Path: ...!2.eu.feeder.erje.net!feeder.erje.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: Phil Hobbs Newsgroups: sci.electronics.design Subject: Re: nice polyfuse Date: Fri, 22 Mar 2024 01:54:02 -0000 (UTC) Organization: A noiseless patient Spider Lines: 107 Message-ID: References: <266nvilu01f0mf8n7kgl3rnbdh9ieqqtav@4ax.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Injection-Date: Fri, 22 Mar 2024 01:54:02 -0000 (UTC) Injection-Info: dont-email.me; posting-host="88b8030969698c63dc8cb4843c4a23b8"; logging-data="2688545"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX1/mof2Yt27/7xbnUKRyU0Rq" User-Agent: NewsTap/5.5 (iPhone/iPod Touch) Cancel-Lock: sha1:bOLIbmjFolKCDrGUFXzX2VhGfqY= sha1:UC2rvXWcx+HYH3zcYj6ni1z4e3E= Bytes: 5032 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. > > 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