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General => General Chat => Topic started by: johnnyfp on May 30, 2012, 09:16:37 pm

Title: PTC Resettable Fuses
Post by: johnnyfp on May 30, 2012, 09:16:37 pm
Hi,

What's the difference between a Hold Current value and a Trip Current Value? So I'm looking for a PTC that will protect my circuirt if my circuit draws more than 4.5amps. IE over 4.5amps I want it to trip. But this Hold Current has thrown me off. For example http://nz.mouser.com/ProductDetail/TE-Connectivity-Raychem/RGEF250/?qs=F6FIpiMdEVaYOIe%2fh1gFSGNIGtf%2f3hvbK0PlSDHOFt4%3d (http://nz.mouser.com/ProductDetail/TE-Connectivity-Raychem/RGEF250/?qs=F6FIpiMdEVaYOIe%2fh1gFSGNIGtf%2f3hvbK0PlSDHOFt4%3d) This PTC has a trip current of 4.7amps which is fine for me. But has this hold current of 2.5amp.
Is this talking about the max current of the PTC after it's tripped?

Cheers

Jfp.
Title: Re: PTC Resettable Fuses
Post by: jimmc on May 30, 2012, 09:57:22 pm
Trip Current is the minimum current guaranteed to trip the device.
Hold Current is the maximum current guaranteed not to trip the device.

See the manufacturers website
http://tycoelectronics.custhelp.com/app/answers/detail/a_id/3844/related/1/session/L2F2LzEvc2lkL0VNNDZuclpr (http://tycoelectronics.custhelp.com/app/answers/detail/a_id/3844/related/1/session/L2F2LzEvc2lkL0VNNDZuclpr)

Jim
Title: Re: PTC Resettable Fuses
Post by: johnnyfp on May 30, 2012, 10:32:53 pm
Cool. Thanks. Had a moment of confusion there.

Jfp.
Title: Re: PTC Resettable Fuses
Post by: dcel on May 30, 2012, 10:41:20 pm
Poly fuses are like circuit breakers, but not. Poly fuses have a dead band where any more current above the hold current and below the trip current is unknown weather it will hold or trip. And it has to do with temp and time at that current. The higher the ambient temp, the closer to the hold current the devicew will trip. If you need absolute protection, choose a circuit breaker.

Chris
Title: Re: PTC Resettable Fuses
Post by: johnnyfp on May 30, 2012, 10:48:51 pm
Circuit breakers are Huge. And I need four of them!

They need to invent micro sized CB's.

Jfp.
Title: Re: PTC Resettable Fuses
Post by: dcel on May 30, 2012, 11:11:26 pm
Circuit breakers are Huge. And I need four of them!

They need to invent micro sized CB's.

Jfp.

Compaired to ptc's they are, and the need for four, well, ptc's may be your only solution.
They make some pretty small circuit breakers, might be worth a look anyway.

Chris
Title: Re: PTC Resettable Fuses
Post by: tom66 on May 31, 2012, 10:47:34 am
You can also use things like these load switches http://www.onsemi.com/PowerSolutions/product.do?id=NCP380 (http://www.onsemi.com/PowerSolutions/product.do?id=NCP380)

Limited to 2.1A but maybe two in parallel could be used or there might be higher current limit devices available.
Title: Re: PTC Resettable Fuses
Post by: Zero999 on May 31, 2012, 08:56:16 pm
You could also add a fuse for protection against the PTC resistor failing.
Title: Re: PTC Resettable Fuses
Post by: dfnr2 on May 31, 2012, 09:35:06 pm
PTC "resettable fuses" will only function usefully in that capacity if you design your circuit properly.  You may have up to hundreds of mA in the "tripped" state, depending on the device.  Also, you need to look at the curves to make sure the PTC will trip in time.  Selection of PTC device vs fuse, and selection of a specific part, will come down to what circuit you are protecting, and what is powering the circuit.  So, what are you putting on either side of the fuse?

Dave
Title: Re: PTC Resettable Fuses
Post by: johnnyfp on May 31, 2012, 10:06:25 pm
Output side is Dc-Dc converters. 12 of them to power LED's. Input side is a 24v Switchmode PSU.

I have 48 in total. I want to make it so that if anything has a catastrophic failure, then the rest is protected. But it's more of an overkill as the PSU has it's own internal protection and each DC-DC converters have there own thermal and overcurrent protection. I suppose a normal fuse would do, but thought a PTC would be more user friendly.

Jfp.
Title: Re: PTC Resettable Fuses
Post by: dfnr2 on June 01, 2012, 01:14:02 pm
Do you have some reverse-voltage and over-voltage protection on the DC-DC converters?
Title: Re: PTC Resettable Fuses
Post by: bfritz on June 01, 2012, 06:25:17 pm
Trip Current is the minimum current guaranteed to trip the device.
Hold Current is the maximum current guaranteed not to trip the device.

Good answers, but note that those ratings are for a given temperature.  At higher temperatures, both those values must be derated.  In other words, at higher temperatures the trip and hold currents will be lower.  You must look at the data sheet to determine how to derate them.
Title: Re: PTC Resettable Fuses
Post by: johnnyfp on June 01, 2012, 08:46:18 pm
The Converters themselves are more or less bullet proof with built in protection against shorts, overload, polarity, heat and excess voltages.

But with so many of them I don't want anything else to go wrong.

Jfp.
Title: Re: PTC Resettable Fuses
Post by: dfnr2 on June 01, 2012, 11:09:05 pm
The Converters themselves are more or less bullet proof with built in protection against shorts, overload, polarity, heat and excess voltages.

But with so many of them I don't want anything else to go wrong.

Jfp.

Good.  So you will have to figure out what kind of current they draw in various scenarios of abuse by an idiot with the wrong power connections.  Then you will look at the data sheets and choose a part that will limit current from your power supply to a level that won't destroy the protection circuitry.  If the converters are smart and draw limited current already under reversed polarity, voltages outside the spec, etc., then I'd question the value of adding a PTC.   

Using one of these devices definitely requires more consideration than, say, a fast-blow fuse.  Unlike a fuse, choosing a PTC is tricky--the numbers listed are really misleading.  You will have to consider the whole temp range of your project, under various scenarios, and look for the trip times and trip currents.  The data sheet curves are often misleading and incomplete as well, and often don't give the whole picture of performance against temperature.  The trick is to keep the current low enough to not destroy the protection diodes or other circuitry.  Ultimately, you can pick some parts that look close, then you will have to test them on the bench under the various failure scenarios, and actually measure the "trip time" and current.  Do this at the ends of your temp ranges, and at 25 deg C. (Or, if it's informal, use a heat gun & freeze spray, or a peltier heater/cooler like Dave Jones demonstrated.

BTW, you can get pretty small SMT fuse modules somewhat smaller than an so16, where you install the socket+fuse from the tape as one piece , but then just replace the fuse if it blows.

Good luck,

Dave