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H-Bridge motor driver not working properly

BlueCerealBox

Sep 30, 2014
27
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Sep 30, 2014
Messages
27
Hi all,

I'm trying to make this long post a little easier to read, and this is the only way I know how. So I apologize if this looks too much like a school report.

What I'm trying to do:

As part of my final year college project, I am trying to build a driver circuit to drive my magnetorquer (Think of it as a solenoid within a metal coil) , the magnetorquer is : http://www.cubespace.co.za/cubetorquer . I intend to supply bi-directional current to it using a H-bridge motor driver, the motor driver chip that I am using is : http://www.mouser.com/ds/2/427/sip2100-519775.pdf
I will be controlling the input pins (InA and InB) using PWM signals from my arduino. As a test, I am just using a simple arduino program that varies the duty cycle of the pwm output in Pin 5, and sets Pin 6 to digital low. The connection of my circuit is the attached image. Note that the batteries would represent my DC Supply(I am using 5V to power my chip) , and the inductor represents my magnetorquer. Also, assume that the bottom left pin of the IC is pin 1.

What went wrong:

I have observed that as I am drawing an unusually low amount of current, compared to what I should get theoretically(V=IR , resistance of the magnetorquer is shown in the link above ). When the duty cycle of my PWM signal is set to 50%, the chip only draws 0.030A of current, in that case the resistor of my magnetorquer would be R=V/I = 2.5/0.03 = 83.33Ω! Which is almost thrice of what is stated in the link above!

I am using an oscilloscope to measure the output voltage across my magnetorquer, when I set my duty cycle to 50%, what I get on the oscilloscope is a square wave that has max and min voltages of 2.5V and -2.5V respectively. If Pin 5(OUTB) of my motor driver is always 0, I should expect to see my output voltage vary from 0-2.5V right?

Also, when I set the duty cycle to 100%, I should be expecting to see 5V on my oscilloscope, but what I see is a very small DC voltage ( about 100mV ) which is effectively 0.

What I've tried:

I tried connected the negative end of my magnetorquer to ground while keeping the positive end at Pin 8 (OUTA) of my motor driver chip, but nothing changed.

I have tried shorting Pin 5(OUTB) of my motor driver chip to ground, didn't work either.

I have also tried connecting a the second channel of my DC power supply ( Set to 2.5V ) to pins 6 and 7 (InB and InA ) of my motor driver chip, when I do this the current jumps up to about 0.150A and doesn't increase when I increase the supply voltage of the second channel of my DC power supply.

I'm currently really out of options, at this point I can really only assume that my motor driver chip is broken. Is there something that I am not doing right?

Thanks and sorry for the long post.
 

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Alec_t

Jul 7, 2015
3,587
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I think you have neglected the inductive reactance of your coil. That will present an impedance dependent on frequency. What is your PWM frequency?
Does your coil have a DC resistance of 30Ω and an inductance of 0.15H, as per the link?
I have tried shorting Pin 5(OUTB) of my motor driver chip to ground
:eek: Why? That may have fried the chip, or if you're lucky the chip's internal protection will have saved it.
doesn't increase when I increase the supply voltage of the second channel
Why did you think it would? Pins 6 and 7 are logic inputs.
max and min voltages of 2.5V and -2.5V respectively
Is your power supply output connected to ground? If so, your 'scope negative terminal (which is grounded) can't be connected to pin 5 or 8.
 

BlueCerealBox

Sep 30, 2014
27
Joined
Sep 30, 2014
Messages
27
I think you have neglected the inductive reactance of your coil. That will present an impedance dependent on frequency. What is your PWM frequency?
Does your coil have a DC resistance of 30Ω and an inductance of 0.15H, as per the link?

:eek: Why? That may have fried the chip, or if you're lucky the chip's internal protection will have saved it.

Why did you think it would? Pins 6 and 7 are logic inputs.

Is your power supply output connected to ground? If so, your 'scope negative terminal (which is grounded) can't be connected to pin 5 or 8.

When I measured my PWM frequency, it was around 800Hz, I can't really remember. Also, I tried using the second channel of the supply voltage as I thought maybe the chip accepted only analog voltages, but your explaination clarified that.

Finally, yes the negative terminal of my power supply is connected to ground, why can't the negative terminal of my scope by connected to pin 5 or 8? Should it be connected to ground then, as even when I tried that, I got the same results.
 

BlueCerealBox

Sep 30, 2014
27
Joined
Sep 30, 2014
Messages
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Because the scope negative is grounded you would be shorting pin 5 or 8 to ground. The chip won't like that.

So if I wanted to measure the voltage across the magnetorquer, where should i connect the negative probe to?
 

Arouse1973

Adam
Dec 18, 2013
5,178
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Dec 18, 2013
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5,178
You use two probes and use the math function of the scope to measure the difference between the two points.
Hope this helps
Adam
 

BlueCerealBox

Sep 30, 2014
27
Joined
Sep 30, 2014
Messages
27
You use two probes and use the math function of the scope to measure the difference between the two points.
Hope this helps
Adam

Didn't know you could do that, I thought the math function was only used for Fast Fourier Transform. So if the oscilloscope is not set to the math function, it just measurements the value of a node in the circuit with reference to ground?
 
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