Path: ...!news.mixmin.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Klaus Vestergaard Kragelund Newsgroups: sci.electronics.design Subject: Re: nice polyfuse Date: Sun, 31 Mar 2024 01:35:32 +0100 Organization: A noiseless patient Spider Lines: 169 Message-ID: References: <266nvilu01f0mf8n7kgl3rnbdh9ieqqtav@4ax.com> <4q9g0jd8odj3tamgprio52hk2u6vk0a1pl@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Injection-Date: Sun, 31 Mar 2024 00:35:32 +0100 (CET) Injection-Info: dont-email.me; posting-host="efd883c2856d1e8e5179d830abba6038"; logging-data="1387032"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19+c7CG/s6GcA3VF+VGZzUZEBWfNoicvtY=" User-Agent: Mozilla Thunderbird Cancel-Lock: sha1:LJdi8ik12PuEqq/C4JSyRIelpTM= Content-Language: en-US In-Reply-To: <4q9g0jd8odj3tamgprio52hk2u6vk0a1pl@4ax.com> Bytes: 8118 On 30-03-2024 17:22, John Larkin wrote: > On Sat, 30 Mar 2024 12:14:47 +0100, Klaus Vestergaard Kragelund > wrote: > >> On 22-03-2024 02:16, John Larkin wrote: >>> On Thu, 21 Mar 2024 16:27:13 -0700, boB wrote: >>> >>>> On Wed, 20 Mar 2024 19:38:38 -0700, John Larkin >>>> 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. >