On Sun, 15 Aug 2010 12:53:42 -0500, "Tim Williams"
wrote:

Yuck!

U1A is awfully slow (~500Hz?), U1D input is being overdriven, Q1 has weak
drive, yet a huge "speedup" capacitor, and U1D's output is pretty much
short circuited into C3-Q1 and C3-(diode). R3 and R4 hardly do anything.
Transformer polarity is not indicated, so we don't even know if you have
it backwards or not, but it hardly matters because D1 shorts it out anyway
(if not for R2-C2 narrowing the pulse width).

Q1's hFE isn't terribly high, and it's got 2N3904-ish current capacity.
It's no MOSFET. That transformer better have at least 2mH primary
inductance. If not, that would be why you observe constant voltage drop,
it's already in the constant current region.

Offhand, you could use a circuit like this:
http://webpages.charter.net/dawill/t..._Generator.gif
The toroid is probably #43. It should really be gapped, in which case a
much smaller toroid (or more likely, E or U core with coil former) will
work, with many more turns of course.

There are other circuits that would do a better job. A proper flyback
supply is easy to make, using a UC3842 in the appnote circuit, or a
(discrete or IC) BCM flyback. Royer oscillator also comes to mind:
simple, low noise, easy to regulate.

In any case, for a watt or two output, figure a thousand turns or so of
rather fine wire (#40). Something resonant or quasi-resonant will help
mitigate the winding capacitance and leakage inductance inevitable in such
a transformer.

Tim


A simple forware converter isn't a bad way to make a low-current HV
supply. It's quasi-regulated all by itself, since the hv Vout is just
the supply voltage times the transformer ratio. It needs to operate at
low duty cycles and short pulse widths, but that's OK for a
low-current HV app.

Note the 2M output resistor!

John