Current consumption of AXE023 board (08 + L293D)

J

JimB

New Member
Hi!

This is my first post after discovering Picaxe a few weeks ago...all I can say is that this stuff is very addictive :) Reading about the projects on here is just amazing, and I love the way the Picaxe creators have made it cheap and accessible to anyone.

So the inevitable questions from a rookie:

I'm playing with an AXE023 motor driver board and a bipolar stepper motor from a scrapped Epson printer. I've got the thing turning backwards and forwards nicely which feels like a major achievement!

The problem is that the current consumption of the motor driver board is about 40-50ma when nothing's happening i.e. looping for input. I also have a 14M project board to experiment with, and the current usage is tiny when all is quiet. I was hoping to run this motor board off batteries for opening and closing a blind (i.e. only operate the motor twice a day) but I guess the batteries would be run down pretty quickly with that constant drain?

Is it the L293D drawing the current when all the outputs are off? Is that because the "enable" pins are hard-wired and not controlled by the Picaxe? (not that there are any outputs spare on the 08 :( ).

How about using another 08M as a controller for the input which then enables the motor driver board? Could this board be driven from the output of a ULN2003A?

Sorry, lot's of questions all at once...thanks for any help...

Cheers,
Jim
 
E

eclectic

Moderator
Jim. Some quick ideas.

"How about using another 08M as a controller for the input which then enables the motor driver board? Could this board be driven from the output of a ULN2003A?"

Depends on the total current of the motor.
It could be possible with parallel inputs and outputs.
(see for example Manual 3 page 6)

Or, a relay, controlled by an 08M, to act as an On/Off switch?
(Manual 3 page 7).
The "controller's" current could be reduced to microamps, when it's resting.

I'm sure you'll get lots more suggestions.

e
 
I

inglewoodpete

Senior Member
Jim, you will find that when the power is connected to your circuit, the armature of the stepper motor is much more difficult to turn than with no power. This option is often used to 'hold' a stepper in a particular position. Of course, the trade off is that a significant current is drawn when the motor is stationary and this is dissipated as heat.

The ULN2003 is a simple(!) darlington array, so you have the option of changing your software to turn off all outputs when the stepper motor is stationary. You may want to introduce a short delay below entering this state, to ensure the the motor is stationary first.
 
J

JimB

New Member
Thanks for the replies so far...

The stepper motor isn't difficult to turn when the code is waiting for input - I'm setting the 4 output pins of the 08 to low at the end of the stepper code, and that seems to stop current going to the pair of motor coils from the L293D.

The board as a whole consumes a lot more power than 40-50mA when the motor is turning : ~300mA.

I've looked at the data sheet for the L293D but that doesn't give me any clues as to where the power is going during the quiet periods...and unfortunately it's soldered into the AXE023 board so not easy to troubleshoot.

As a workaround I'm going to try controlling the power to the AXE023 board from another Picaxe 08M with a relay. Bit of a shame to be over complicating things, but that's life I guess :)

Cheers,
Jim
 
S

slurp

Senior Member
also note the "quiescent current" drops significantly when the "enable" pins taken to a low voltage. I'd not needed this but I'll remember it for another time! :)

regards,
colin
 
E

eclectic

Moderator
I've just done a quick continuity test on an AXE023.

Pins 1, 8, 9 and 16 are hard-wired to V+.
(Similar to the circuit on page 12 of Manual 3)

Both enable pins are high, leading to higher quiescent current.
(As mentioned by Jim in post#1)
e
 
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J

JimB

New Member
Thank you all...that explains what I'm seeing.

Just goes to show that the data sheet is only as good as the person reading it! :)

Cheers,
Jim
 
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