On Tue, Feb 11, 2003 at 12:39:12PM -0800, Robert Ritchey wrote: > Thanks for the feedback on my options. I am not familiar with the > Triac/zener circuit. In fact, I have never used a Triac. Is it just > as you describe it below, Polyswitch in, Triac with a 4.5V (or close) > zener on the gate with a 1nF cap?
Right. (Low power) Zener between A2 and gate, 1nF between A1 and gate. Very simple. Your circuit would never get killed; but however the wrong power supply could be cooked. Dimensioning for 5 volts +-10% is difficult. Regulated power supplies delivers 4.5 to 5.5 volts; choosing 5.1 zener +0.2...1.3 volts Triac threshold will result in 5.3...6.4 volts for the triac getting fired. So your device could be switched off from your normal supply too. I would choose a 5.6 zener. (check the datasheets and the tolerances of zener and triac). M. > > At 12:14 PM 2/11/03 -0800, Matthias Weingart wrote: > >On Tue, Feb 11, 2003 at 07:59:27AM -0800, Robert Ritchey wrote: > > > >The solutions with zeners and polyswitch are not the good way. I tested it > >with different currents and there is a big range of current that can flow > >and dissipate too much power in the zener. The zener will blow and convert > >in most cases to a "permanent crow bar"; you need to replace the zener. The > >solution with a fuse and zener is better, but the user/service also have to > >exchange the fuse. > > > >My favourite solution is a combination of Polyswitch, Triac and Zener at the > >gate of the Triac. This will protect from reverse power (<1.5V) and > >overvoltage (zener voltage + 1V triac gate threshold). Do not forget a > >capacitor (1nF?) from gate to ground, to protect from firing because of fast > >rising input voltages or very short spikes. In case of fault, the current > >flowing is only at the rated level of the Polyswitch and the power in the > >triac is very low (1.5volts * polyswitch current). > > > >I remember there was a component called TRISIL that worked in this way. > >But I think this is no longer available (why?). > > > >However I would always use a regulator on the board for the 5 volts +-10% > >for a wide input voltage range and use a unregulated wall socket. That also > >solves problems with the sometime quite high voltage drop over the poor > >poor DC plugs. > > > > Matthias > -- Author: Matthias Weingart INET: [EMAIL PROTECTED] Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
