Path: Xl.tags.giganews.com!local-2.nntp.ord.giganews.com!nntp.supernews.com!news.supernews.com.POSTED!not-for-mail NNTP-Posting-Date: Thu, 21 Mar 2024 23:27:14 +0000 From: boB Newsgroups: sci.electronics.design Subject: Re: nice polyfuse Date: Thu, 21 Mar 2024 16:27:13 -0700 Message-ID: References: <266nvilu01f0mf8n7kgl3rnbdh9ieqqtav@4ax.com> User-Agent: ForteAgent/8.00.32.1272 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Lines: 47 X-Trace: sv3-KyNDcZhoYGxzsz1otZhwhXot12QAmaulo2NGqC5BYiRnta6A3wwaVflTVBVpjZPnhK3HgJoiqiG5t7W!vMfW7e/T9D7UlGTB9TaIt1a+LSANYQ/DQ2e7v1Y42aw7wWdovxrQvPw2JDcHYs1G33wMuBYZZbjj!w+SJ4A== X-Complaints-To: www.supernews.com/docs/abuse.html X-DMCA-Complaints-To: www.supernews.com/docs/dmca.html X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 Bytes: 2790 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 ? If 1 Ohm 5 watts limits the voltage across the PTC then it's probably good. 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