DIY PSU build query...

[Doddy]

I use 45v secondary's when wired in serial and rectified gives me 72v without load and 68v with load.

I also have a 50v one I don't use which gives 78v without load and 73v with load, I bought 2 because when Maplins went bankrupt they sold them 1/2 price and that's when I built the PSU.

Also I blew up the first transformer I bought which I paid full whack from RS for, wired the rectifier the wrong way round I seem to remember. So buying 2 was my insurance policy lol.

Sorry, pedantic mode (it might be important for someone blindly following this to avoid damaging drivers etc.):

Wired two 45V windings in series and you get 72V DC off load?, there's something amiss there. Perhaps in parallel? - I'd expect a 45Vrms winding to present around 64Vpk-pk, probably 62V after rectification and smoothing. Add 10% for the 220VAC rated primary and the nominal 240VAC UK supply and you're getting within a gnats of the 72V.

Two 45V windings in series you should expect 90VACrms, leading to around 128Vpk-pk, around 126V after rectification and smoothing. And then you can play games with the 220VAC primary versus nominal UK supply.

For reference, I also bought a toroidal tx from Maplin on closure...



That pokes out 72V off-load, rectified and smoothed, measured with cheap Chinese current/voltage DMM panel meters. Two secondaries driving two separate circuits - but this should give the same voltage as two secondaries in parallel (not series). The maths for mine = 50VAC * SQRT(2) - 2 * Vf(rect) = 69V - within about 5% of measured - I'm happy with that.