my DMM, and they they all tested tested fine-and yes, I've got the
pins correct.)


Like I said, the polarity works, but the output is far too low. Any
ideas on what's going on and how to fix it?

Thanks a lot.

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

'm guessing that the transistors I used are the problem since the rest of
the device is just wires;CHA CHING!!!!!!!!!!!

So what's the problem and how do I fix it?



(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.)

Alec S. wrote:

On Oct 9, 3:40 pm, Meat Plow wrote:

On Mon, 09 Oct 2006 12:03:33 -0700, Alec S. Has Frothed:


'm guessing that the transistors I used are the problem since the rest of
the device is just wires;CHA CHING!!!!!!!!!!!


So what's the problem and how do I fix it?





You'll want to read up on basic transistor theory, that should clear
things up a bit. Essentially, bipolar transistors are current-driven,
that is in the case of the NPN type you show you need to have a voltage
on the base which is positive in respect to the emitter with the current
limited by a resistor to turn the transistor on. Often these days
MOSFETs are used instead for applications like this which are voltage
driven and a little simpler to work with. If you look up H-bridge there
should be lots of examples out there you can simply copy, there's even
single IC's which incorporate an entire bridge for you.

"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:

'm guessing that the transistors I used are the problem since the rest of
the device is just wires;CHA CHING!!!!!!!!!!!

So what's the problem and how do I fix it?

(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.

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.

How much current does your motor require?

What are you using for a power supply?

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James Sweet wrote:
Alec S. wrote:
You'll want to read up on basic transistor theory, that should clear
things up a bit. Essentially, bipolar transistors are current-driven,
that is in the case of the NPN type you show you need to have a voltage
on the base which is positive in respect to the emitter with the current
limited by a resistor to turn the transistor on. Often these days
MOSFETs are used instead for applications like this which are voltage
driven and a little simpler to work with. If you look up H-bridge there
should be lots of examples out there you can simply copy, there's even
single IC's which incorporate an entire bridge for you.

Actually, I need more advanced transistor theory, I've already got the
basics: a switching device that controls electric flow. What I need is
more information on the different types, the internals (what's causing
the drain), etc.

I'm aware of all the things you mentioned, but it won't help because I
don't have the money or resources to buy anything. I'm using
components that I'm scavenging off of old electronic equipment.

I'm currently looking around for some MOSFETs that I can salvage.

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.

Actually, I need more advanced transistor theory, I've already got the
basics: a switching device that controls electric flow. What I need is
more information on the different types, the internals (what's causing
the drain), etc.


That's all part of what I'd consider basic transistor theory, there's
lots of information out there.


I'm aware of all the things you mentioned, but it won't help because I
don't have the money or resources to buy anything. I'm using
components that I'm scavenging off of old electronic equipment.

I'm currently looking around for some MOSFETs that I can salvage.



You know some companies will send you free engineering samples, Analog
Devices is good about this, as is TI, National Semiconductor and
Dallas/Maxim, just try not to abuse the system and request only what you
will actually use. Companies that are pleasant to deal with and liberal
with the samples I tend to recommend frequently to engineers at work,
what goes around comes around.

If you want to salvage parts, old CRT computer monitors usually have a
decent supply of power semiconductors in them.

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


You need the resistors, the base to emitter drop is only about 0.8V, any
higher potential sees a more or less direct short to the emitter and
you'll toast the transistors or overload whatever is driving them. Look
for examples of similar circuits online if you don't feel like
calculating the correct values.





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


We're talking about the drive voltage to the base, it HAS to be higher
than the potential at the emitter or the transistor won't turn on.



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



Measure it, any $20 multimeter will measure DC amps.

"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.

Alec S. wrote:

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

Ohhhh. A tr b/e junction is just like a diode, so your lower tr will
short the drive line down to 0.6v above the 0v line. With that voltage
on the upper tr base, theres no way it will conduct. The upper tr needs
to see 0.6v above its own emitter, which will be near the + rail.
You've got to put base resistors in.

I dont know what you've got driving that rail, but running trs with no
resistors may well have killed them.

The other question is one of current and hfe. And also tr ratings!
Ratings first. Toy motors might eat over an amp while they start up,
and a litttle plastic jellybean tr will not be amused if asked to
deliver that. In fact it'll likely die in a matter of milliseconds.
Really I'd use 3A rated trs as a minimum.

Next question is hfe, which is transistor current gain. Since you dont
know the motor ratings, we can at least make a vague guess to see if
your circuit is in the right ballpark. Lets say the motor is 6v 0.5A.
During startup its liable to eat maybe 10x that, so 3A. I dont know the
hfe of whichever trs youre using, but power types (3A) might have hfe
of anything from 2 to 50 typically. Letts again make a vaguess guess
and say yours are hfe = 20. So to pass 3A C-E you need base current of
3/20 = 150mA.

More guesses... lets say youre dropping a whole 1v across the base
resistor supplying that upper tr. 500 gnomes on 1v (V=IR) gives 2mA. So
your tr drive current would be inadequate by a factor of 75 or so.

To tidy this up, you'd need to show us what circuit youve got driving
those tr drive lines, and also give us hfe figures for your chosen
power transistors. Just google the tr number and 'data'


A universal power adapter with polarity and voltage control.

what are its ratings/settings.

Once you've got this one working we can explain why youd do better with
2x pnps in there, but later. Which reminds me, your trs will need
heastinks, esp the upper ones, to survive. Ie random lumps of metal
bolted on.

Oh, the biggest problem with H bridges is timing. Trs dont switch off
instantly, so if you switch one line from 0 to 1 and the other from 1
to 0 at the same time, you can get a dead short that fries stuff. You
need to always have a little dead time, where both trs are off, before
switching one on. (There is a bodge way round that if its just for
play.)

Re getting parts, scrap will give you enough bits to build projects for
years. One thing you do need to troubleshoot is a multimeter. If youre
seriously not got even $5 for one, make one from scrap, find a tape
deck with moving needle meters and make one. I have on occasoin done
repair jobs without one, but its far more difficult, and you need what
information you can get in order to understand the circuits and figure
out how to get them working.


NT

I just noticed 10x0.5 is not exactly 3, but its near enough in this
case