[Amps] HV PS transformer

[Paul Decker]

Hi Ray, 



Interesting, I don't know if he has a kit for this or not, I haven't found much.   It is pretty basic design and the parts are completely un-obtainable so I've had to do quite a bit of rework. 



At first glance, the transformer seemed very unsafe to me, but it's not my area of expertise by any means.   One change that I was thinking about doing was to use RG-58 coax cable rather than the 3/8 ID pipe.  If I could wrap the coax through the core material, all I would then need to do is "short" the braids to each other reduce the number of primary turns.    This technique is more in line with an RF transformer anyway and the RG58 coax is rated at 1900V,  but I still think you would be hard pressed to get a 10:1 turns ratio through those small inductors. 



I'm going to do some experiments with my square wave generator (lower voltages) to see what sorts of turns ratio I need, I assume that the regular turns ratio equation holds true for square wave, and that it's linear i.e. 5v in 10v out is the same number of turns as 50v in for 100v out. 



Anyway, thanks for the insight and confirmation on the transformer.   I'm sure there is a way to make it "safe", I just need to find the right magic. 



73, 

Paul 






----- Original Message ----- 
From: "Raymond Mack" <ray.mack at sbcglobal.net> 
To: "Paul Decker" <kg7hf at comcast.net>, amps at contesting.com 
Sent: Wednesday, March 11, 2009 12:16:51 PM GMT -05:00 US/Canada Eastern 
Subject: Re: HV PS transformer 



Paul: 

If this is the article I remember, I have serious issues with his transformer design.  Is this the one where he also sells a kit to replace the supply in SB220 and SB200? 

Your evaluation is correct; you need significant voltage capability for the wire.  First you need to wrap the cores with tape to isolate the primary from the cores.  Second, you need to wrap the primary to get the isolation between primary and secondary.  Then you will need to probably wrap each layer of the secondary (if you have multiple layers) to isolate the windings. 

There are very good reasons why there is so much insulation in a commercial transformer.  I was doing a switcher type power output stage at 100 kHz and about 500W at work.  I was doing a quick and dirty prototype using magnet wire on a toroid.  We very easily got above 1500V across the toroid.  The end result was current completely blowing through two layers of magnet wire insulation (one layer on each conductor) and arcing like crazy. 

Ray 




From: Paul Decker <kg7hf at comcast.net> 
To: amps at contesting.com 
Sent: Wednesday, March 11, 2009 11:02:08 AM 
Subject: HV PS transformer 




GM gents, 


Part of my (3cx800A7) amplifier project is to do something "new".   Well OK, it's old, but new to me. 



I'm trying to reproduce the switch mode power supply outlined by Tim Hullick W9QQ (SK) in Feb 1991 QEX.   Things are fairly strait forward, but I'll throw a question to the list. 



In the article Tim says:   "...The secondary winding consists of as many turns as required, looping through the pipes to provide the necessary secondary voltage.  For any high secondary voltage, Teflon (r) - insulated hookup wire should be used because of it's superior insulating properties at high voltages.   Any small gauge Teflon wire can be used as long as the required number of turns will not be too tight in the pipes.   I used standard no. 20." 



Q.  So does that mean the secondary wire needs to have a minimum insulation voltage rating of the secondary voltage i.e. greater than 2200V? 



I have found Teflon coated wire with that high of insulation ration, however I have doubts he could get 50 or so turns through a 3/8 id pipe. 



Q.  Any ideas on what Tim might have been getting at and what suggestions for wire and sources? 



BTW, parts placement is going along, what a chore to get everything to fit! 



Thanks and 73, 

Paul kg7hf