Re: gEDA-user: Switching regulator question

[DJ Delorie <dj@xxxxxxxxxxx>]

> > 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.


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