is there such a thing as a current limiter that looks like a disk capacitor ???

On 9/8/2009 5:21 PM Martin spake thus:

Nope. Small apacitors are typically used on cheapo DC motors like that to reduce RF noise, as a courtesy to those who might experience interference. They just filter out (shunt) high-frequency AC.

If it's shorted, just snip it out.

--
Found--the gene that causes belief in genetic determinism

No,

I know what you're talking about, and those are in parallel with the brushes and/or connecting the brushes to the case. (and this actually has one of those too, right at the motor, which I neglected to mention)

This thing looks just like one of them (small disk cap) but its in SERIES with the motor and the rheostat.

Also, the non-blown-up one has one too, and it also reads as a 1 or so ohm resistor.

Could be a resettable fuse, like a Tyco (Raychem) Polyswitch, Bourns Multifuse, Littelfuse Polyfuse, or Optifuse. See

Most of the radial packages are rectangular, but some are discs. Does the part have any markings?

TM

How about it being a plain old thermistor ? Some control boards for drinks machines that I repair, have a disc thermistor in series with the supply to the drink whipper motors. When the whip units get clogged, and the motors struggle, the excess current heats the thermistor, whose value then rapidly rises, limiting the excess current to a value that won't burn out the motor. Heat it with the tip of a soldering iron, whilst having your ohm meter across it. If it reads 1 ohm at room temperature, however, I think it's fairly unlikely that is your problem

Arfa

while measuring the good one, apply a little heat to it(solder iron?), if the resistance changes fast upward, it might be a limiting fuse. On overload,they turn high resistance, and recover when cooling down.

| > 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. See |

Most of the radial packages | are rectangular, but some are discs. Does the part have any markings? | | TM

That's my guess too. Here's a pic:

Martin schrieb:

Maybe, the overvoltage hurt the motor, it draws to much current, therefore the thermistor gets hot.

It could be a poly fuse, who knows. could even be a thermistor, but I don't see a use for it there.

See

Most of the radial packages

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

I considered suggesting this, then concluded that there's no (straightforward) way to discern between a failing Polyswitch and one that's responding properly to a partially shorted motor winding, unless the heat itself is a failure indication. I'd replace the Polyswitch with a 1 ohm resistor, see how the motor runs, and compare the current draw between the two units (by measuring the voltage across the resistor).

TM

Yep. Good thoughts and a good suggestion.

Arfa