As promised, attached is my “crowbar” circuit I mentioned a few days (weeks?) back. I could not find my original design, so this is what I recollect from memory. The circuit draws no power (just the reverse bias current through the zener and R – not enough to trigger the SCR.) Then if the power supply voltage exceeds 6 volts, the zener conducts hard, there is sufficient current to trigger the SCR, and when it does, it shorts the 5V supply to 0V and blows the fuse (or circuit breaker if you like.) I’m confident that put on the output of almost any switching supply (don’t use the fuse), the short will shut down the power supply. I know I inadvertently shorted the 5V on one of my MeanWell QP-200F’s and it shut down instantly - I had to turn it off and back on to reset it back to normal operation. The zener is not particular, 1/2W will do. R, if I remember correctly was perhaps 1K. The SCR has to be rated for at least the maximum output current of the power supply. But since its power dissipation is non-existent, and when it triggers, though it may pass 20 or 30 Amps momentarily, the time is so short that there is no heat sink needed. I believe I will build this soon and test it. Then I’ll most likely put it on the output of my MeanWell PSUs and feel just a bit safer running my S-100 systems on 5 Volts without on-board regulators. Bob Bell From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of J. Alexander Jacocks John, Yeah, I read your advice, and I have a 7.5 and a 15vdc Meanwell PSU. I was just hoping to find a smaller combination unit, as I am trying to create a custom small S100 chassis. Thanks! On Mon, Feb 16, 2015 at 12:11 PM, John Monahan <mon...@vitasoft.org> wrote: On S100 bus power supplies, unless you are extremely energy conscious it’s easier to supply S100 bus motherboards with 8V and 16V switching power supplies rather than 5V & 12V and pulling each and every voltage regulator on every S100 board. A suggested source is here:- http://s100computers.com/My%20System%20Pages/The%20Box/The%20Box.htm#The_Power_Supply there are countless others. I would also be careful getting a “cheap” 5V unit. If the is a glitch to a higher voltage, there goes each and every board in your S100 box if you don’t have a V regulator on each board. John From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com] On Behalf Of J. Alexander Jacocks I have noticed several folks are using the Meanwell QP-200F PSU, recently. I looked it up, and it is a bit expensive, but it appears to be much smaller than my current 8/16vdc pair of units. I assume that the unit can be adjusted for the proper voltages? Do folks who have them think they're worth the money? Thanks! - Alex -- -- -- |
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Over-voltage Crowbar.pdf
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