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Current limiting with a thermistor?

Discussion in 'Electronic Design' started by Guy Fawkes, Sep 17, 2006.

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  1. Guy Fawkes

    Guy Fawkes Guest


    I'm trying to protect a battery from overloading when too much current is
    drawn from it. Is it possible to do so with a thermistor? Or are there
    better ways of doing this?


  2. 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
    Look at devices like:

    Best Wishes
  3. John Fields

    John Fields Guest

    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?
  4. Phil Allison

    Phil Allison Guest

    "Guy Fawkes"

    ** The ideal device is a " Polyswitch " .

    ........ Phil
  5. mkaras

    mkaras Guest

    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
  6. Phil Allison

    Phil Allison Guest

    ** Mad Groper ALERT !!


    Tripping current is strictly related to device temperature.

    ........ Phil
  7. Ken Smith

    Ken Smith Guest

    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

    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
  8. Phil Allison

    Phil Allison Guest

    "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

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

    ........ Phil
  9. Don Foreman

    Don Foreman Guest

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

    Don Foreman Guest

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

    Phil Allison Guest

    "Don Foreman"

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

    Try learning to read sometime.

    ** Irrelevant.

    ........ Phil

    ......... Phil
  12. Don Foreman

    Don Foreman Guest

    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.
  13. Guest

    Besides the polyfuse, you may want to look into
  14. Ken Smith

    Ken Smith Guest

    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.
  15. Ken Smith

    Ken Smith Guest

    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.
  16. Phil Allison

    Phil Allison Guest

    "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
  17. Guy Fawkes

    Guy Fawkes Guest

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

    Ken Smith Guest

    For example: MF-SMDF050 is rated at 10A
    I use them mostly on battery powered stuff where there is lots of current
  19. John Fields

    John Fields Guest

    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?
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