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i have an EI core transformer with a 3.15-0-3.15 secondary whose rated amperage is 7.5 amps, whose three wires are 14 gauge. i have a toroidal transformer with a 3.15-0-3.15 secondary whose rated amperage is 4 amps,
whose three wires are 14 gauge, this toroid also has a 0-6.3 v secondary
rated at 4 amps whose two wires are 20 gage. can some explain why such divergent ratings/wire gauges ? also what maximum operational amperage should be be expected from the three windings?
Follow Ups:
There are other factors to consider besides current density in secondary wire.
First is maximum copper loss - a transformer design may be close to maximum loss, so one winding is changed to reduce it.
Regulation depends on wire resistance, and a particular winding may need better regulation, so would use heavier wire. Toroids have a longer wire length than an EI transformer and may use a heavier gauge for the same rating. If a 6.3V winding ended up with an odd number of turns, then a 3.15V winding would either be too high or low by half a turn. Using heavier wire would give less voltage drop, staying within rating.
Another reason is safety. A unit may be required to pass a safety test where each transformer secondary is shorted; it passes if the primary fuse blows each time. The smaller the secondary, the poorer the coupling and the less primary current that's drawn. Oversize wire may help, as may changing the winding sequence of the secondary layers.
The lead size does not necessarily match the winding size. Most transformers are wound with 700cm of copper per ampere. Range from 500cm to 1000cm has been used over the years. The winding current rating is same as what the transformer manufacture specified.
When some DIYers want to push to limits, should have at least 6 volts with connected load and many consider that voltage under load the safe outer limit, but opinions will vary. Personally, I usually do not push more than 10-15% over specified current specifications. And, I never push HV secondaries. I prefer to load to 50% of the HV current rating and especially so with capacitor input filter.
so you are saying that the EI core trans was designed at about 500 cmil
per amp, but the toroidal was designed at 1000 cmil for the center taped filament and at 500 cmil for the other filament. it seems strange to me that the designer of the toroid would use a 1000 and a 500 cmil spec in the same transformer, when both taps are rated at 4 amps???
All transformers used 500 to 1000cm of copper wire per ampere. Older transformers used larger wires due to lower temperature rating of the wire insulation. Standard was 700cm. The core type has nothing to do with wire size.
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