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Author Topic: Question on PTC Resettable Fuse  (Read 470 times)

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Offline praveen_khmTopic starter

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Question on PTC Resettable Fuse
« on: July 01, 2013, 08:21:23 AM »
Hi,

Just back to SOR after a long time with a quick question.

I need to add a PTC fuse to one of my board. It states that "Hold" is 500mA and "Trip" is 900mA. Now, if I use a regulator with an output of 5V/800mA, how does it affect the PTC?

Also, what happens if I add 1A regulator? Will it trip the moment I switch on the board, or will it wait until components try to sink in atleast 900mA...

-Praveen
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Offline Graynomad

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Re: Question on PTC Resettable Fuse
« Reply #1 on: July 01, 2013, 08:37:18 AM »
Quote
or will it wait until components try to sink in atleast 900mA...
Yes, it doesn't matter what the regulator is, it's the current being drawn by the components (including the regulator of course but that's a tiny amount) that counts.
Scattered showers my arse -- Noah, 2348BC.

Offline jwatte

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Re: Question on PTC Resettable Fuse
« Reply #2 on: July 01, 2013, 10:14:06 AM »
Current isn't really "pushed" into circuits. Circuits "draw" power.

The regulator will have a certain amount of power delivery capability. If you try to draw more than that, one of three things will happen based on what the regulator is:
1) The regulator will drop the voltage until less power is drawn
2) The regulator will go into thermal shutdown
3) The regulator will overheat and be destroyed

The reason to put a PTC in a circuit is to protect the power delivery part (such as the regulator) from overheating, as well as providing a "fail safe" in the case of accidental shorts or failures somewhere in the circuit, perhaps preventing various components on the power path from overheating and failing.

The PTC you describe is constructed such that, if you draw no more than 500 mA, it will never "trip." Meanwhile, it guarantees that if you draw 900 mA, it will "trip." (This is likely in 25C ambient temperature -- derate if the air is hotter or the PTC is not mounted in free air.) When "tripping," it will dramatically increase its resistance, thus dropping a lot of voltage, thus reducing (but not totally eliminating) the current through the circuit.

Now, if the PTC is guaranteed to trip at 900 mA, but the regulator is rated for 800 mA, then there is some chance that the circuit could draw, say, 850 mA, which might damage the regulator, but the PTC may not trip. Thus, the conservative design would be to upgrade the regulator to one rated for 1A -- that way, you know the PTC is guaranteed to protect the regulator under all circumstances.

Note that the PTC is a resistor. There will be some voltage drop across it. If you put it after the regulator, then the actual voltage seen by your circuit will sag a little bit as it draws more current. This may cause instability for the microcontroller (assuming that's what you're driving.) As you're using a linear regulator, the current into the regulator is the same as the current out of the regulator, so the right place to put the PTC is before the regulator. A voltage drop there will just mean the regulator has to burn off slightly less power to regulate the voltage, so it won't affect the circuit much except slightly raising the "recommended minimum voltage" input to your device. For example, if you use a 1.2V drop-out regulator, and you expect the PTC to drop 1.5V at 500 mA, and you want 5V out of the regulator, your minimum input voltage will be (5+1.2+1.5) = 7.7V.


Offline praveen_khmTopic starter

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Re: Question on PTC Resettable Fuse
« Reply #3 on: July 01, 2013, 10:21:00 AM »
Thanks to both of you. Really helpful information written in an easy to understand manner.

Cheers,
Praveen
Robot building is all about sharing and learning
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www.robotplatform.com : Beginners guide to Electronics & Robotics

Offline waltr

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Re: Question on PTC Resettable Fuse
« Reply #4 on: July 01, 2013, 05:11:10 PM »
Most linear regulator will current limit on their own. For this reason I don't protect their output. Do check the data sheet of the regulator you are using.
Where I do like using a PTC fuze is the input line from an external source. And if you put a diode across the power lines (cathode to pos, anode to neg) after the PTC then you have reverse polarity protection as well.

If has saved more than a few circuit from me mis-connecting a PS or battery.

 


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