Re: nice polyfuse

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