ef37a wrote: Folderol wrote:
Come on Will, noboby uses a two diode circuit these days for a single supply!
I do, frequently. Check up on RS & Farnell. It's virtually impossible to get a single secondary transformer in the 3 - 20VA range!
Oh! You old fashioned thing! I just parallel them and use a bridge!
If you are forced to use a 2 winding transformer for a single output you should get one with half the required voltage and wire in series.
My old colledge tutor would have revelled in this! He would have immediately jumped in with, "The only transformer windings that should be paralleled are bifilar or dual bobbin ones". Both of these are rare as hens teeth and damned expensive.
The reasoning being, any other winding form will not give identical halves. The copper length (hence resistance) will be different, as will be the flux density - less of an issue with toroids. So, the current distribution in the windings will be different and the total power less than you'd expect - for some small value of 'less' :)
Conversely with series layered windings the effect is as if there was no break. There are no differential currents and the winding resistance is simply the sum of the two.
In a rectifier situation (push-pull v bridge) the winding differences mean that, under load, there will be slightly different voltage peaks between half-cycles, whereas these will be equal coming from a bridge.
Because I've sometimes had more free time than is good for me, I've actually tested this, and if you create an approriate DC offset then wind a 'scope up to maximum sensitivity you can indeed see the difference.
I found yesterday's heat so exhausting that today I'm doing as little as possible to recover - aren't you the lucky ones :)