[Amps] Choosing a Plate Transformer

[Gary Schafer]

Hi Scott,

Here are some numbers to keep in mind when transformer shopping.
The full load voltage with a bridge rectifier will be 1.25 x the transformer
ac voltage. With a doubler it will be 2.5 times the transformer ac voltage.

The voltage that your filter capacitors must withstand is the no load
voltage. No load voltage will with a bridge rectifier will be 1.4 times the
transformer ac voltage. With a doubler the no load voltage will be 2.8 times
the transformer ac voltage.

The current rating of the transformer can be pushed some on ssb from the
continuous rating. But a transformer rated at 800 ma with a bridge can only
supply 400 ma with a doubler. It is a mater of total power delivered.

Easiest way to keep track of transformer power is to look at the ac voltage
and current rating. If it is say 800 volts at 1 amp that is an 800 watt
transformer. With a full wave rectifier you will get 1000 volts full load
out of it. You can draw 800 ma from it to equal the 800 watts of rated
transformer power.
With a doubler on the same transformer you will get 2000 volts at full load.
You can draw 400 ma from it to again give you 800 watts.

Newer transformers have lower resistance in the windings and have much
better voltage regulation than the old stuff with high resistance windings.

The MFJ catalogue has some very nice transformers in their catalogue that
should be just what you are looking for at reasonable prices.

73
Gary  K4FMX


> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of KB0NLY
> Sent: Tuesday, March 10, 2009 11:30 AM
> To: amps at contesting.com
> Subject: [Amps] Choosing a Plate Transformer
> 
> I'm thinking about slowly scrounging parts to build another amp in the
> future, it will be a ways off so in no hurry, and i was wondering how one
> chooses a plate transformer.
> 
> I know the minimum i'm shooting for, 2500v 600mA.  This will be to run a
> single GI7B.
> 
> I was thinking about using those Far Circuit boards for constructing the
> doubler and rectifier.  It looks like the average factor is 2.8.  Which
> means for 2500v i would need a transformer with a secondary of at least
> 893 volts (rounded up).  But what should i do on the current capacity
> side, should i look for something as high as a 1 amp transformer?  Most of
> the ones i am finding on eBay for example are 500mA or smaller.  I could
> probably get away with .500 mA.
> 
> The question is, does one loose ampacity with the double circuit, or does
> the current remain constant?  I can't seem to find any notes in the
> handbook articles on the subject of choosing a transformer.  This is why
> the last amp i built i used a donor FL-2100B so i wouldn't have all the
> questions on constructing the power supply.  But the next time around i
> want to build the power supply from scratch and learn more about HV
> supplies!
> 
> The 6M amp i just built is working great by the way.  It will do 500w but
> i keep it around 400w.  I checked into a net this weekend that a nearby
> club holds on 6m, they were suprised how loud i was as i'm usually the
> most distant and sometimes one of the weekest for them to hear.
> 
> 73,
> 
> Scott KBØNLY
> 
> www.kb0nly.net
> 
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