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Stepper Motor

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netbug

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Hi,

I am using an unipolar stepper from AstroSyn model SST0017. The specs of this stepper are 12 V / 0.4 A / 30 ohm. The holding torque is 2.4 Kg-cm and rotor inertia 27 g-cm2. This stepper is connected to a ULN2803, and it works fine. The only problem is that it gets very hot, as well as 7812 that is connected to it. Can I use some power resistors in series with the stepper to limit current ? Will this degrade torque ? Do you know another way to solve this ? This circuit will be closed in a wood box, so I don't want it to burn !!

Another drawback I found on this stepper, is that it is too much expensive for the project I am finishing. I would like to know, if anyone knows another brand of steppers, with similar specs but with a lower price ?

Best regards,

Pedro
 
If you want to reduce the temperature - add heatsinks and fans. Sealing anything in a wood box will cause it to overheat at some point. Now if you want to reduce the power going in, you can reduce the current going into the system by:

1) reducing the current/torque - (current and torque are proportional).
2) PWM the thing so that it isn't drawing so much power when it is idle.
3) using a better switch than the ULN2803 - this chip will have a ~1.5V drop when on, which means that the chip will be dissipating close to 2Watts and will probably need a heatsink.
4) !don't put the motor on a regulated power supply! What is the actual transformer voltage? The regulator is going to dissipate 0.8A * (voltage drop) which could easily be a few watts. Hook the motor up directly to the unregulated power supply and use a current limiting resistor to keep the current at .4A.

Dunno about Europe, but there are a lot of surplus steppers in that torque range for ~US$15. There are also a lot of import (ie made in China) ones as well that you can get from companies that sell motion control equipment.

James
 
Hi HJames,

Thank you for your reply. The idea of this project is to make a WatchWinder, so the circuit will be closed in a box. The stepper will rotate a wrist watch, to make it charged. I want to use a low consuming stepper, but otherwise a stepper that can't be stopped easily, that is if you apply some pressure with your fingers the stepper shouldn't stop.

The other goal of the project is to make a cheap watchwinder, so I must use a cheap driver like ULN2803, and I am looking for a cheap stepper that fits my project. Any suggestion for stepper specs that fits this project ?

Best regards,

Pedro Cardoso
 
This is for watches that are "charged" by being shaken?

I assume you don't need this thing to spin over a couple dozen RPM...

Don't even bother with a stepper. I'd suggest just getting a normal DC brushed motor with a worm-gear or other gearbox. Throw in a limit switch or optical sensor to determine how many rotations it's gone through. Is this a production product?
 
Hi,

Thanks again. This is for production. At beginning I thought it would be easier to count turns with a stepper, rather using a DC motor. Besides the stepper is very accurate and I can program it to run 1000, 1200 or 1500 turns per day. With a DC motor, speed increases with voltage, and the only way to count the turns will be, like you said, using a simple encoder. But will be this approach better to a product that intends to be produced ? Will this be cheaper ?

Well I think with a stepper I won't need a gearbox, because I can run it very very slowly. The main thing is that I need a simple and cheap solution, taking into account motor + driving IC.

Best regards,

Pedro Cardoso
 
Well, put it this way: where do you see stepper motors in (current) mass produced products? Printers .. and that's about it - steppers require more silicon to control, are more power hungry, don't inherently move smoothly - they will vibrate and make noise, and are inherently more expensive.

A DC motor that spins in one direction only needs 1 transistor to drive, and 1 sensor to sense velocity/position. Put the two in a feedback loop, and you can have a constant speed motor. As for actual pricing, I don't know your local suppliers, so I can't really comment...
 
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