[Amps] PS Theory ?

[Ian White, G3SEK]

Bill Turner wrote:
>On Wed, 27 Aug 2003 06:23:40 -0400, "Mike, K8LH"
><k8lh at kramerairtool.com> wrote:
>
>>How can you determine the power handling capability of a transformer???
>
>_________________________________________________________
>
>The power handling capability of a transformer in a steady-state
>condition is determined mainly by it's internal temperature.
>Excessive heat buildup reduces the breakdown voltage of insulation,
>both of the wires and the insulation between windings.
>
>If you know nothing at all about the transformer, I'd recommend
>letting it get no hotter than about 140 degrees F - slightly hotter
>than your hand can stand to touch it.  Transformers with ordinary
>insulation should handle that ok.  If you want to be conservative, run
>it even cooler and it will last even longer.
>
>For test purposes, a string of 120 volt light bulbs of the appropriate
>wattage can be used as a dummy load.  They have the advantage of
>increasing resistance with increasing current, so they have a sort of
>built-in regulation.  I'd recommend using a variac to bring the
>transformer up to full power to avoid the initial surge when the lamps
>are cold.  I have done this with transformers in the few hundred watt
>range, but never at the kilowatt level.  Be careful!
>

What Bill says is correct, but in the non-steady-state conditions of 
amplifiers, we also have to think about voltage regulation.

Voltage sag is almost always the practical limitation on the power you 
can extract from a transformer in a particular amplifier. When you 
increase the RF drive, the anode current goes up but the anode voltage 
comes down, and beyond a certain point the RF power output doesn't 
increase any more. In practical terms, you can't extract any more 
*useful* power from that transformer in that power supply in that amp.

In even more practical terms, you need to stay well away from that 
situation, because the amplifier is "limiting" and its IMD performance 
will be terrible!

A reliable way to estimate the performance of a transformer in a power 
supply is to use the 'PSU Designer' program. All you need are the 
off-load secondary voltage, the primary resistance and the secondary 
resistance, and the program easily lets you set up various kinds of 
rectifier/smoothing configurations - simply click and point to what you 
want.

Then 'PSU Designer' will accurately predict... well, just about 
everything you want to know about output voltages, currents etc.

The program was written by Duncan Munro for audio amps, but it works 
fine for our kinds of power supplies too - our numbers are just bigger! 
The program can be set up for 115/230V AC mains, 50 or 60Hz. There's a 
link from my website, at:
www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#psud

I've also included a high-voltage rectifier file that you'll need. Do 
read the Help file, particularly the sections on 'Modelling your first 
power supply', because it isn't entirely obvious where to click and 
where to double-click. But once you've got the hang of it, you're 
flying.

VERY highly recommended!


Returning to Bill's points about internal temperature, obviously these 
are very important, but they are more related to the question: "Will it 
work for a whole contest weekend without cooking the insides?"

'PSU Designer' will answer the question that comes before that: "Will 
this transformer be *any* use for my next amp?"


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek