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Author Topic: Resettable Fuses - Tutoring Please  (Read 419 times)

Tug Fanatic

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Resettable Fuses - Tutoring Please
« on: February 15, 2020, 09:35:25 AM »

Microgyros introduced the idea of Resettable Fuses into the topic below. I have posted this as a new thread so as not to take over the exisitng thread.

https://www.modelboatmayhem.co.uk/forum/index.php/topic,64467.msg683258/topicseen.html#msg683258

Resettable Fuses:
https://www.ebay.co.uk/itm/16V-3A-4A-5A-6A-7A-8A-9A-10A-11A-12A-14A-PPTC-Resettable-Fuse/232445358290?

Wikipedia Explanation:
https://en.wikipedia.org/wiki/Resettable_fuse

As my electronics was learnt in the stone age I have never heard of them before but they sound really useful as they allow power back to the circuit rather than leaving me stranded in the middle of the pond etc.

My problem is how do I specify them?

Voltage appears simple. My guess is that as long as the voltage of the fuse is higher than the power supply then it will be OK but I then need to specify the current. I am unclear as to whether amps or watts is the important characteristic.

For a motor fuse do I ignore watts and simply specify an amp figure above full load but below stall or do I need to consider wattage for the fuse to work properly? 16v seems to be the common fuse rating but lots of models run around 7v - a big difference.

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microgyros

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Re: Resettable Fuses - Tutoring Please
« Reply #1 on: February 15, 2020, 03:55:59 PM »

The key figures in https://uk.farnell.com/c/circuit-protection/thermistors/pptc-resettable-fuses are holding current, tripping current and voltage rating.
After whittling down the relationship between the three figures is clearer.
I picked filters (1) Manufacturer = LITTELFUSE (I know they are priced cheap) (2) Thermistor Mounting = Through Hole  (3) Holding Current= 3A (4) Packaging= Each
https://uk.farnell.com/w/c/circuit-protection/thermistors/pptc-resettable-fuses?brand=littelfuse&thermistor-mounting=through-hole&holding-current=3a&packaging=each
Use the Sort ascending/descending buttons and you see the lowest acceptable voltage is physically smaller, trips quicker and at a lower trip current.
You can connect in parallel & you can't use a rated voltage lower than your supply.

 
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Tug Fanatic

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Re: Resettable Fuses - Tutoring Please
« Reply #2 on: February 15, 2020, 04:16:42 PM »

Thank you.
Please clarify what "holding current" is. I can take a guess but I am probably wrong.
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microgyros

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Re: Resettable Fuses - Tutoring Please
« Reply #3 on: February 15, 2020, 05:19:07 PM »

Thank you.
Please clarify what "holding current" is. I can take a guess but I am probably wrong.
From one manufacturer: IH is defined as the maximum current at which a device will not trip at temperature (20℃ to 25℃ depending on product family) in still air.
Two back-to-back leds + bias resistor will work as a trip indicator. I have some little circuit boards to make this task neater.
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Tug Fanatic

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Re: Resettable Fuses - Tutoring Please
« Reply #4 on: February 16, 2020, 10:08:49 AM »

Thank you.
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Martin [Admin]

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Re: Resettable Fuses - Tutoring Please
« Reply #5 on: February 16, 2020, 01:56:53 PM »


Video of what's going on inside a semi-conductor resetable fuse:
https://youtu.be/x7BP9o2goPE
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"This is my firm opinion, but what do I know?!"    -   Mayhem FaceBook Group!

Akira

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Re: Resettable Fuses - Tutoring Please
« Reply #6 on: February 21, 2020, 01:39:38 PM »

Resettable fuses have been in the hobby world for a long time. Model railroaders have long had them as part of the transformers controlling there trains. This goes back 50+ years and I am probably out of date now, but I use these fuses instead of expendable fuse regularly. They get you home. Over here, they are readily available from auto parts stores and electronic suppliers.I know this may not be the correct way to choose one, but I simply determine my motor's max running current draw under load and choose a re-setting fuse that is close to but slightly greater in current capacity. That way the weeds and such ( smaller, slower boats, submarines ect) that I may happen to run over will cause the fuse to trip. At a few bucks each, moving to a higher capacity setting will not break my bank account.
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microgyros

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Re: Resettable Fuses - Tutoring Please
« Reply #7 on: February 21, 2020, 02:17:41 PM »

Resettable fuses have been in the hobby world for a long time. Model railroaders have long had them as part of the transformers controlling there trains. This goes back 50+ years and I am probably out of date now...
Those ones are bimetallic with a coiled wire to give additional heat. These are also sold by electronic distributors.A few years back in another thread,  Martin mentioned yet another bimetallic resettable fuse (Ebay search) which had a reset button.

Your selection method works well with the newer, semiconductor breakers. It just needs a step up the rating (or add in paralle) if full throttle trips the device.
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roadrunner440

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Re: Resettable Fuses - Tutoring Please
« Reply #8 on: February 23, 2020, 06:12:40 PM »

G.M. makes some 15 amp reset-able circuit breakers for power seat's they are a standard 90's era mini fuse size..thats what i use

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Tug Fanatic

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Re: Resettable Fuses - Tutoring Please
« Reply #9 on: February 23, 2020, 06:36:56 PM »

I think that you might be talking about something different. The things that this thread is about are small solid state devices that reset automatically as they cool down after tripping.

There are also fuses that you reset by pushing a button, or something similar. These are not what I was asking about. 
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Akira

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Re: Resettable Fuses - Tutoring Please
« Reply #10 on: February 23, 2020, 07:18:25 PM »

Not to throw a wrench in the discussion, but over here there are available auto reset breakers that plug into an automobile fuse box in place of a one time fuse. They do auto reset. Work great.
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Tug Fanatic

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Re: Resettable Fuses - Tutoring Please
« Reply #11 on: February 24, 2020, 08:40:47 AM »

Akira,

Thank you for telling me about something else that I had never heard of.  :-))  :-)  :-))

A quick search of ebay found the automatic resettable blade fuses. I hope they become cheaper as they become more common.
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colh107

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Re: Resettable Fuses - Tutoring Please
« Reply #12 on: February 25, 2020, 06:46:11 PM »

Beware with PTC's - they appear great, but they have an innate 'on' resistance of maybe a few ohms - typically bigger the higher the current capacity. You can end up with quite a voltage drop. Don't just look at holding/tripping current - check what the datasheet says for its resistance, and apply Ohm's law.
There's 'solid state' [MOSFET] current limiters available - I don't know who sells such things for modellers, but I've used these IC's in designs at work, and they can limit without a significant voltage drop.

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microgyros

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Re: Resettable Fuses - Tutoring Please
« Reply #13 on: February 25, 2020, 07:08:36 PM »

Beware with PTC's - they appear great, but they have an innate 'on' resistance of maybe a few ohms - typically bigger the higher the current capacity. You can end up with quite a voltage drop...
The resistance is about one hundredth of what you are saying and the voltage drop is negligible in this application.
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colh107

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Re: Resettable Fuses - Tutoring Please
« Reply #14 on: February 25, 2020, 07:32:49 PM »

The resistance is about one hundredth of what you are saying and the voltage drop is negligible in this application.


If FET-based, yes. But if a plain simple cheap resistive 'PTC' then it can be significant. Not slating any specific suggestions on this thread - just saying watch out, and check the spec  ;)
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colh107

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Re: Resettable Fuses - Tutoring Please
« Reply #15 on: February 25, 2020, 07:40:15 PM »


If FET-based, yes. But if a plain simple cheap resistive 'PTC' then it can be significant. Not slating any specific suggestions on this thread - just saying watch out, and check the spec  ;)


I know we probably have more headroom in this hobby with battery volts over min circuit voltages - just saying, bear it in mind

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microgyros

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Re: Resettable Fuses - Tutoring Please
« Reply #16 on: February 25, 2020, 08:59:01 PM »


If FET-based, yes. But if a plain simple cheap resistive 'PTC' then it can be significant. Not slating any specific suggestions on this thread - just saying watch out, and check the spec  ;)
I disagree.
I say Pptc resistance is the equivalent to FET based solutions and with the advantage of being two terminal and easier to retrofit.
If you parallel connect Pptcs to the quivalent trip current of the smart FETs the resistance comes out the same.
I crunched those numbers and started using them before ROHS was introduced in 2006. The clear advantages of Pptcs is why I never sold FET based breakers but, instead, gave Pptcs out like sweeties.

To put it in a way for those who can be misled by emotive statements, I pointed to a RGEF300 equivalent as a suitable device for Trucker in the original thread. The normal working resistance is around 0.035Ω. That makes 0.1 volt drop and not noticeable compared to NiMH battery resistance. If you look inside Ansmann 5-cell sub-c NiMH packs what do you see? - answer (Pptc protection)

The anecdotal mention of unsuited 100volt devices without important data is not worth chucking in any more than for 100volt Fet. Unless you made assumptions and didn't allow enough time to check.
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colh107

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Re: Resettable Fuses - Tutoring Please
« Reply #17 on: February 26, 2020, 06:24:15 PM »

Not at all anecdotal - based on real-world customer designs. BUT, admittedly, with much different battery voltage, load current and circuit voltage. I went to a FET-based limiter because all PTC's lost me too much of my meagre battery overhead. (Thinking about it, I think it was having a 'tight' trip current that maybe made my life difficult in finding a PTC solution - e.g. spotting a LiPo charge current that has gone even slightly out of norm)
But hey, anyway, I wasn't looking for an argument.
I was just trying to point out the pitfalls of people picking 'any old thing'. If people follow your advice and experiences then they should be ok.
I'll just say, 'follow his advice and [especially] specific part recommendations'  :-)

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