# More power through a transistor

Discussion in 'Electronic Basics' started by Julia Goolia, Feb 4, 2004.

1. ### Julia GooliaGuest

Hi,

I'm trying to pump more current through a transistor. Can anyone tell
me the relationship between (in an NPN transistor):

1. The voltage between the base and emitter VS. current through the
collector to the emitter

2. The current between the base and emitter VS. current through the
collector to the emitter

Basically, I want more voltage and current so I may power a motor.
Does anyone know where I could find a graph of these relationships?

Thanks,
juliabean

2. ### John PopelishGuest

First you need to determine if it is the transistor that is limiting
the current or the load (the motor winding, in this case). When the
transistor is in the on state, measure the voltage drop between the
collector and the emitter. If this voltage is only small fraction of
the total supply voltage, then there is little that the transistor can
do to increase the current, since most of the supply voltage is
already across the load. However, if a significant part of the supply
voltage appears across the transistor, then it is not fully on, and
more base current is needed to put more voltage across the load.

Did you read my post about center tapped motor coils and the use of a
zener diode?

3. ### CFoley1064Guest

From: John Popelish
the power zener with the anode (side with the line) connected to pin 16, and
the cathode connected to your motor power supply. Remember, the inductive kick
of the motor will cause current to go through that pin backwards.

From the several posts over the past few days, it seems you're indexing a
stepper with a BASIC stamp, using a ULN2003 as the driver. Possibly you could

* Stepper motor -- How about telling the manufacturer, model number, voltage
Steppers come in all flavors. You could be driving a 12V or 24V stepper motor
with 5V for all we know.

* Power supply -- Are you using the Stamp on-board 5V, or using a separate
power supply for the motor? If you're using the Stamp +5V output, there's no
chance your setup will work, no matter how small the stepper. That little
SOT23 regulator on the Stamp will chuckle in disbelief at the prospect of
powering even the smallest motor -- it's good for something like 40 mA max.
Also, what's your external power supply voltage and current rating? If you
overload a current limited power supply, the voltage (and your motor power)
will obviously decrease.

* How fast is the motor going? Steppers have a phenomenon known as mid-range
resonance, where the mechanical resonance of the motor interferes with the
progression of the steps, leading to a dramatic reduction of torque at certain
velocities (usually around 1 to 2 rev/sec). Also, have you determined the
rotational inertia and mass of your load? These are things that are covered in
any elementary physics textbook. You have to make sure your motor is capable
of doing the work of accelerating and decelerating the load before you start.
a different speed (especially slower speed)?

These don't sound like trolling posts -- you sound like you have a problem, and
are legitimately interested in getting a solution. Waiting -- can't really

Good luck
Chris

4. ### John PopelishGuest

(snip)

Whoops. The cathode end has the band.

5. ### CFoley1064Guest

From: John Popelish
Whoops -- how did I say that? A senior moment? Thanks!

Chris