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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: nice polyfuse
Date: Fri, 22 Mar 2024 01:54:02 -0000 (UTC)
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John Larkin <jl@997PotHill.com> wrote:
> On Thu, 21 Mar 2024 16:27:13 -0700, boB <boB@K7IQ.com> wrote:
> 
>> On Wed, 20 Mar 2024 19:38:38 -0700, John Larkin <jl@997PotHill.com>
>> wrote:
>> 
>>> I've been designing relay-matrix switch modules (how the mighty have
>>> fallen) and I don't want the customers igniting my PC boards or
>>> welding my relays by ignoring our 2 amp max current spec.
>>> 
>>> Polyfuses are usually terrible, but this Yageo part is pretty nice.
>>> 
>>> https://www.dropbox.com/scl/fi/w7x8rvqgrdua8boqmxg7y/BK60_1-1955033.pdf?rlkey=xpavzb8b8movr2xd4o5amkvx9&dl=0
>>> 
>>> https://www.dropbox.com/scl/fi/czk9ids5bj68ytcimcrb5/BK60.jpg?rlkey=77lrxc915it7y083quld9ectb&dl=0
>>> 
>>> It (slowly) trips at 2.5 amps in still air at room temp, 3.2 amps with
>>> some air flow. It survived 120 volts DC, which is all I had available
>>> on my bench, pulling about 25 mA. 
>>> 
>>> The good part is that its cold resistance is only about 0.07 ohms.
>>> 
>>> The next question is, if I put it in series with a 1 ohm 5 watt WW
>>> shunt resistor, does the poly protect it from, say, a stiff 60 or so
>>> volt source?
>>> 
>>> And does it absolutely protect an inner-layer 50 mil wide 2 oz PCB
>>> trace? I need to do a multilayer board that's 1 oz on the outsides,
>>> for BGAs and stuff, but the board houses don't mind making all the
>>> inner layers 2 oz copper.
>>> 
>>> 
>> 
>> The series power resistor idea is interesting.  Are you wanting to put
>> the PTC in a circuit with voltage above its rating ?
> 
> I plan to spec the instrument for 2 amps and 60 volts max, which is
> the poly rating, but I did verify that the Yageo part survives 120
> volts.
> 
>> 
>> If  1 Ohm  5 watts  limits the voltage across the PTC then it's
>> probably good.
>> 
> 
> My intent was to have the polyfuse protect the 1 ohm current shunt
> resistor, not the opposite.
> 
> https://www.dropbox.com/scl/fi/7efsvz7ba7wq4ebdxmpcp/P948A4_Shunts.jpg?rlkey=3sw5o5j2uxjnmog4md8lrgisu&raw=1
> 
> 
> I connected the poly and the 1 ohm 5 watt WW in series and then
> connected them to a 60 volt, 5 amp power supply. The resistor smoked
> and then unsoldered itself and survived. The next idea might be to put
> a couple of giant diodes across the resistor. Maybe the poly will blow
> them up too.
> 
> I will have a series relay to engage the 1 ohm shunt, and an ADC
> across the shunt to measure current, so we could software protect it,
> open the relay before the resistor falls off the board.
> 
> Or an optocoupler across the shunt to sense too much voltage.
> 
> 
>> 0.07 Ohms, cold, is good.
>> 
>> We use a 250V 1/2 amp PTC as well as the 1206 size ones.
>> 
>> One problem we had with a 1206 15V PTC was that sometimes (rarely) it
>> would burn through and short to the next layer down which was 5V.  A
>> thicker PCB should fix that as well as getting rid of copper just
>> below the PTC on the next layer down.
>> 
>> boB
> 
> We found the surfmount polyfuses to be really bad.
> 
> Polys are interesting. Given constant current, at some current they
> begin a slow self-heat thermal runaway and (eventually) go hi-z into a
> basically constant-power mode with surface temp around 100c.
> 
> I think that with a constant voltage drive, they become a sort of
> constant-temperature regulator.
> 
> I did find that if you run them hot for a while, their cold resistance
> goes up, permanently.
> 
> I wish there was a really good 2-terminal current-limiting device. A
> real fuse does that, once.
> 
> The real surface mount fuses are bad too.
>

One approach is to increase the thermal coupling between the resistor and
the polyswitch. 

Cheers 

Phil Hobbs 
> 
> 



-- 
Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /
Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics