Resettable fuse equivalent to 500 mA glass fuse ?

[Jolly 7]

Modern resettable PTC fuses have some advantages over traditional glass fuses. Can anyone please advise a suitable replacement for a 500 mA glass fuse ? I want to choose a suitable resettable fuse for lithium batteries from 4.2 to 8.4V which will power DIY projects that consume no more than 300 mA of current.

[toshiba tony]

Sorry to ask but what is a PTF fuse please. I was with LG and Sony for fifty years and I was a regular contributor to this site, but due to a stroke and dementia certain abbreviations elude me.

[Nickthedentist]

PTC = positive temperature co-efficient I think. They protect then automatically reset when they've cooled down, I think.

e.g. https://www.littelfuse.com/products/...ble-pptcs.aspx

[Uncle Bulgaria]

I have used a 'Polyswitch' (e.g. available here at CPC) for a car battery isolator bypass that feeds a clock radio. With battery isolated, the radio is still powered, but if an attempt is made to start the car the excess current blows the Polyswitch. Turning off resets it.

I've found it very reliable and got the circuit from a reputable book, so perhaps one of those would suit your application. A variety of voltage ratings and trip currents are available, down to the tens of milliamps.

[toshiba tony]

i remember the properties of ptc thermistors well enough, that's what threw me, that's my plan now for the next puzzle about life, thanks!

[Jolly 7]

Apologies for not been clearer with my abbreviation. Yes, PTC stands for Positive Temperature Coefficient. PTC fuses are also known as polyfuses.

[Uncle Bulgaria]

For clarity, CPC refer to them as 'PPTC' meaning 'Polymeric Positive Temperature Coefficient'.

[Jolly 7]

https://youtu.be/sF0KOVWj9p8?si=55V3h48j2g4Iyvt0

Here is an excellent video from Great Scott on YouTube about this type of fuse.

[toshiba tony]

No, it's me. Need help with mail now. Ta.

[trobbins]

I use the Littlefuse RXEF range that are available at a few well known outlets - they don't cost much so postage may be an issue unless you have a long shopping list. https://www.littelfuse.com/media?res...rxef-datasheet

I'm guessing the lithium battery would have quite a high prospective fault current. Is it plausible for faults to cause high current flow in excess of say 10x your max operating current ?

Do you have a good awareness of what the max operating current could get to - you say 300mA - does that include all tolerances etc ? Do you have a good awareness of the local ambient temperature range that the PTC will sit in ? They would be the two main design variables for choosing a particular PTC part for your widget.

[Marconi_MPT4]

Experience stems from using original polyfuse components. Other manufacturers might use similar technology.

Used in 24V battery systems for a short period revealed they are not the golden panacea for every application. They degrade slightly after every trip by which the fuse resistance increases over a period of time making them far more sensitive. Leaving a polyfuse in a tripped state for a length of time increases the problem. Often sensitive to ambient heat, so will require careful placement. Occasionally had a few that lost the reset feature and remained permanently open or high resistance.

OK for situations where the polyfuse might be activated one or two times over the product lifetime.

Rich

[Jolly 7]

Originally Posted by trobbins

I use the Littlefuse RXEF range that are available at a few well known outlets - they don't cost much so postage may be an issue unless you have a long shopping list. https://www.littelfuse.com/media?res...rxef-datasheet

I'm guessing the lithium battery would have quite a high prospective fault current. Is it plausible for faults to cause high current flow in excess of say 10x your max operating current ?

Do you have a good awareness of what the max operating current could get to - you say 300mA - does that include all tolerances etc ? Do you have a good awareness of the local ambient temperature range that the PTC will sit in ? They would be the two main design variables for choosing a particular PTC part for your widget.

I am powering a white star LED with a forward voltage drop of around 3.4V from a 500mAh rechargeable lithium battery. I have limited the current through the LED to 220 mA using a resistor. The LED will tolerate up to 350 mA, so I want the fuse to trip around 300 mA. Essentially, as this is a DIY project, I am assuming the worst case scenario of a dead short which can certainly lead to fire. The environment is a plastic enclosure being used indoors. The LED does have a heatsink that gets very hot to the touch after around 5 minutes of use.

[djsbriscoe]

In my limited experience with these, it's probably best to study the data sheets of a selection of them and then actually test them to see how they operate whilst taking measurements. You need them to operate reliably at the holding current, whilst tripping COMPLETELY at your required fault current. They don't act immediately, so may allow some transient current peaks through. I've used a 500 mA one in the past to protect a 1A circuit. There are a few guides online if you search with "Polyfuse".

[trobbins]

Can you clarify why you said 4.2 to 8.4V in post #1, and how you determine the max 220mA operating current through the PTC, given its 3.4V voltage drop ?

Is it just (8.4V-3.4V)/22 ohm = 220mA ?

Are you saying the battery and the PTC will be in the same enclosure (or close thermal vicinity) as the LED with its heatsink ? Is all that interconnect wiring internal, or is there an external wiring link/plug/... that could accidentally cause a short ?

How are you charging the battery, and would it be charging with the LED on, and is the PTC to be in-line with the charging current ?

[Jolly 7]

@trobbins:

1. My question is whether the same value of polyfuse

[Jolly 7]

Apologies for the incomplete reply. I am currently having an issue with posting replies.

[Jolly 7]

My question is whether the same value of polyfuse could be used if I were to use two batteries in series i.e. 8.4V. Bench tests allowed me to choose an optimal current of around 220 mA , as measured, when powering the LED from one battery. I relied on the Ohm's law equation and calculated the resistor to be 3.3 ohms ,1 watt roughly. I want to charge the battery with a TP4056 module from an external USB charger, with the LED off. All the interconnect wiring will be internal. I intend to place the PTC in between the LED resistor and battery positive terminal.

[trobbins]

Sorry for all the queries - just making sure I appreciate the application.

Given the likelihood that a fault would be internal, and worstcase could be a short across the battery then a PTC should 'mitigate' the outcome, and hopefully then prompt an investigation, and as indicated above, best to replace the PTC at the same time. These are physically small parts, and I've seen them in ss amps where repeated exposure to fault currents have damaged the PTC - so somewhat easy to faultfind !

The part's Hold current rating needs to be derated due to local ambient, and any other tolerance, to avoid operational hassles. Colder temps shouldn't alter a fault outcome, but hotter ambients could cause hassles. For that RXEF range, as an example, a 500mA Hold part should be ok for 220mA nominal out to 70C.

If you use a different battery config, with a different operating current, then assessment gets a little more onerous as to what the max operating current could be, as that is what determines the PTC part choice. I'd imagine that any battery config would cause a worstcase fault current that is > 10x the part rating, so that aspect may not change.

Just be mindful that all fault scenarios may not cause a short circuit of the battery through the PTC, as some faults may include series resistance that limits the fault current to a level where the PTC may not operate as intended.

[Tractionist]

Hopefully not 'off topic' : I'm currently working on a Sony CDP-H300 (CD Player) power board whereat the transformer primary appears to be open. The manual schematic shows a fuse on the primary rail - but gives no part number or type. Further, the fuse isn't visible on the board and ergo must be inside the transformer casing. Does anyone know whether this [plastic] casing is openable? Maybe Jolly 7 can comment?

[Jolly 7]

@Tractionist: To the best of my knowledge, fuses buried inside the mains transformer could be hidden under insulation tape windings and are thermal fuses, not PTC types. I once had a similar problem with an open winding primary for a wall wart type adapter. I cut open the insulation tape around the primary but could not find any thermal fuse.

There was some discussion on the forum earlier this year in relation to hidden mains transformer fuses. I may have started a thread too a while ago with photos, but currently cannot locate it.

Please watch the YouTube video below from Grants TV repair. It might be helpful.

https://youtu.be/4HoHERN_-DE?si=4ezw5osnApJTpzAV