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Fume check

4/24/2000 10:04 PM
Don Symes
Fume check
Been playing with Duncan Munro's PSUD-II and have noticed something _very_ odd.  
A given choke-input supply yields about 310V.  
Adding a 1uf cap to the front of the choke takes it to about 325V. B+ rises (non-linearly) with the size of this cap all the way to 460V or so leaving all else alone, including the 100mA load.  
Is this a real effect, an error in PSUD-ii, or some combination? Did I just stumble on a secret way to set B+ over a >100V range?
4/25/2000 12:43 PM
Stephen Conner

Yes Don, it is a real effect. By adding a large capacitor in front of the choke, you are changing it back to being a capacitor-input filter, and you get the higher voltage and poorer regulation you would expect from this type of filter. The choke and second capacitor act like an additional filter stage giving you extra smoothing.  
Smaller values of cap give you a kind of halfway house between the two circuits. I suppose this would be a good way to adjust B+ by switching in various sizes of cap. To reduce surges and stress on the rectifier, it would be wise to pre-charge all the switchable caps by connecting 100K resistors across the switch contacts.  
P.S. Most choke-input circuits I've seen include at least a 1 uF first filter cap to damp ringing in the choke.  
Steve C.
4/25/2000 1:26 PM
Unfortunately the intermediate voltages that  
are available are poorly regulated. One way  
to think of it is this: the voltage drop in the choke depends strongly on how much ripple is  
in the voltage it receives... and with a small  
cap in front of it, that depends strongly on  
the current drawn from the supply.
4/25/2000 2:52 PM
Don Symes

I _knew_ it couldn't be all good.  
Thanks guys.

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