"Martin" wrote in message
...
On Sep 8, 9:22 pm, Tony Matt wrote:
Martin wrote:
I was trying to run one of the kids toys from a bench supply instead
of batteries.

Instead of hooking it up to 5 volts I accidentally hooked it to 17
volts ...

So now I'm trying to troublshoot.

The toy is a hotwheels "shooter" ...
There is a motor driving some gears that then drive two foam wheels
about an inch apart.
when the motor and foam wheels are spinning, you feed in a hotwheels
car and it shoots out the other side at high speed.

The circuit driving the motor is (all in series):
Battery positive,
something that looks like a disk capacitor,
a wirewound rheostat,
the motor,
battery negative

I have two of them, the one that is blown up will kind of run for a
bit, but then pretty much stop.
At that point the "disk capacitor" will be pretty hot to the touch.

Both of them act/measure the same when the batteries are removed.

It didn't make sense to have a capacitor in the circuit in series with
everything else, since that would have kept any DC current
flowing ... but maybe it was it was some kind of wierd reactive RC
circuit with the motor ... I dunno ...

So I measured it with a capacitance meter ... and it measured the same
as connecting the probe tips together in a dead short ... so probably
not actually a disk capacitor after all.

Which made sense, since a capacitor there would have made no sense.

Measureing it with an ohm-meter gives a value of about 1 ohm, for both
the dead and the good one

So what is it ... I'll guess some kind of current limiter to protect
the motor if the thing becomes jammed.
I would guess the resistance goes up if the current gets too high,
which then reduces the current to the motor.

Anyone know of anything like that, and what to search under in a
catalog.

Could be a resettable fuse, like a Tyco (Raychem) Polyswitch, Bourns
Multifuse, Littelfuse Polyfuse, or Optifuse.
Seehttp://en.wikipedia.org/wiki/Polyswitch. Most of the radial packages
are rectangular, but some are discs. Does the part have any markings?

TM

Thanks all,

It seems like it is one of the above.

Now to see if I can find a local source, so I don't pay $5 shipping on
a 30 cent part.

Martin

If it is a thermistor or a polyfuse as seems likely, you can save yourself
the costs of buying and shipping one by first determining for sure if it's
faulty. With the *correct* voltage applied to the circuit, just short it
out. However, if it really does read 1 ohm now as you stated earlier, it's
unlikely that it actually is faulty, unless something really obscure has
gone wrong with it such that it opens or goes high resistance at a much
lower overload point than intended. An alternate way of determining just
what it's doing, might be to set the toy up normally, with the correct
voltage applied, and then measure the voltage drop that's occuring across
this device. If you have another which is working, as I think you said you
did, compare by doing the same test on that one

Arfa