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Author Topic: Is this right? Trying to use Ohm's law  (Read 1595 times)
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Hyperspace
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I have a stepper motor I want to limit current too. Voltage is 24 volts and I want the current limited at 1 amp. if R=V/I then I need a 24 Ohm resistor, right?
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No, that is not how you do the math.  You need to take into account the voltage drop of the motor and its resistance.

However, the better question is, why are you trying to limit the current?
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Hyperspace
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Well, because I just fried a brand new SN754410 H-Bridge chip. I have a 24 volt stepper motor hooked to an Adafruit motor shield. I'm giving it 24 volts from a 'remodeled' atx power supply. The Y axis did not fry, it worked okay, the X axis snap crackle popped almost immediately. As you can see from my original question, I have no idea how to use Ohm's law. I really need to know this and this is really the first time I've had a pressing reason to learn. I could just ask what resistor to use, but I need to learn how to do this. This is like the third time I've asked this and people always tell me, but without a real world project to try it on I don't get it. This is my chance to learn it with a real world project to make it stick.

Please, help me get this.
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Worst case, assuming you had an ideal motor with 0 resistance and ideal switches with 0 resistance when turned on, then yes, 24V/1Amp = 24 ohm.
Real world, the motor will have finite resistance, the H-Bridge chip will have some voltage drop across the high side transistor and the low side transistor.
So a 24 ohm resistor would result in less than 1 amp of current flow.

There's also a limit on how much power the part can handle.

http://www.ti.com/lit/ds/symlink/sn754410.pdf
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Current is load dependent. All you need to take care of is the Voltage.
If you need to understand this more clearly. Consider the Voltage to be a water tank. Your load is another empty tank. The pipe connected between them is your current. Now your question is, "How to limit the flow of water in the pipe". Why would you want to do that?. Basically, this is not how it works. The flow of the water in the pipe depends on the empty tank. On its capacity, on where it is placed (Theoretically). It also depends on the water tank. the pressure given by it.
So like I said, Current is  Load dependent.
 What you should take into account is the voltage required to drive the motors. Read the specs of the IC before you use it. I think you will find your answer there.
(In the Electrical world. There is no other Law which is as important as Ohms law. Learn how to use it.)
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With respect, you are failing to observe what happened.  One side of your H-bridge failed "almost immediately"  I would suggest you have a major wiring or component problem that needs addressing before you go any further.
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Hyperspace
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I guess I was fooled because at 12v it ran for a long time, or several minutes, with no problem. When I tried 24 is when it burned. So, what am I supposed to think?
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You need to know the resistance of the stepper motor windings to calculate the ballast resistor value.  You also need to calculate the power dissipation in the the ballast resistor and choose the right rating (hint it will be high).

If you have a "24V stepper motor" and its already taking more than 1A, then its got to be a large beast indeed - are you sure its a 24V motor?
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Two of these http://www.dz863.com/pinout-811088463-M49SP-2K

When I ran them 12 volt, even though it ran the IC got hot as hell. I don't know how I could have the wiring wrong. Its four wires that go in to the motor shield.
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The quoted device is a printer stepper motor designed to be driven by pulses of current.   Judging by its size (and manufacturers use recommendations) I'd suggest it is designed to be either turning under stepped power or stopped and de-energised.  Are you by any chance leaving the power applied to each coil all the time, even when not stepping.  This being the case your coils will pull around 4 amps so demanding some 100watts from the h-bridge drivers.  If so then it's little wonder things went "pop".
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Hyperspace
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As I said, I'm using an Adafruit motorshield.
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Hyperspace
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Here is exactly how I have it hooked up. I do have the Ext power jumper removed.


* motors.png (30.79 KB, 675x497 - viewed 39 times.)
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Rapa Nui
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..you have to change your approach.. instead of guessing what happened you have to measure - EE is about measurements (as we cannot see the electricity), otherwise you will just smell the smoke all the time.. smiley
Take a voltage source 1-20Volt, 4 amperes, add 1ohm/1W resistors into each motor wire, take an oscilloscope, connect to the resistors and observe what is up there.. When everything looks ok - ie. voltages, currents, phases, durations, periods - than start to increase the voltage - and so on..
« Last Edit: August 12, 2012, 03:13:10 pm by pito » Logged

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I'm trying to learn how to do that. I've heard this advice before but I don't know how to actually do it. I'm getting discouraged again. I just don't know how to do what you guys say I have to do.
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..so at least try to compare, go step by step - minimal steps, record what ypu have measured, observed: connect the motor which worked well to the channel where it popped (after you replace the chip)  and do compare whether that axis works well. Change the axis channel (single motor only) and observe. Go with 4-6Volts only and measure at least the total current (with single motor, ammeter into adafruit's power rail). Learn how it works, observe the current, temperature of the chips, speed, torque, rotation direction, noise, etc. All done with one single motor at lower voltage. Then start the same experiments with the second motor on both channels (do disconnect the first motor), mind the difference if any (temperature, current, speed, direction, noise). Do not connect both motors yet. Do not jump directly to full throttle smiley
PS: Buy more spare chips smiley.
« Last Edit: August 12, 2012, 04:09:02 pm by pito » Logged

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