specification for picking a PTC

Discussion in 'General Electronics Chat' started by dcd528, May 5, 2018.

  1. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    42
    1
    Hi all,
    I have a resistance heater powered by a lithium ion battery, nominal 3.7 volts. This draws up to 10 amps. I want to add a PTC fuse to protect the battery in the event it was shorted. I can't find any info on what the trip current should be to protect against a short.
    I picked this one RHEF1500HF-ND (https://www.digikey.com/product-detail/en/littelfuse-inc/RHEF1500/RHEF1500HF-ND/5029799) the hold current is 15A and the trip current is 28A. My guess is the trip current is too high with this one.
    Any help would be greatly appreciated. Thanks for looking
     
  2. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    812
    Selecting over-current devices is a remarkably difficult task sometimes. You've see a typical situation - you have a spec for the running current. You need to avoid nuisance trips but you have to be sure there is sufficient current available to actually trip or blow the protection device. Toss in issues of performance over temperature and a few other things and it gets messy.

    Depending on how your system operates (ON time of heater, ambient temperature, etc.) you can probably select a PTC with hold current closer to 10 amps e.g. RGEF1100, which has a nominal hold current of 11 A and trip current of about 19 A. There is variation in the ratio among the series from any particular manufacturer.

    Littelfuse acquired Raychem's PolySwitch line of PTC devices some years back. Raychem published quite a lot of ap notes on how to select and apply devices. Last time I looked, most of that was available on the Littelfuse website. I'd also suggest looking at the websites of some of the other PTC manufacturers carried by DigiKey, such as Bourns and Belfuse.

    Actual fuses can be a better choice, but they aren't especially easy to spec in difficult circumstances either.
     
  3. dl324

    AAC Fanatic!

    Mar 30, 2015
    8,757
    2,119
    You don't specify the maximum current you can safely draw from the battery.

    The general selection process is to determine the maximum current you want to allow, including surge, and maximum ambient temperature. That will allow you to determine the minimum Ihold. Then you consider whether the Itrip for any device you consider is low enough to protect the battery. If you can't find one that meets your requirements, you design your own electronic fuse.

    Some offerings from Littlefuse:
    upload_2018-5-5_9-30-58.png

    Temperature derating:
    upload_2018-5-5_9-33-57.png
     
  4. dcd528

    Thread Starter Active Member

    Jul 28, 2011
    42
    1
    Thanks to both of you. As dl324 pointed out I didn't specify the maximum amperage. That is the information I need to pick the best PTC. Great table thanks much.
     
  5. Hymie

    Active Member

    Mar 30, 2018
    701
    184
    Based on the tabulated data for the Litttelfuse devices, I’d be concerned that the cell would be unable to cause the ptc to trip – even under a worst case short circuit conditions.

    If you were to select the device with the 10A Ihold rating, it could pass 60A for up to 6 seconds.

    It is worth noting from the table notes that the Itrip current is the minimum current at which the device will trip at 20C, so the device could pass over 17A (continuously).

    Based on the tabulated data, you might be better selecting a device with an Itrip of 6, 7 or 8 amps, which should not trip passing 10A.
     
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