Current limiting with a thermistor?

Look at self resetting thermal fuses. Depending on the actual current levels required you will need such a larger ratio of resistances from the 'normal' to 'protecting' state, that something with a sharper cutoff, and a higher ratio between the 'break' current, and the current then needed to keep the device switched of, is preferable. Look at devices like:

Best Wishes

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There are better ways.

What is the battery voltage, the nominal load current, the current
limit, and do you want the circuit to limit the current into the
load or disconnect the battery if the current limit is exceeded?

"Guy Fawkes"

** The ideal device is a " Polyswitch " .

....... Phil

Yes a "polyswitch" or a "polyfuse" is a good solution for current limiting in a battery circuit. But do be aware that these devices are far from ideal. The most annoying characteristic of these devices are that their 'trip current level" will change to a higher level with each successive trip of the fuse device. You may expect to see the actual trip level change by 2X to 3X through initial trips. This of course needs to be taken into account when designing the usage senario.

- mkaras

** Mad Groper ALERT !!

** What ABSOLUTE BULLSHIT !!!

Tripping current is strictly related to device temperature.

....... Phil

PTC thermistor devices can work in some applications but not all. It doesn't take much of a battery to be able to output enough current to destroy the PTC device if the load side shorts. The ceramic ones have more resistance and thus hold the current at a lower value while they warm up.

Making sure that there is enough wiring resistance to protect the PTC is the easiest way to protect the PTC from over current. Using wiring resistance spreads the heat over a large amount of material.

The trip point of a plastic PTC moves around depending on its history. For the Bourns ones at least, the trip point is higher on one that is fresh out of the box. When it is soldered into the PCB its trip point tends to decrease slightly. If you take a fresh one and use clip leads and a power supply to trip it, you will see a reduction in the trip point after the first trip or perhaps two. After that it settles out to a final value. You have to be careful to let the thing cool for a long time. A small difference in temperature makes difference to the trip point.

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kensmith@rahul.net   forging knowledge

"Ken Smith"

** Raychem suggests that 40 amps is safe for their smaller devices.

If a *dead short* produces more, then a fuse in series is the go.

** But then go back up because of the heatsinking effect of the PCB . ** Direct opposite of what "mkaras" just claimed out of thin air. ** The trip point occurs at the same temp for a given device.

The actual (rms) current value to reach that temp obviously depends on the prevailing ambient temp plus any heatsinking of or air flow round the device.

The maker's "I trip" spec is for free air and room temp (20 C ?)

....... Phil

Fuses aren't exactly precision devices either ... and of course they vary quite a lot between first trip and subsequent trips!

Mkaras didn't say which way it varied, nor what cooling time he allowed between trips.

"Don Foreman"

** Yes he did - the direction and the amount too.

Try learning to read sometime.

** Irrelevant.

....... Phil

........ Phil

Yah, you're right, he did say increasing. Back to the ABC blocks for me re reading skills.

Wrong. Perhaps I write no better than I read so I'll not expound further. I've used and tested these devices. I seem to be able to read lab instruments well enough to get things right most days.

Besides the polyfuse, you may want to look into

[...]

If you take a look at the Bourns products, you will find some with a limit of only 10A.

..or adding resistance to if that can be done. This is often a better way to go than a fuse. In many situations, a blown fuse is "broken".

[....]

Yes, but I only really can say for sure about Bourns parts. They are the ones I use. If the ones "mkaras" uses are made in someones garage, all bets are off.

It also depends on the resistance of the device. This is the one internal thing that varies. The material undergoes a phase change at a specific temperature. It does not however perfectly recover when it is cooled.

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kensmith@rahul.net   forging knowledge

In article , Don Foreman wrote: [...]

Yes and if you need a 7A fuse you can't parallel a 2A with a 5A to get it.

BTW: With fuses rated for about 30V and operating at about 300V, the second trip can be to the stock room to get a new holder.

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kensmith@rahul.net   forging knowledge

"Ken Smith"

** The Bourns ones I use are all rated at 40A .

** I was not complaining about *you* ! ** His absurd libel was against one and all such devices. ** Not done enough test cycles to have observed a significant change in the apps I have used them in.

( Mostly low voltage transformer winding protection)

....... Phil

"John Fields" schreef in bericht news: snipped-for-privacy@4ax.com...

Voltage = 12V Current = nominal 200mA, transient 1.5A (50ms) Current Limit = 300mA Current Limit (no disconnect)

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Posted via a free Usenet account from http://www.teranews.com

For example: MF-SMDF050 is rated at 10A

I use them mostly on battery powered stuff where there is lots of current available.

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kensmith@rahul.net   forging knowledge

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I\'ve posted a circuit for you on alt.binaries.schematics.electronic,
but it occurs to me that you may need that transient to get through
for some reason, and my circuit will squish it.

Can you provide a little more detail about your application?