Limiting current output

I am working on a project using a Windows CE computer.
Input power can range from 10-24 VDC (which the computer already does). I have parallel and TTL outputs available (and also RS232/422/485, 10V D/A and others) from which to select.

In the past, I had a simple circuit using a 2N7000 that could light up an LED indicator or drive an audible alarm, but now I would like to have external outputs for such devices and need short-circuit protection to prevent damage in the event of a problem in the outside world.

The common power supply handles several pieces of equipment and can supply almost 4A at 24VDC (short-circuit protected) or the system can run from a car battery which can give all its got until the fuse blows.

Oh yeah, I need as simple of circuit as possible (without PC board if possible), just enough to keep the thing running.

Thanks,
Dale

I'm not sure exactly what you're after, but if it's just a short circuit protection a normal fuse will do, but I'd go for a polyswitch because it'll take huge surges and automatically reset when the power is dis connected and reconnected.

I wasn't looking for a fuse because I did not want users to have to open the controller box. Just want a simple circuit that will limit current or shut off the output in the event of a short circuit and automatically restore when the short has been fixed.
Thanks,
Dale

That's exactly what a polyswitch, or pollyfuse does, it goes high impedance on a short circuit then gets reset when the power is disconnected.

This is a pretty crude but common method. R7 is adjusted to the desired limit based on Q4's Vbe. Q3 is current starved as you reach the limit. Note only the current limiting portion is shown.

Just use clamping diodes.

Clamping diodes only protect agianst back emf, or an input exceeding the supply rails. They won't protect against overcurrent at all.

I found what I needed. It is something called a PTC resettable fuse. It looks like a thermistor or a disc capacitor. When there is a fault, it automatically opens until the fault condition (and power) is removed. When power is restored it resets and again conducts. Just a single inexpensive part.

Hero999, I checked and I think you hit the nail on the head. I think a polyfuse is the same thing, but that is not what it was called in the cataloge I had. After a quick search, I think polyswitch(tm) and polyfuse(R) are trademark names of Raychem and Littelfuse respectively.

Thanks,
Dale

Thanks for pointing that out. I'll remember to call them PTC resistors from now on.

Hero999 said:

Thanks for pointing that out. I'll remember to call them PTC resistors from now on.

Click to expand...

Hero999,
That's PTC resettable fuses not PTC resistors. I also discovered that Bourns uses the trademark name MultiFuse for their PTC resettable fuses.

Thanks,
Dale

Is there a difference between PTC fuese and PTC resistors?

Are they not simply PTCs thermistors? The warmer they get the higher their reistance becomes.

Hero999 said:

Is there a difference between PTC fuese and PTC resistors?

Are they not simply PTCs thermistors? The warmer they get the higher their reistance becomes.

Click to expand...

I'm just now learning about these devices, but as I understand, a PTC resistor changes resistance proportionally to temperature whereas the PTC fuse maintains its resistance then rises sharply when the trip current is exceeded to a point where it limits the current (to about 50% of the trip current) to 'hold' the device (i.e., keep it 'hot' enough to maintain its high resistance state) until the power is completly shut off to reset the device to its low resistance state.

Dale

But as they pass more current, they get hotter causing their restance to rise so they dissipate even more power and get even hotter until barely any current flows at all.

Hero999 said:

But as they pass more current, they get hotter causing their restance to rise so they dissipate even more power and get even hotter until barely any current flows at all.

Click to expand...

I'm not quite sure about which device you mean in your reference to "they".

The PTC resistors have a gradual linear increase in resistance (proportional to temperature) whereas the PTC resettable fuse goes from its low resistance state to a sudden sharp increase to its high resistance state once the trip current is reached and holds the high resistance state until the power is shut off to allow it to reset its state.

I suppose one could use a PTC resistor to protect a circuit, but I would think you would not want the increase in resistance within the acceptable current range.
Dale

When I ment they I was reffering to the PTC resistor, polyswitch etc.

I am right, PTC resistor and resettable fuse are the same thing.

https://en.wikipedia.org/wiki/Resettable_fuse
https://en.wikipedia.org/wiki/Thermistor#Applications

The reason why a PTC resistor's impedance rises so sharply is due to a runaway effect. The current increases which generates more heat so its resistance increases, causeing more heating until it reaches a point when only a small current flows, at this point it it said to have tripped.

Hero 999,
Sorry to disagree, but though a poodle is a dog, not all dogs are poodles. A PTC resettable fuse is a specialized version of a PTC resistor but not all PTC resistors are PTC fuses--they are NOT the same thing. You should consider the temperature vs. resistance curve of the device for your application.

Other applications for PTC resistors (where a PTC resettable fuse will NOT work) are for temperature sensing, self-thermostating temperature control, and power reduction at operating temperature (for fast warm-up to operating temperature) among other things. Another application is to provide a current burst through the demagnetizing coil when a television is first turned on but it seems to me that this behavior is sort of like a time-delayed PTC resettable fuse.

I would consider a PTC resistor as a type of thermistor whereas the PTC resettable fuse acts as a fuse (but is resettable).

(I don't go to wiki for answers because they are given by self-proclaimed "experts" and are subject to errors and entries are subject to being incomplete or inaccurate. It only works as well as those that are making, reviewing, and correcting the entries.)

Dale

I agree with you, a resettable fuse it a type of PTC resistor but not all PTC resistors suitable to be used as current protection devices. The Wikipedia is also correct, it doesn't say that all PTC resistors are resettable fuses but if you don't think it's clear enough,then you know what you can do about it.

Only idiots go to Wiki for answers, it is however an excellent site to go for information, as long as that's kept in mind it's an extremely useful first stop.