smoking primary

[mike3124]

Hi folks;
for whatever reason (incipient madness, personality disorder etc) I decided I wanted to wind my own mains transformer.

This had a 240 v input at 50 cycles which I wound on an EI 84 laminated core.

I wanted 350 - 0 - 350 on the secondary

a ratio of 1:2.9

I also intended to wind another secondary having 3.25 0 3.25 at 5 amps for valve heaters

So I used the equation relating core area to flux density and number of turns and for the HT primary came up with 1663. too much for the winding window using 0.5 mm wire.

In the end I opted to try the same ratio but with less turns - 500 and the secondary adjusted accordingly.

on test output voltages were right but the primary did start smoking. Obviously reducing the turns is the culprit but I don't quite understand why.

Observations greatly appreciated while I go source a larger lamination size. Also does anyone have any views on the necessity of wrapping each layer with Kraft paper? It does increase the windin size quite a lot.

Thanks

Mike

[CCH Concepts]

by reducing the turns you have reduced the resistance and there for increased the current the primary can draw, possibly beyound the rating of the wire, this might be a reason. is the wire burning off-load or on?

off the top of my head having never made a transformer, of this have to do with the inductance. by changing the turns to have altered the inductance, i suspect this would have been set according the the 50hz ac source. but im not sure what effect this would have.

persaonlly i would try using a thinker wire for your primary to be able to carry the current. because assuming an ideal transformer its going to be carring 2.9 times the current of the secoundary, but if your doing well and getting 85% effienceny then you looking at like 3.4 times more.

hope this helps as i said never made one before, only cover the theroy of them.

[Weazel]

use a cople of computer psu transformers and use the pwm chip that comes with them be alot better if u dnt knw about winding transformers u need to read alot of papper work to do so u have ratio inductane inductive reactance volt turn and/or volt cm and mre transformer winding is not for the dummy and its got to be wound flat to work the best