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In Reply to: RE: You sure about that?... posted by Steve O on January 09, 2017 at 20:28:44
This is pretty basic stuff. To establish the rated full load current of the windings of a two winding single phase transformer, take the VA rating of the transformer and divide it by the rated winding voltage.
For example, for a 50VA transformer with a 120V primary and a 6.3 volt secondary, the rated full load currents are: Primary, 50/120 = 0.42A, Secondary, 50/6.3 = 7.9A.
So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary in subtractive polarity (120-6.3 vs 120+6.3) and you needed it to be capable of supplying 3 amps to your load then the transformer would need to be rated 360VA minimum (120V x 3A) to have a primary winding capable of 3 amps.
To verify, I took a 135VA transformer, 120V pri, 6.3V sec. With my line voltage I got 128V additive polarity & 114V subtractive. Wired in subtractive polarity I connected a 60W light bulb and measured 474mA in the wire to the bulb. In the incoming wire to the transformer primary from the wall outlet I measured 480mA - a little higher as expected because of the transformer magnetizing current.
Follow Ups:
... "So, if you wanted to make a buck transformer out of this by connecting the secondary in series with the primary"This isn't a conventional circuit for a stand alone bucking transformer. Conventional arrangement places primary directly across the mains and the secondary in series with the load with the combo across the mains.
I too set up a circuit using a tiny 115V - 6.3V/1.5A filament transformer in buck mode supplying 100V/1.5A to a resistive load (150VA or W). Measurements were just as I described previously and the 10VA transformer handled the 150VA load for 1hr w/o issue. i.e. no transformer buzzing and normal operating temp.
Will edit in circuit diagram and meas later.
Test circuit:
Edits: 01/10/17
Yes, that would be great if you could post the circuit diagram as our discussion is a little in the dark without it.
Hi Steve. I'm not seeing your posted diagram but no matter. I have set up my buck circuit again & retaken measurements. Results - I'm more than a little embarrassed. My statement regarding calculation of rated winding currents of a transformer is correct - the rest, regarding current thru the primary 120 Volt winding in the buck configuration is not correct. I apologize to all. With the 60W 120 volt bulb bucked with the 120 / 6.3 V transformer to operate at 114V from the 122V incoming, the lamp current thru the 6.3V winding was 474mA and the current going into the 120V winding was 58mA. I'm scratching my head a little on this but again I apologize to all. Jack
I see your diagram - same circuit as mine. Thanks Steve.
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