Horay for the Basic Stamp it did not blow up!

metron9

Playing around with the darlington ULN2803 and an old floppy stepper running at 5 volts The stepper is rated as 12 so I decided to put 12 volts to it. Hmmm I said thats funny the green light glows brighter on the stamp when I hook this up. Yikes I forgot to disconnect the +5 rail to the darlington when I pushed 12 volts back at it. It shut down, I felt a bead of sweat on my forehead but then unplugged the battery and replugged it in and all seems well. Whew

Now I know steppers should run at at least 2x the rated voltage and you can use resistors to reduce the current.

I measure 85 ohms of resistance between the common and one of the coils (170 per coil) (5 wire stepper MSJE200A53 No. 1Y21)
If I double the voltage to 24 volts should I use 85 ohms power resistor in series
or
can you use PWM at a 50% duty cycle and create a chopper drive but perhaps the darlington is not fast enough switching?

Thought I should ask before I make smoke

Bean

Metron,
If the stepper has a rated current, then what you want to do is connect your supply and adjust the resistor value to get that amount of current. The "chopper" circuit is much more effecient, but more complecated because it is actually a current regulator (you need feedback from the circuit).

The reason for all this is that the coil of the stepper has a large inductance. And inductance resists change in voltage. So when you first apply power to the coil the current is low (so too the magnetic force), then increases as the coil saturates. The resistor is a crude attempt at current regulation that can waste many times the power supplied to the motor (but sometimes that's not a problem). The chopper is basically "full-on" initially then starts chopping the power as the coil saturates (and the current increases).

Bean.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
"SX-Video·Module" Now available from Parallax for only $28.95

http://www.parallax.com/detail.asp?product_id=30012
Product web site: www.sxvm.com
Available now... SX-Video OSD module $59.95 www.sxvm.com

"I'm a man, but I can change, if I have to, I guess"
Red Green
·

metron9

Thanks bean, your reply has quite a bit of information that summerises most all of the salient points. It'a juat the feedback I need as I explore individual components. Kind of like learning how to program in various languages, each has it's strong points and some can do what others can't. So I am really all over the map with questions about so many various topics, but I have my notebook handy as I gather various bits of information about each particular component. That way when I have a task/project I wish to make, I can draw on that knowledge base. I shall spend some time looking at currrent limiting circuits that have feedback loops. I see the speed of the stepper I am using tops out at about three to four hundred steps per second. I understand that is a function of not enough voltage getting to the coil to saturate the coil thus the stepper just vibrates at higher frequencies. Of course I am wondering now about the high limit of voltage on stepper motors. For example in theoritical terms, would it be possible to use 1000 volts and a very fast chopper drive to run a stepper rated at 12 volts? Exploring the boundrys of the physics involved in just a coil of wire can be fascinating. For example harmonics at higher frequencies would come into play at some point. Some people simply want to know how to hook something up, I just like to explore the limits, after all it is in breaking those limits that we make discoveries.