Re: gEDA-user: Switching regulator question

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

On Sat, Jun 16, 2007 at 09:53:39PM -0400, DJ Delorie wrote:
> 
> > 24VAC RMS means 34V peak,
> 
> 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?  I
calculated a peak-to-peak voltage of 37.3V allowing for 10% head room,
and then when you subtract diode drops 36V ought to do it.  I'm
planning to put a 40V MOV or similar on there to handle transients, of
course, which will no doubt exceed the normal sine wave peaks.  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.  Or
else your mains is >110% of nominal even after line losses, or else
your rectifier has impossibly low Vf drops.  Or I guess the
transformer could have bad tolerances on its winding count.


> > so I would ideally aim to have the largest input ripple possible,
> > which coincidentally allows me to choose a smaller, cheaper input
> > capacitor.
> 
> What I did was put in four smaller caps in a row, which reduces the
> ripple current through each.  That helps keep cost down too.
> 
> 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?


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