68.3.231.220
In Reply to: RE: Heater Current and Transformer Ratings posted by dbishopbliss on May 04, 2009 at 17:01:40
Most transformers are rated into a resistive load. This means that yes, the filament is easy. Add up the currents with 20% margin for line and part variation and you are done.
The B+ is harder. The rms of a capacitive input filter can be many times the output current. Duncan Amps PSUD can help with this. An easy way to figure the RMS current from PSUD is to take square root of the peak current times the average output current and divide it by 1.5. This isn't 100% accurate, but it is good enough.
Example: 100mA average out with 2.5A peak = sqrt(0.1 * 2.5)/1.5 = 0.33A rms
If the layout is crowded or you want the transformer to run cool, you'll want to over size it quite a bit. If you've got more than an inch all the way around the transformer, oversize it just a little (that is if you don't mine the transformer being too hot to touch.)
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.. . .Oh!. . .Remember: Modifying things voids their warranty.
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