[sdiy] Integral scope on VCO

[Tony Clark]

>  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...

   With modern switching techniques, you can use much smaller caps 
because the switching frequency is typically 18KHz or higher.

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

   Sure, I tried about everything suggested in other posts, from using 
mains transformers in a reversed configuration (works, but draws a lot of 
power, but useful if you want to get higher currents out).  And I did 
look at flyback transformers, but I could not for the life of me find 
anyone that makes those things.
   Instead, I discovered that there's a much more common circuit that is 
being used that can be modified to give very nice results.  This type of 
supply is used primarily for powering florescent backlights.  So doing a 
search on CCFL (cold-cathode florescent lighting) power supplies yielded 
a lot of useful information.
   The main problem is that the output from these supplies is AC.  Take a 
look at this one:

   http://www.integritytechnology.net/CCFL/CCFL-circuit-BOM.gif

   As you can see, the two transistors switch the power through the 
primary side and generate a massive HV AC signal on the secondary side.  
   If you modify this circuit a bit, you can optimize it for getting DC 
out.  The way I did this was to inject 12V into one end of the primary 
(not the center tap), and use a mosfet at the other end of the primary to 
switch it to ground.  Drive the mosfet with a 555 timer that has as 
narrow a pulse as you can get and then tune the frequency of the 555 
until you achieve the lowest power consumption.
   On the output, just use the common voltage doubler/tripler technique 
to get any number of HV DC out.
   The real trick is to get a transformer that has the proper turns 
ratio, AND has as high a primary impedance as possible.  I found this to 
be a serious problem, but fortunately I found an excellent supplier:

   http://www.jwmiller.com/pdf2/PM61300.pdf

   Check out PM61300-5, it has turns ratio of 125 and a primary impedance 
of 2.34 DCR.  The cool thing, it's $4.71 from Digi-Key, so it won't break 
the bank.

   Hope that helps,

   Tony

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