Quoting Philip Pemberton : > I had a play with WEBENCH, and the stupid thing kept crashing out > (usually taking Firefox with it). The parametric search thing isn't much > better... Webench is actually awesome, but I've noticed that the latest release = of Firefox has been crashing quite often. I really like Firefox, but = don't know how to stop the automatic updates, as I'm not an early = adopter of any technology. But I digress. I'm on Webench right now, = and it works fine. It tells me that the lowest-BOM-cost circuit is = for a LM25085 circuit @ $2.69 ea. f/1K units. BTW, National's support is awesome. Even when I've told them in the = past that I was building one unit for myself, they were very helpful = in explaining things to me. > ... > At this point I seem to have a few options: > > - 5V 5A external PSU. Cost ~=A325, comes with standard power plug. > Problem scenario 1: someone plugs in a 12V PSU and blows the power > sequencer / SMPSU chip and potentially the FPGA and PIC too (if > the FET shorts out). Not good... I really wouldn't do this -- I've learned that now matter how strong = the warnings, if there's a chance that some user will wire up = something incorrectly, they will. > Problem scenario 2: PSU plugged in has the wrong polarity. Same > result as above. A nice big Schottky diode should stop this > happening... Reverse-polarity protection is easy. Not your real issue here. > Potential solution: custom power connector. I don't even want to > think how much THAT would cost. Why not? I generic molex type connector with pins should be under = US$1. The labour (<-- UK friendly spelling) to wire it up will be more. > - 12V 3A external PSU and LM2678. Cost ~=A325 for the PSU brick, =A312= ish > for the National SMPSU chip and support components (and an RPP > Schottky). > Problem scenario 1: Someone plugs in a 12V PSU with reversed > polarity. Add a Schottky diode across the power plug to push the > PSU into short-circuit protect mode. Should work as long as the > PSU is smaller than ~50W (i.e. not a Nemic-Lambda EWS600 or > something equally brutish). Why short-circuiting for reverse protection? Why not just inline? Option 4: How about a small circuit (PIC 10F? with zener regulation = perhaps) that comes on when the power connector is plugged in. With a = voltage-divider, it can monitor the power actually plugged in, and if = correct, switch on the power to your main circuit. Haven't actually = tried this, but it works awesome in my head right now. :) > In an ideal world I'd buy a box of Cisco ADP-20s and use them -- they're > ... Not familiar with those, but also look for PS'es from oil inkjet = printers -- they put out quite a bit of power. >> Alternatively, isn't there some type of protection mechanism you could >> put in? Or perhaps change to a custom power connector? > > ... > To put it simply: I don't want to have units coming back from the field > that have been damaged (needing a new crowbar SCR, Zener, etc counts as > "damaged") just because some complete fool plugged the wrong PSU in. > ... But how about a simple fuse. Design for say 3.3V, make it 5V = tolerant, but at 5V, there'd be enough current to pop the fuse. = Again, I haven't drawn this up, but off the top of my head I can't see = why it can't work. How many of these thingamajigs are you planning to make anyway? Cheers, -Neil. -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist