[Robotgroup] Motor Controllers and Fuses

Wed Apr 23 10:38:23 PDT 2008

I'd use a 3 amp slow blow fuse in series with the battery. You can get these
at allelectronics.com:
http://www.allelectronics.com/cgi-bin/item/FSSB-3/245300/3A_SLOW_BLOW_FUSE_.html
You can probably find them at most auto parts stores also.

Fuse holders can be found at either of the above places. I just got some
from Allelectronics.

Since each motor draws 1 amp and controller has a max of 4 amp, you would be
protecting the controller. A motor typically draws more at stall/start-up,
so the fuse can handle ~1.5A each. Slow blow because it's more forgiving,
meaning it can handle 150% of the rated current for a small period of time.

If I was doing this, I'd use a current limited power supply and a meter to
figure out the actual current. Then I'd get a fuse that's a bit above the
peak current. If I didn't have a power supply, etc I'd start with a 3 amp
slow-blow and be prepared to blow a few fuses in the process.

BTW a cheap ($3-$7) Harbor Freight DVM (digital volt meter) does have a 10
Amp current scale. If you smoke that, you'll also need a 10 A fast blow fuse
to replace it with.

Paul

On Wed, Apr 23, 2008 at 11:44 AM, Gray Mack <gray_mack at yahoo.com> wrote:

> Think of a fuse as just a thin wire. If you connect it
> across the two terminals of your battery, it gets hot,
> it's a better conductor than any other circuit
> conected to the battery and thus steals all the power
> until it burns up. You want to connect a fuse in
> series with something to prevent that something from
> being able to pull too much current.
>
> Same thing with measuring current with an Amp meter.
> When your multimeter is in a current measurement mode
> and you have switched the red plug to the amp jack it
> is going to measure how much current goes through a
> wire inside the meter. So you always have to put it in
> series with a load to be measured.
> If when you are done you forget to move the plug back
> to the volts-Ohms jack, then the next time you turn it
> on and stick the probes on a power supply, (shorting
> the supply through your meter) the meter will blow an
> internal fuse or catch on fire.
>
> how to measure current
> http://www.scienceshareware.com/how-to-measure-DC-current-with-a-dmm.htm
>
> couldnt google a diagram on how to fuse a circuit.
>
> -Gray
>
>
> --- Doug Evans <DEvans at compubotics.com> wrote:
>
> > Technical term for fuse placed in parallel with the
> > battery: Short Circuit
> >
> > :)
> >
> >
> > -----Original Message-----
> > From: robotgroup-bounces at puremagic.com
> > [mailto:robotgroup-bounces at puremagic.com] On Behalf
> > Of LHudson
> > Sent: Wednesday, April 23, 2008 10:43 AM
> > To: The Robot Group Mailing List
> > Subject: Re: [Robotgroup] Motor Controllers and
> > Fuses
> >
> > Very funny.  :D  That made me laugh.  But actually
> > it's more like this:
> >
> >   1.  L298 PCB caught on fire due to Soldering
> > Deficit Syndrome.  It was not
> > attached to a motor at the time, only to a 6v
> > battery.
> >
> >   2.  Went to Fry's to see if they have comparable
> > dual H-Bridge MC in order
> > to save $8 on S&H.  Received unsolicited advice from
> > a non-affiliated
> > pre-teen, who recommended a 5amp slo-blo fuse,
> > connected in Parallel to the
> > battery.
> >
> >   3.  Asked non-affiliated pre-teen if it should be
> > in Series rather than
> > Parallel.  He seemed convinced that he was correct.
> > Asker was not sure
> > because asker knows very little about electronics
> > (see item 1).
> >
> >   4.  Asked non-affiliated pre-teen if there should
> > be 2.5amp fuses
> > connected to the motors as well, or only to the MC.
> > Responder theorized
> > that three fuses would be better.
> >
> >   5.  Asker came to the realization that although
> > the responder seemed very
> > bright, TRG would probably have better answers.
> >
> >
> > Vern Graner <vern at txis.com> wrote:
> >   Andre Lamothe wrote:
> > > can you sum up in a paragraph what the question
> > and problem is exactly on
> > the table?
> >
> > Hmm ok, hows this:
> >
> > Problem:
> >
> > L298 + advice from Frys employee = "the pcb caught
> > on fire"
> >
> > Question:
> >
> > "What went wrong?"
> >
> > ;)
> >
> > Vern
> >
> >
> > --
> > Vern Graner CNE/CNA/SSE | "If the network is down,
> > then you're
> > Senior Systems Engineer | obviously incompetent so
> > why are we
> > Texas Information Services | paying you? Of course,
> > if the network
> > http://www.txis.com | is up, then we obviously don't
> > need
> > Austin Office 512 328-8947 | you, so why are we
> > paying you?" CVLG
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