Subject:
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Re: High current speed controllers
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Mon, 8 Dec 1997 18:47:54 GMT
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Original-From:
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Mike Vande Weghe <vandeweg@parlance-ncs!stopspammers!.com>
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Viewed:
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769 times
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As for your question about controlling a motor with a PWM signal versus
an analog level controlled by a power transistor, the issue is
efficiency and power dissipation. If you use a power transistor to set
the voltage at the motor, it will have to dissipate the remaining
power. For instance, you said your motor was 12V and could draw up to
25A. If you want to drive it at half-voltage, 6V, with a 12V supply,
the power transistor will have the other 6V across it (Vce). The same
current will flow through both, so if the motor gets stalled you will
have up to 12A going through the transistor. That will produce 72W (6V
* 12A) of heat, which is a tremendous amount of heat. The transistor
will burn up in no time.
Power transistors are almost never run in their linear region, but
instead are turned either full-on (saturated) or full-off. When
saturated, the transistor is at full current, but with almost no voltage
across the device (0.4V). When off, it has the full voltage across the
device, but zero current. In each case, the power being dissipated
across the transistor is either zero or nearly zero. By quickly
switching on and off for varying amounts of time, the average power can
be controlled, but in a way that is not nearly as lossy. That's why PWM
control schemes are so popular with any sort of linear control of high
power devices, such as motors, automotive engine components, and
lighting.
- Mike
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Message is in Reply To:
 | | High current speed controllers
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| Hello, I was wondering if anyone could recommend a speed controller circuit for controlling a 12VDC motor which could draw up to 25A. I'd like to use the Handy Board to control its speed. Cost is not a big concern but compactness is. I would (...) (27 years ago, 6-Dec-97, to lugnet.robotics.handyboard)
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