Subject:
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Re: reverse ploarity circut protection??
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 14 Jul 2006 14:43:16 GMT
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Viewed:
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2810 times
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In lugnet.robotics, Ross Crawford wrote:
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In lugnet.robotics, Ondrew Hartigan wrote:
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hey electronic wizards,
i am working on a new lego project to rid the new IR train electronics from
the bulky train base and i’m looking to build a circut protector for it.
since i am planning on using standard lego conections 2x2x 2/3rds this is
very important. if anyone has a solution i would greatly appreciate it.
thanks, ondrew
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A bridge rectifier would work, very simple, but I’m not sure it’s the best
solution.
ROSCO
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Yes, though the things to think about are the voltage drop across the diodes or
other devices and the amount of power required.
If only low power devices were being driven, such as the IR chip on its own, a
simple bridge rectifier made from four 1N4148 diodes would be best, since their
voltage drop is low (as little as 0.2V), giving 0.4V drop in total. Similar
diodes (in surface-mount packages) are used in RCX and NXT sensors, rectifying
9V at up to 15mA.
However, if the protection circuit has to drive motors (possibly 2 train motors
and 2 technic motors), requiring up to 1 Amp, this would be too much for
1N4148s. The standard alternative of 1N4001s or a one-piece bridge rectifier
would have a voltage drop of 0.6-1V per device, a significant loss of power and
speed from the driven motors.
A power bridge rectifier could be made with four power transistors, shorting the
base of each one to its collector in order to keep them turned on. This could
achieve a voltage drop as low as 0.2V per device, 0.4V total. However, the
maximum base-emitter reverse breakdown voltage must be considered. If this is
5V then additional protection is required in a 9V system.
The sort of circuit required for more power is a controlled bridge rectifier.
I’ll have a look in my books. I think there’s a circuit in one of them that has
transistors in this configuration with resistors crossing the base connections
over somehow.
Mark
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