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[AMPS] Transformer Current ratings

To: <amps@contesting.com>
Subject: [AMPS] Transformer Current ratings
From: jflanders2@home.com (Jerry Flanders)
Date: Sun, 16 Dec 2001 12:58:48 -0500
Thanks for the recommendation. "PSU Designer" is a pretty neat program. I 
downloaded it and also got the psu files from G3SEK's site and played with 
them. I can see where this program could really help.

One thing confuses - I have never worked with the "series resistance" of 
the capacitor. Is this a parameter I can measure on the bench? How? The 
help file doesn't.

I presume your article on PSU Designer is not on the web. I wish it was.

None of what has been discussed in this thread relates to how hot the 
transformer runs in service, but isn't that the _ultimate_ determinant of 
its KVA capacity?

Thanks to all who helped.

Jerry W4UK

At 03:00 PM 12/15/2001 +0000, Ian White, G3SEK wrote:

>Jerry Flanders wrote:
>>
>>For "ordinary" old power transformers:
>>
>>Is there a rule of thumb to help estimate transformer KVA capacity from 
>>the iron (or total) weight? If not, why?
>>
>>Prior to hypersil, weren't most transformers made from similar iron in 
>>similar configuration? If so, wouldn't capacity be directly related to 
>>core weight?
>
>Only indirectly. Weight is a rough indication of core cross-section, which 
>is important but still only one factor in the transformer's overall 
>performance.
>
>The rule of thumb was "Size of a six-pack = 500 watts, size of a 12-pack = 
>1500 watts, too heavy to lift = 2500+ watts!" This of course is very 
>variable, and older transformers tend to be bigger and heavier for their 
>power rating than modern ones.
>
>However, I have been bitten several times in the past by building a whole 
>PSU - or even a whole amp - around a transformer that passed the 
>rule-of-thumb weight test but was quite unsuitable.
>
>A *much* better way is to measure the winding voltages and resistances and 
>look at the electrical performance. Rich gives some useful electrical 
>rules of thumb, but before you commit to building a whole power supply 
>around a particular transformer, you'd do well to run it through the 'PSU 
>Designer' software.
>
>'PSU Designer' is Windows freeware from an audio firm called Duncan 
>Amplification and it's an easy way to put some real 21st-century accuracy 
>into PSU design. Duncan's amps use relatively small PSUs by our standards, 
>but the software  works just fine for big PSUs too.
>
>I wrote a short article on it in my monthly magazine column, and there are 
>links from: http://www.ifwtech.com/g3sek/in-prac/   Slide down to August 
>2001, where there are links to the download site and some extra files to 
>download.
>
>The extra files relevant to AMPS are the circuit files '3kV Bridge.psu' 
>and '3kV Doubler.psu', and you'll also need the expanded RECTIFIERS.TXT 
>file which contains a high-voltage rectifier.
>
>The two .psu files are a basis for modeling normal high voltage supplies. 
>Double-click on any item in the schematic to alter its properties, and 
>then click Simulate to get a simulation over a chosen time period. You can 
>change the load current by altering the current sink connected to the output.
>
>Then click various items on the form below to see graphs over time. The 
>most interesting item is probably V(C1) - the output voltage.
>
>As you get further into using PSU Designer, check the Help files for more 
>information.
>
>A note about transformer modeling: PSU Designer only lets you model the 
>secondary resistance of the transformer. You can include the effects of 
>(primary + mains supply) resistance by multiplying those values by the 
>square of the step-up turns ratio and adding the answer to the secondary 
>resistance. There's more information if you double-click the transformer 
>symbol, and then choose Help.
>
>This really is a great program! It takes some practice, but I wouldn't 
>ever build another PSU without running the components through PSU Designer.
>
>Coming back to Jerry's original question about current ratings, if the 
>winding resistances are low enough to give good voltage regulation at the 
>currents you want, you pretty much have your answer about current ratings too.
>
>--
>73 from Ian G3SEK


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