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I have some small devices that draw 20mA. I'd like to add a resettable (PPTC) fuse to them but I am not sure what value to select.

My thought is I need to find a value that will tolerate the startup or inrush current when the circuit first turned on, but I don't know how to measure that. (My meter isn't that fast, but I do have an analog 100MHz oscilloscope.)

The devices have a capacitance of about 122μF.

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  • \$\begingroup\$ A related, useful question on PTC fuses and low-current applications: electronics.stackexchange.com/q/50251/2028 \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 2:59
  • \$\begingroup\$ Does your scope have a persistance refresh mode (some of the old tek scopes do for example - I have one here) Either that or maybe set up a camera for multiple "sports" type shots (or video) The other option is to work it out using the basic formulas (or simulate it) \$\endgroup\$
    – Oli Glaser
    Commented Dec 2, 2012 at 3:04
  • \$\begingroup\$ @Oli I'm assuming it does not have persistence refresh. It's a BK 2190B. \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 3:08
  • \$\begingroup\$ No, you're right it doesn't unfortunately, I just checked the manual. I think I'd try the video/sim options and see how you go there. \$\endgroup\$
    – Oli Glaser
    Commented Dec 2, 2012 at 3:39

3 Answers 3

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LittleFuse provides a very nice app note with a lot of details for the design of these kind of devices.

As for measuring the current, you could use a very small resistor (1 or 0.1 ohm resistor) and use an oscilloscope to measure the current. Be careful not to ground the device. You need to use two probes for doing this measurement and subtract the value of one from the other - DO NOT CONNECT THE GND OF THE PROBE TO EITHER SIDE OF THE RESISTOR- you will in effect cause a short circuit if you do this. Just get 2 probes, connect the tip to each side of the resistor, and the grounds to the actual ground. use oscilloscope math capability (or just yourself) to do a difference between the two channels and this will be your current.

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  • \$\begingroup\$ This current measurement will be the startup current? I'm still concerned that the event will be over far more quickly than I can measure. \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 8:36
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    \$\begingroup\$ Sure, you can connect the oscilloscope to the board and only then boot up the board. Even if it's fast, a good oscilloscope can give you microsecond resolution which is more than enough. \$\endgroup\$ Commented Dec 2, 2012 at 9:30
  • \$\begingroup\$ The link is dead and I couldn't find a replacement. Also the company name (first word) should be Littelfuse with 'el', not LittleFuse. Can't edit as its less than 6 chars. \$\endgroup\$
    – robsn
    Commented Jan 24, 2020 at 8:46
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The good thing about PTC fuses, in this context anyway, is that they take a bit of time to heat up enough to "blow" i.e. become an open circuit. If your inrush current is brief, the PTC does not get enough high-current time to react, before current has settled into steady state.

If the capacitors are not being charged through any significant resistance, then inrush should be quite brief. A good strategy to follow in such cases, is to estimate the current at which either the power supply, or some malfunctioning part of your device, is likely to suffer damage. Then pick a current between the expected steady state and this estimated overcurrent, and use a PTC of approximately that current rating.

If you have (or can request samples of) several PTCs spanning a range up to your power supply rating, this becomes even simpler: Use a fuse two levels higher rated than the highest PTC that trips on start-up.

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  • \$\begingroup\$ I have several PTCs of different values on order for doing exactly what you suggest. I'm not entirely sure what current levels could occur, or be damaging. The device is intended to run from 7.2 to 15 volts on batteries. If a user connected the wrong voltage somehow (not sure what the most likely scenario is) I just want to prevent the device from exploding/causing a fire. :) \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 3:11
  • \$\begingroup\$ A PTC isn't ideal to protect equipment from power supply incorrect voltages. For that, you need to perhaps consider an overvoltage protection IC or circuit. My personal strategy is to simply use a cheap, high input voltage range buck regulator on the input line. The ones I use (Fulree, $1, 3-pin, 5Volt / 2 Amp) work fine with input voltages up to 22 Volts. \$\endgroup\$ Commented Dec 2, 2012 at 3:18
  • \$\begingroup\$ An EE friend recommended a transient voltage suppression (TVS) diode or "transorb". I'm getting confused with so many solutions as to what is best. I'm principally interested in preventing things from overheating/exploding due to potentially common user errors. I am anticipating reverse polarity (the power connection is two screw terminals, clearly marked, but easily reversed) and incorrect voltage chiefly. I'm OK with things exploding if someone attempts to hook up a device to mains power, for example. :) \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 8:41
  • \$\begingroup\$ A related question on a similar project: electronics.stackexchange.com/q/48831/2028 \$\endgroup\$
    – JYelton
    Commented Dec 2, 2012 at 8:51
  • \$\begingroup\$ The each have their uses and depend on what you need to accomplish and what your board might face. The wiki for Tranzorb (which is just a brand name for TVS diodes) is useful: en.wikipedia.org/wiki/… \$\endgroup\$ Commented Dec 3, 2012 at 19:34
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At my last job, our switch mode power supply wasn't working on our USB device. We checked it on the 100Mhz scope and everything looked fine. We then went to the vendor and she had a 1Ghz scope. There was a 3ns 6A spike at power on we didn't see with our scope. Most likely, the spike was from all the bypass caps we had. We changed the switcher design to use an older IC from a different vendor and it worked.

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