Alt-BEAM Archive
Message #11953
To: beam@sgiblab.sgi.com
From: "Ben Hitchcock" beh01@uow.edu.au
Date: Thu, 16 Mar 2000 22:04:52 +1100
Subject: [alt-beam] Re: Current draw of DC Motors
Hi,
> All we really wanted was something that would show Current draw on a DC
> motor, that seems to be the only feature his current meter is lacking.
> The salesman was very willing to show us a range of meters from $20.00
> to well over $200.00 but once we got talking he told me a little way
> around the meter problem without any need to up grade the meter.
>
> He said to place a 10 ohm 10 watt power resistor between the plus
> termianl and the motor. Measure the voltage across the resistor and use
> Ohm's law to find the current.
>
> I = E/R
>
> Thus 1.21 volts across the resistor would be:
> I = 1.21 / 10
> I = .121 Amps or 121 mA
>
> Is this correct? Or am I way out to lunch here?
You're absolutely right. Incidentally this is how analog (and probably
digital, I haven't checked) multimeters measure current as well. Inside my
first meter (well my father's actually) was a loop of thick wire that went
from the Negative terminal to the Current In terminal. That was the
resistor - the wire! The multimeter then measured the voltage across the
wire, to figure out the current going through it.
One thing to be careful of, however, is that when you measure a voltage on
your resistor you are dropping the voltage available to the motor. So when
you connect your motor back up to where it was before, it will have more
voltage available to it so will pull more current. What I do is change the
resistor until I get a reasonable voltage drop across it (0.2 volts or so),
then use Ohms law to figure out the current. This won't affect your accuracy
that much.
I own a couple of digital multimeters that can measure current but I am
still forced to use this method with very low currents (such as standby
current of a circuit). I use a 1k resistor for this. Using a 1k resistor
has the advantage of volts = milliamps. So a 0.5 volt drop means that you
are sucking 0.5 milliamps through the resistor. For higher currents the
voltage drop becomes a problem and you have to use a lower resistor value.
hth,
Ben
11954 Thu, 16 Mar 2000 13:14:57 +0000 [alt-beam] BEAM fish? "Beam List (E-mail)" Ken Warren Check out the following link for the latest Japanese robot pets. Look
familiar don't they!!
http://news.bbc.co.uk/hi/english/sci/tech/newsid_679000/679642.stm
11955 Thu, 16 Mar 2000 14:30:34 +0000 [alt-beam] UK BEAMers "Beam List (E-mail)" Ken Warren Hello fellow UK BEAMers. This might be of possible interest to
you, so set you video recorder now!
Robocritters
Thu, 23 Mar, BBC2, 2130-2220
Philippa Forrester finds out about the new generation of robots
based on
animal designs, marrying the best design features of modern
technology with
those inspired by the natural world.
11956 Thu, 16 Mar 2000 15:01:10 GMT [alt-beam] beambotix update beam@sgiblab.sgi.com "Mike Kulesza" Im terribly sorry for letting you guys down for 8 straight days. I was away,
and had no access to the Internet whatsoever. But, once again, beambotix is
providing you with the latest BEAM and other Robotics headlines.
www.geocities.com/beambotix
______________________________________________________
11957 Thu, 16 Mar 2000 07:15:00 -0800 [alt-beam] Re: Rat Shack FLED "'beam@sgiblab.sgi.com'" Wilf Rigter The few Rat Shack FLEDs I tried worked fine. Some were cosmetic rejects with
off-center led chips but perfectly adequate for SE use. When used in an SE
like FRED, you have to be quite unlucky to "overload" them.
wilf
> -----Original Message-----
> From: Daniel Grace [SMTP:lifebytes_98@yahoo.com]
> Sent: Wednesday, March 15, 2000 4:20 PM
> To: beam@sgiblab.sgi.com
> Subject: Re: Rat Shack FLED
>
> I got a red FLED from there not long ago. If you
> overload them at all (voltage or current either one)
> they STOP responding. I couldn't get the
> characteristics to change in light either. Maybe it's
> just me. Anyone else?
>
> ~Daniel
>
> --- walkau wrote:
> > a question:
> >
> > are rat shack FLEDs reliable or should they be
> > avoided?
> >
> > thanks
> > niels
> >
>
> =====
> ICQ # 39402143
>
> __________________________________________________
> Do You Yahoo!?
> Talk to your friends online with Yahoo! Messenger.
> http://im.yahoo.com
11958 Thu, 16 Mar 2000 07:33:36 -0500 Re: Once more, a reminder for the Lego MotorsMini Korus
> I'm just wondering... I know you have to order these from the lego shop at
> home service, but how would I go about ordering them from Alberta, Canada?
> I.e., where can I get a catalog, order form, etc?
>
> thanks.
>
> ~Roger~
> TenFour
> http://korus.iwarp.com/
>
> >Perhaps one of the best uses for these tiny gearmotors is in the
> >consturction of tiny Stryder and Walkman type bots.
> >
> >dennison
> >
> >
> >
> >Hey, $11.00 for any usable gear motor is a good price. It's hard to find
> >new motors under $20, and that is still low. That price is almost like a
> >surplus price, except for a new item! I'd be very happy to pay $11 for
> >quality motors!
> >
> >Hopefuly these motors will become very useful for us!
> >
> >
> >BUDSCOTT@aol.com wrote:
> >>
> >> The U.S. part ain't a problem, and i can stand shipping but sheesh,
> $11.00
> >> that is a bit pricy, but i suppose thats the price you pay for a good
> bot!
> >Do
> >> they have any other models that are cheaper? Size doesn't matter too
much
> >to
> >> me, i just like to find some kind of servol or motor that is consistant
> >and
> >> "price efficient" (I can only go without eating for so long!) ThAnKs! I
> >like
> >> your .giff, lots smaller than any .mov or .ram!
> >>
> >> -Spencer
> >
> >--
> >
> >
> >Richard Piotter The Richfiles Robotics & TI web page:
> >richfile@rconnect.com http://richfiles.calc.org
> >
> >-- Make Money by Simply Surfing the Net or responding to E-Mail!!!
> >-- Click below!!!
> >
> >http://www.alladvantage.com/go.asp?refid=ATL147
> >http://www.spedia.net/cgi-bin/dir/tz.cgi?run=show_svc&fl=8&vid=329630
> >
>
>
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