Subject:
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Re: when they say "fresh" batteries, they mean it!
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 13 Aug 1999 16:22:27 GMT
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Viewed:
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600 times
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In lugnet.robotics, Alex Wetmore writes:
> From: John R. Galloway, Jr. <jrg@dbengines.com>
> > I was having problem getting any comm working between the
> > IR tower (on my Mac) and the RCX. I was using unused batteries,
> > but the 9 volt in the IR tower was old enough that it was only
> > providing 8.25 volts with no load. Replacing it with a brand
> > new one did the trick. Seems like a piss poor design to be that
> > sensitive to battery power. But at least now its working. A
> > USB design that took power from the USB seems like a better
> > plan.
>
> I believe RS232 is in the 9v range (or maybe even 12v), so they are probably
> fighting that spec there. They could use a voltage up-convertor, but at a
> loss of efficiency. Two 9vs might have made sense.
>
> USB really wasn't an option a year ago. It has only recently begun to take
> off. NT still doesn't support it.
>
> They could have powered off of the keyboard port or some hack like that, but
> it would have been ugly.
Actually it is common practice these days to power small peripherals off of
the supply voltage pins on RS-232 ports as long as you're not trying to pull
tooooo much juice. And even a device with a 9-volt power supply can produce
higher voltages using step-up transformers or other nifty circuits. Not to
mention that even if RS-232 expected a 9-volt signal, anything above say, 4.5
volts should still do the trick. (Engineering tolerances being what they are
-- you've got to expect that some day, somebody will try to drive an 11-ton
truck over a 10-ton bridge.)
But that's all water under the bridge, since the IR tower is what it is.
The important thing to remember is that, when a battery shows less-than-spec.
voltage at its terminals when it is only connected to a voltmeter, it is
pretty much dead. Voltmeters are designed to draw very minimal current,
otherwise the load would compromise the voltage reading. And batteries
provide very flat voltage output until the juice is almost completely drained.
So if you're only measuring 8.75 volts, it's probably time to replace the
battery.
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Message has 2 Replies:
 | | Re: when they say "fresh" batteries, they mean it!
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| (...) Yep. The measured voltage is the idle voltage. The normal approximation to a battery is an ideal voltage source at 9V and a Thevenin resisance of some ohms, say 7 ohms, in series. This will give a stall current of about 1,25A at a terminal (...) (25 years ago, 13-Aug-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: when they say "fresh" batteries, they mean it!
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| From: John R. Galloway, Jr. <jrg@dbengines.com> (...) I believe RS232 is in the 9v range (or maybe even 12v), so they are probably fighting that spec there. They could use a voltage up-convertor, but at a loss of efficiency. Two 9vs might have made (...) (25 years ago, 13-Aug-99, to lugnet.robotics)
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