Electronics > Beginners
Transformer Ratio Design Tip
ArthurDent:
Where I used to work they made instrument transformers to measure current going through high tension lines. The smaller transformers were toroidal ‘donuts’ with the current carrying conductor going through the center of the transformer and the ratio was determined by the number of turns on the core (secondary). Sometimes these ratios didn’t work out quite right so one of the engineers came up with an idea which he patented. Say you needed 50.5 turns to get the ratio you needed. What was done was the secondary was wound with two windings, one with 50 turns and another with 51 turns and these were put in parallel to give the equivalence of 50.5 turns. It might be a little more inefficient because of the small difference in voltage between the two windings, but it was simple. Of course these are line frequency iron core transformers and not R.F. but it illustrates another way to get odd ratios.
T3sl4co1l:
Uhhh yikes! That's not how that works!
Erm, if you mean "in parallel" as in feeding a common burden resistor, with resistance between windings, or not actually on the same core, that will work however.
Possibly the windings had enough resistance to begin with, that external resistors weren't needed.
In any case, that sets up a circulating current of one turn over, whatever the impedance of that loop is. Which acts in parallel with the burden resistor (if this is all connected normally). So it's like getting 50.5 turns but you get a burden resistor that's 2% lower than you thought (say if the windings are ~25 ohms each and the burden is 1 ohm).
Tim
Wimberleytech:
Hmmm...I checked out at NON-RPN...lol
ArthurDent:
--- Quote from: T3sl4co1l on February 19, 2020, 03:27:13 am ---Uhhh yikes! That's not how that works!
Erm, if you mean "in parallel" as in feeding a common burden resistor, with resistance between windings, or not actually on the same core, that will work however.
Possibly the windings had enough resistance to begin with, that external resistors weren't needed.
In any case, that sets up a circulating current of one turn over, whatever the impedance of that loop is. Which acts in parallel with the burden resistor (if this is all connected normally). So it's like getting 50.5 turns but you get a burden resistor that's 2% lower than you thought (say if the windings are ~25 ohms each and the burden is 1 ohm).
Tim
--- End quote ---
There were indeed 2 secondary windings wound together turn by turn on the same core with the ends of the secondaries connected together and no resistors other than wire resistance in the secondaries. Like I said, losses were a little higher but it worked. That is also basically the description you gave of the variac where you have one or more turns connected to the same point (through the graphite wiper). The patent dated from the 1940s or 1950s and I don't know of any production transformers ever being made.
There have been other schemes including one described in this patent from HP called "Transformer providing half-turn secondary windings". https://patents.google.com/patent/US3768055A/en .
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