transistor array, more power wanted

[Julia Goolia]

hi!

i am trying to pump more power through a transistor array
(8-darlington high-current(500 mA) driver array, part no:ULN2803A) for
a step motor. i don't understand why more current will not go
through... i am plugging in a 12V, 200mA power supply (from a
transformer plugged directly into the wall) to the COMMON power supply
of the IC. then I turn on the inputs with a basic stamp, but very
little power gos through the outputs.

Is it better to ground the COMMON and plug the load wires into the +
of the power source? How can I get more power through a transistor?

Thanks!
juliabean

 

[John Popelish]

Julia Goolia wrote:

hi!

i am trying to pump more power through a transistor array
(8-darlington high-current(500 mA) driver array, part no:ULN2803A) for
a step motor. i don't understand why more current will not go
through... i am plugging in a 12V, 200mA power supply (from a
transformer plugged directly into the wall) to the COMMON power supply
of the IC. then I turn on the inputs with a basic stamp, but very
little power gos through the outputs.

Is it better to ground the COMMON and plug the load wires into the +
of the power source? How can I get more power through a transistor?

Thanks!
juliabean
Have you read the data sheet?

http://www.controlord.fr/CONTRIB/uln2803.pdf

The positive side of the supply should connect to all the loads and to
the diode common (com). The other side of each load should connect to
one output. The negative side of the supply connects to the chip's
ground which also has to be connected to the logic ground of the
signal source driving the inputs.


--
John Popelish

 

[John Popelish]

Julia Goolia wrote:

hi!

i am trying to pump more power through a transistor array
(8-darlington high-current(500 mA) driver array, part no:ULN2803A) for
a step motor. i don't understand why more current will not go
through... i am plugging in a 12V, 200mA power supply (from a
transformer plugged directly into the wall) to the COMMON power supply
of the IC. then I turn on the inputs with a basic stamp, but very
little power gos through the outputs.

Is it better to ground the COMMON and plug the load wires into the +
of the power source? How can I get more power through a transistor?

Thanks!
juliabean

One other detail: If you are driving center tapped step motor
windings, you connect the center taps to the positive supply (possibly
through current limiting resistors) and the diode common to a zener
diode cathode. The zener diode anode connects to the positive supply
rail. The zener should have a breakdown voltage about equal to the
positive supply voltage. This is needed, because when one driver
pulls one end of a center tapped winding to ground, the other end of
the winding pops up to twice the supply voltage, like a see saw. It
wastes power to clamp the swing lower than that. But there is an
additional positive voltage spike generated by any end when it is
first turned off, and you want only that clamped by the internal
diode, the zener and the supply all in series.

--
John Popelish

 

[Julia Goolia]

John,

Thank you so much for your reply! Sure enough, I had it totally
backwards... so now, thanks to you, I have a working step motor!
Still, the amount of power that I get is small. It doesn't work with
a 9V battery. It only works with my 12V 200mA transformer that I plug
directly into the wall. The output is only about 4V and about 100mA
on each pulse. But at least the motor turns!

thanx again!
juliabean


John Popelish <jpopelish@rica.net> wrote in message news:<401ED138.D00A5720@rica.net>...

Julia Goolia wrote:

hi!

i am trying to pump more power through a transistor array
(8-darlington high-current(500 mA) driver array, part no:ULN2803A) for
a step motor. i don't understand why more current will not go
through... i am plugging in a 12V, 200mA power supply (from a
transformer plugged directly into the wall) to the COMMON power supply
of the IC. then I turn on the inputs with a basic stamp, but very
little power gos through the outputs.

Is it better to ground the COMMON and plug the load wires into the +
of the power source? How can I get more power through a transistor?

Thanks!
juliabean
Have you read the data sheet?
http://www.controlord.fr/CONTRIB/uln2803.pdf

The positive side of the supply should connect to all the loads and to
the diode common (com). The other side of each load should connect to
one output. The negative side of the supply connects to the chip's
ground which also has to be connected to the logic ground of the
signal source driving the inputs.