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From: Klaus Vestergaard Kragelund <klauskvik@hotmail.com>
Newsgroups: sci.electronics.design
Subject: Re: nice polyfuse
Date: Sun, 31 Mar 2024 01:35:32 +0100
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On 30-03-2024 17:22, John Larkin wrote:
> On Sat, 30 Mar 2024 12:14:47 +0100, Klaus Vestergaard Kragelund
> <klauskvik@hotmail.com> wrote:
> 
>> On 22-03-2024 02:16, John Larkin 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.
>>>
>>
>> SW protection is nice, but what happens during power up or when the
>> customer only applies power to the relay section, not the unit power.
>> Then there's no protection.
> 
> I'm designing a FITS module, a fault insertion box, aka guillotine
> box. If someone has two boxes that are connected by a cable, they can
> chop the cable in half and run the hacked ends through the FITS
> module. Now they can route all the signals through, open any, or short
> any to any other. That's the classic function. I'm adding shorts to
> ground, soft ground faults, and current measurement through any wire,
> and voltage measurement/waveform acquisition between any two wires or
> any wire to ground.
> 
> So I have two current shunts that can be inserted anywhere. I'm
> protecting every one of my connector pins with a polyfuse, so nobody
> blows traces off my board, but the polys work too slow to protect my 1
> ohm 5 watt WW shunt resistor.
> 
> We have an isolated ADC to measure the voltage across the selected
> shunt, an ADUM7703. So our FPGA can sense overload on the shunt and
> open the series relay before the resistor fries. The SMW51 5 watt
> wirewound is rated for 8 kilowatts for 1 millisecond. Some FPGA
> algorithm should mostly protect the resistor and various relay
> contacts.
> 
> The classic FITS module, designed by my customer, has become
> impossible to make now, and had no protections hence lots of relay
> failures. We'll include BIST.
> 
> Big companies used to design their own test gear but the guys who did
> that have retired and weren't replaced, and kids these days know how
> to type but not solder.
> 

I was thinking, that you could do a solid state switch with current 
sensing for each wire, but that would probably be a nightmare in time 
and parts.

A PTC should work, if you have a trace which is a couple of mm wide, it 
can handle more than 10A at 60degrees delta. So you should be able to 
find a PTC that will protect the trace (I am guessing this is low 
voltage DC)


>>
>> What do you need the series resistor for, maybe I missed the point?
>>
>>> 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.
>>>
>>> If I want to keep blowing things up, which I do, I need a giant power
>>> supply. This looks good:
>>>
>>> https://siglentna.com/product/sps5082x/
>>>
>>
>> Looks nice. Now I have to buy one, hoarding instruments you know.
>>
>>> That ain't cheap, but we can expense it and get basically a 45%
>>> discount from tax savings.
>>>
>>> The user interface looks typically bizarre.
>>
>> I have a lot of Siglent stuff.
>>
>> https://www.electronicsdesign.dk/tmp/Lab2023.jpg
>>
>> Good instruments, but horrible PC software.
> 
> We use their dummy load boxes a lot, and my kids have written a Python
> library to control them in test racks. I'll probably use the giant
> power supply manually, mostly to blow things up. I could add an
> outboard mosfet switch to apply programmable pulses to victims.
>