I need power
kenwtn
Posts: 250
Hi All,
·· I am stil trying to get 3 stepping motor I have to work. The motors are fairly small but require 1.2 amps per coil I have seen others on YOU TUBE and they do not seem to have a huge power supply. I have alot of wall plug in power supplies but none put out that many Amps and I have several RC 7.5 Volt batteriers but again I do not see that they put out that kind of Amps. Does any one have suggestions of something I might have laying around that could work. I do have power supplies from some desktop computers but that seems like over kill.
4 Volts are needed
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When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !
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·· I am stil trying to get 3 stepping motor I have to work. The motors are fairly small but require 1.2 amps per coil I have seen others on YOU TUBE and they do not seem to have a huge power supply. I have alot of wall plug in power supplies but none put out that many Amps and I have several RC 7.5 Volt batteriers but again I do not see that they put out that kind of Amps. Does any one have suggestions of something I might have laying around that could work. I do have power supplies from some desktop computers but that seems like over kill.
4 Volts are needed
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !
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Comments
·· I am trying to built a small CNC Mill. I attached a stepper to 2 UNL2803 today and connected it using 3 inputs to 3 outputs for each phase, but smoked·both ULN2803's
·· I can only get the motor to jitter
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When you spend 10 times what it cost to build it yourself - That's when it becomes a HOBBY !
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Post Edited (kenwtn) : 8/17/2008 9:27:04 PM GMT
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Chris Savage
Parallax Tech Support
Your digital control is designed from the top down, and your power requirements are designed from the bottom up. Start with how much power you minimally need [noparse][[/noparse] 4 volts x 1.3 = 5.2 Watts per stepper ].
A ULN2803 at 0.5 watts maximum was never intended to provide enough power for 1.3amp coils. Thus you burn it up by trying to suck too many amps through it.
You have to add at least one other stage of amplification between it and the steppers -- a MOSfet that is rated at 3 amps or more might be a good choice [noparse][[/noparse]usually we add extra protection because motors get stalled and suck even more amps]. Or you might just replace the ULN2803 stage with individual MOSfets that can respond to direct digital signals. There are special MOSfets that do so.
And of course, you have to have an adequate power supply in you 'bottom up' design approach. At 5.2 Watts per stepper you need 15.6 Watts or more. 15.6 Watts / 5 volts = 3.12 amps. Lets say you need a 4 amp supply.
You can still have the ULN2803s drive the MOSfets as a way to protect the microprocessor. And you can actually provide a higher voltage output through using the ULN2803 so that you can use both cheaper, more common MOSfets and higher voltage stepper motors [noparse][[/noparse]usually required for more power].
Do your stepper motors require 4 volts maximum? Or do they require 4 volts minimum? This is going to decide exactly what components are best to use. If they require 4 volts maximum, you are better off just using MOSfets that are directly driven or driven by 74LS06 as protection for the microprocessor.
I hope that all makes sense to you.
The 'smoke' always comes when you try to drive a powerful watt-sucking device with a teenie-tiny solid state current source. The jitter is just a death rattle from the ULN2803.
Rather than starting with E=IxR [noparse][[/noparse]the top down approach], start with P=ExI [noparse][[/noparse]the bottom up approach].
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It's sunny and warm here. It is always sunny and warm here.... (unless a typhoon blows through).
Tropically, G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
Post Edited (Kramer) : 8/19/2008 8:23:36 AM GMT