Electronics > Beginners
Power VA and secondary windings.
Brumby:
In reducing the secondary from 30V to 20V, you can still pull 100VA from such a transformer - provided the gauge of the wire used in the secondary is capable of handling a higher current.
Temperature rise should also be considered carefully in doing so - since excess heat is one of the foremost causes of failure in components (from my experience).
If you really want to maximise the current output capability to 5A @ 20V avoiding thermal and current problems, you can add another secondary winding of your own and connect it in phase and parallel to the shortened one. Just make sure you use the number of turns to exactly match the voltage of the (shortened) existing secondary and use a wire capable of carrying around 2A or so.
T3sl4co1l:
The offset in primary winding power dissipation will be enough to take, eh, maybe 7% more from the secondary. (~30% reduction in VAs --> ~15% reduction in primary dissipation, ~7% increase in current because (7% increase)^2 ~= 15% increase.)
There is a nonzero gain available... but it's nothing important.
Reminder that, if you're connecting this to a FWB and cap-input filter, the power factor sucks (~0.6 at best) so for say 3A RMS current rating, you can only draw maybe 1.8A DC. (This partly adjusts for the fact that output DCV is higher than ACV RMS, but it's worse than a one-to-one trade so you are losing some capacity.)
Also consider using a buck transformer: get a ~40V transformer, wire it to subtract from 120V so you're supplying 80V to the primary, and you get 20V out of the 30V secondary. (Or 80V for 240V mains.) This transformer only needs to handle the bucking VAs, so it can be smaller than the main transformer is.
Though, not much smaller in this particular case. It's definitely a more useful strategy for small changes. Also needs an oddball 40V transformer, though 36 is probably common enough.
Note that, although the current rating is not changed in this configuration, you still have all the impedance (DC and AC resistance, and leakage inductance) of the full-voltage transformer, but its drop is a bigger fraction of the total: in other words, regulation will be about 30% poorer. If your load expects a stable voltage (say +/- 10% over full current range), you will be further restricted to a proportionally smaller current range as well (i.e. 20V 2A).
Upside to the bucking approach: core loss is much lower, so the transformer runs very cool and draws less idle power. This is just about mandatory to reuse highly-saturated designs like microwave oven transformers (which are still perfectly good for say 300 VA when rewound with a safer secondary voltage).
Tim
davelectronic:
Some very interesting facts there, things i didn't think of or take in to account. Thanks for giving these stats a mention, if its not a ridiculous amount of turns, i might consider just rewinding the complete secondary instead of just part secondary windings removal. The only slight off putting is the closed loop of the transformer makes it harder to wind than an I E bobbin of a chassis laminated transformer.
Gyro:
Ha, yes, it can be a pain. You can construct a long thin shuttle (basically a strip of wood with a notch at each end to hold the wire) to thread through the core.
Last time I wound a secondary with thick wire though, I ended up doing the winding while my wife ran backwards and forwards from the other corner of the room with the other end to avoid kinks! ;D
It's surprising how quickly the wire length builds up, make sure you measure correctly to ensure that you allow enough.
Gyro:
--- Quote from: T3sl4co1l on December 15, 2019, 03:46:47 am ---Also consider using a buck transformer: get a ~40V transformer, wire it to subtract from 120V so you're supplying 80V to the primary, and you get 20V out of the 30V secondary. (Or 80V for 240V mains.) This transformer only needs to handle the bucking VAs, so it can be smaller than the main transformer is.
...
Upside to the bucking approach: core loss is much lower, so the transformer runs very cool and draws less idle power. This is just about mandatory to reuse highly-saturated designs like microwave oven transformers (which are still perfectly good for say 300 VA when rewound with a safer secondary voltage).
--- End quote ---
A little OT, but why does everyone refer to (and configure) these transformers as bucking transformers. To me, it makes much more sense to wire them as autotransformers instead. It doesn't need to be any bigger, the load current still only runs through the secondary winding, but the voltage across the primary is lower, reducing core temperature and losses there too.
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