Comments on PTC resettable fuse leakage current
Parent
PTC resettable fuse leakage current
Intro
Hi everyone, I have a question about the practical use of a PTC (positive thermal coefficient) resettable fuse. I've never used one before, but I'd like to add one to a DC motor circuit to prevent it from overheating when running continuously.
More features being used
I already have two motor protection features in place: current limiting and timed shutdown upon continuous current draw. Current limiting makes sure that the motor is not allowed to go anywhere near stall, before which the plastic gears in the gearbox become stripped due to the torque and the motor fails. The other feature: if there's current for a certain period of time, the microcontroller tells the motor driver IC to cut off power to the motor. I'm considering using a PTC as a third feature, just as an added safety measure to prevent the motor from heating up if anything goes wrong and current is allowed to just proceed through the coil for hours.
Background to question
I was looking at Littelfuse's PTC selection guide, and it mentions how that the resistance of the PTC increases greatly once the PTC is tripped. It also says that only a "low leakage level" of current of up to a few hundred milliamps is allowed through. However, when selecting PTCs on Digikey, I saw that the tripped resistance is only 2–4x the amount of the normal resistance, which is on the order of a few hundred milliohms to a few ohms to begin with.
The question
What exactly can I expect for the PTC tripped leakage current? How is it calculated? Is it just a matter of using the tripped resistance with any other series resistances (motor), the source voltage, and Ohm's Law (V=IR)? Or is there some other phenomenon in play with PTCs where the current is limited to a certain range? I read that PTCs allow the leakage current through to keep it in its tripped state, but there was no word as to what this amount is or how it's calculated.
Thanks for any help!
Post
That's a marketing blurb. Look at the real datasheet.
I just checked a polyfuse datasheet, and see that there is usually only about a 2.5 to 3.0 ratio of trip current to hold current. That should be enough to keep the motor from overheating.
Polyfuses work on heat. When cold, they have low resistance. Over-current heats them up, which makes the resistance go up. It then requires lower current to maintain the temperature that maintains the high resitance.
The main downside of polyfuses is the relatively large series resistance. They may not be appropriate for your motor application for this reason.
An electronic fuse, as you seem to have, is a better choice. You can then control directly how much current for how long you want to allow. With a properly working electronic fuse, you can use a plain old melting link fuse as a backup, since it should never trip unless there is an outright failure in your electronics somewhere. A stuck motor won't blow the fuse since the electronic system already limits the current and/or shuts down the motor.
But if the motor has say 200mA running through it for hours at a time without completing any work (i.e. just sitting there and heating up), wouldn't that cause some heat damage to it?
Not necessarily. Motor heating comes from I2R losses in the coils. We'll ignore heating of the bearings from friction for this case, since that goes to zero when the motor is stalled anyway.
If we consider the coils resistance fixed, then electrical heating is proportional to the square of the current. Note that how fast the motor is turning has nothing to do with this.
The reason we worry about a stalled motor overheating is because this assumes the motor is being driven with a fixed voltage. When the motor isn't turning, there is no back EMF opposing the driving voltage. All the driving voltage appears across the coil resistance. When the motor turns, the effective voltage across the coil resistance is less, so less power dissipated in that resistance.
If you are controlling the current directly, then the back EMF doesn't matter for heating purposes. When the motor is stalled, it will automatically take less voltage to cause the current. Either way, though, the dissipation in the coil resistance is still proportional to the square of the current only.
For some motors, the motor motion is also used for air cooling internally. In that case, a stalled motor is less able to get rid of heat, and you have to reduce the current thru the motor.
To get back to your question, if the motor doesn't air-cool itself and it can handle 200 mA continuously when running, then it can handle the same current continuously when stalled too.
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