[sdiy] Integral scope on VCO

[Thomas Dunker]

On Tue, 13 Apr 2004, Tony Clark wrote:

>    It's quite possible to make very tiny high voltage supplies for your
> application.  I just finished designing just such a thing for a product
> my company is making.  I was able to come up with a design that
> will generate a good 1200V DC (approximately) and fit in a 2x2x0.5
> space.  It runs on 12V at 120mA and the best part is the total parts
> cost is less than $10.00 (in quantity, probably about $15.00 for a 1-off)
>    Don't know what kind of current the CRT requires, but the power supply
> will definately give you a good zap!

 As far as I can tell from the data sheets on the CRT, the beam current is
virtually in the microamps range, but I also need a reasonably high
voltage supply for the deflection amplifiers (need about 250V pk-pk for
full deflection on either axis) and it seems most realistic to get the
supply voltage for these amp stages from the same supply. The deflection
amps can be high impedance things so don't really need to take a lot of
current. I just emailed a Norwegian ham who built a scope module with this CRT,
so it'll be interesting to find out how he did the deflection amps. I thought
about using diff stages with 12AX7 type tubes which work fine at 0.5mA per
section, so two diff stages would typically draw about 2mA. But for the
sake of saving space it would be nice if I could use transistors in the
deflection amps. I don't know precisely how that would work out in terms
of current consumption. I may have to sink some current into a voltage divider
and some pots and things on the same supply, unless I can get a HV
transformer with sectioned secondary and make a stacked supply. In any
case I will be needing something like -50V, 0V, 500V and 800V taps.

 It's not like I don't have high voltage transformers in my stash, but
they do take up a lot of space (relatively) and so do high voltage filter
caps when they have to take care of 100Hz ripple current. It's cool when
it's a classic tube amp for the stereo where size and weight doesn't
matter (to me, anyway) and one can liberally build pi filters with 10 henry
iron core chokes and that sort of thing, but this is a little different...

 I was looking at some high output voltage DC-DC converters in Farnell's
catalog, but they were far too wimpy on the current for my purpose. We're
probably looking at a 10-20W supply, whereas the module available from
Farnell was good for 1 watt...

 A buddy at work says I'd probably have to use a flyback type converter if
I want to go that way. I'm just a total idiot when it comes to switched
power supplies, since I've always avoided them in audio apps, but for the
scope they'd sure come in handy. It would be a good excuse to learn a
little more about them... I guess if I need multiple voltage taps I could
feed like three separate ferrite core transformers from the same switcher
thing? Oh, and it would definitely be most convenient for me to make an
AC/DC supply with 230VAC input at this kind of power, otherwise I'd eat
away at the synth's low voltage supply's available current real fast.

 Can you say a little more about the type PSU you had in mind?

Thomas