> > HA! 24VAC means 38V peak (i.e. after the bulk caps) on my board. Had > > to change modules because I designed for 34V. > > Eeep, that concerns me. What's the Vf drop on your rectifier?
Under a volt. KBP201G The first modules were Acon's CX13S2405V's, input 18-36v. Had to switch to CX13S6005V's, input 20-60v. > I calculated a peak-to-peak voltage of 37.3V allowing for 10% head > room, Here's your mistake. Allow for more headroom. A lot more. Power supplies are not the place to be "just over spec". The input voltage should end up in the *middle* of your allowed range, not at the edge. > I wonder if your HVAC system might be wired wrong -- most of them > have two ways to hook up their 24V transformer, one for 208V and one > for 240V, I think. You might be running 240V into the 208V > terminal. I'm running 120v into the 120v terminal. USA wiring. > Or else your mains is >110% of nominal even after line losses, Nope. Actually, it's a little low. > or else your rectifier has impossibly low Vf drops. Not impossible, but it is low. > Or I guess the transformer could have bad tolerances on its winding > count. Bingo :-) (or it's designed for a much higher current load, since it's also driving all the furnace bits - it's 24v nominal, but higher when unloaded, so I have to plan for fully unloaded AND fully loaded) > > http://www.delorie.com/house/furnace/pcb2/board-full.html > > Wow.. 4x820uF = 3.3mF total input capacitance. That's motor-start > capacitor territory there. How much current are you intending to > draw? The supply is designed for 2.6 amps output, or about 0.5 amp input. I'm probably only using about half an amp output on average. The design rule was "I have a half an inch strip. How much capacitance can I squeeze into it?" ;-) At first, I had four 470uF caps but then I found those tall skinny ones. Plus, the bigger caps have better ripple current ratings. _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
