I'm hoping someone out there can help me figure out what is wrong with my driver circuit. I'm an undergraduate student, and my electrical engineering skills apparently suck. =/
The chip is the Allegro 3967. It's a chopper driver. A simple google search yields a schematic and parts list that I've attempted to model on a breadboard. Before I get into the details of the mess I've made with the circuit design, let me say that the problem I'm experiencing - and hence the numerous hours spent tweaking things - is the motor output seems to draw less than .06 Amps.
The motor I have is rated at 4.6V and .350A current per phase. Testing the leads shows the motor windings have a resistance of 13ohms, which seems to be fine mathematically. Applying 4.6V straight to the motor draws .350A, so everything seems fine with the motor electrically.
The circuit, on the other hand, does not do what I want it to do.
I'm using a power supply in my school's lab, I've set it to 10V and I am using a 5V regulator to supply power to a 555 timer (to drive the steps) and the logic inputs for the 3967 chip. The 555 happily sends about 4.5V pulses to the step input, and with the step motor disconnected I see the voltage of the motor output lines from the chip fluctuating as if they are attempting to step the motor.
I have a 10K potentiometer in series with a 30K resistor to adjust the reference voltage levels. I've tried slowly sweeping the potentiometer across its whole range in conjunction with .47 ohm, or a 10 ohm, or a 560 ohm resistor with little effect on current draw, but I get different voltage levels on the motor output lines.
When I use a .5ohm 5W resistor for the sense resistor my voltmeter indicates the voltage is about .04 volts. It fluctuates a little bit, but not much. If I put on a 560ohm resistor the voltmeter reads 3.4V and varies from 0 to 3.4 as the chip recieves step inputs. As soon as I connect the motors in any resistor configuration the voltage no longer fluctuates, but the motors don't do anything particularly impressive. With no power applied the motors free spin easily, with the power applied its a little sticky to spin. The power supply has a digital output for the voltage and amps drawn, and indicates that no matter what I do it is drawing .06Amps. Even when the motors are not connected.
Can anyone suggest why my configuration is drawing such a tiny amount of current?
Something to mention is the chip is a surface mount chip which I've brutally soldered tinned leads to and set in a 24pin DIP socket, which is then plugged into the breadboard. I've tested all the pins from the chip to the breadboard and there are no shorts or breaks. The resistances seem quite low. And despite the Allegro documentation suggesting not to use a socket adapter and to keep the sense resistor as close to the chip as possible, I really didn't have any other way to prototype an SMT chip. The ohmmeter can't really measure .5ohm resistances particualarly well. It says the .47 ohm resistor is 1.4 ohm when I directly test the resistor, and when I test the resistance from the chip's pin through the socket and across the other side of the .47 it still says 1.4ohm. I don't know how accurate this measurement is.
One thought I had was that soldering the leads onto the chip may have damaged the chip. But testing all the logic inputs produce the correct effect. For example, enabling the sleep mode results in the current draw to read 0 amps on the power supply. Disabling the 'enable' pin results in no voltage to the motor output. Step inputs change the motor output. etc. The chip seems to work fine. Except it couldn't power a nano-scale motor if it's life depended on it.
I appreciate anyone's input as to what is wrong, and suggestions on what I should do instead.
Thanks in advance,
Michael Kurth