I had to log in just to click a, "Like" for that one.

 

all the above post answered my query well, especially electrician's analysis and guide to transformer derating was amazing.

that was a good thing, now the bad part is, so goes my plan through the window, to build a constant voltage efficient led driver for home lighting without a bulky transformer. anyways, i will do some searching, explore my options and make a new thread after a few days asking suggestions for it.

 

To understand where the 50% (could be as much as 62%) number comes from see:

http://forum.allaboutcircuits.com/showthread.php?t=68839

ok, finally i need a little confirmation, back to my old 6-0-6 500mA transformer, with reference to your post no 2 in the above link :

AC component of bridge rectifier :
--------------------------------------------
Vpk = 12V AC
Irms = 0.5A
= 6VA

DC output with 1000uF cap & bridge rectifier:
---------------------
Vdcpk = 12*1.4 - Vdiode= 16V
Idc = 0.5/1.63=0.3A
Vavg = Vdcpk -(Idc/4*0.001*50) = 16 - 0.3/0.2 = 14.5V
=14.5*0.3=4.35Watts

so, i can expect ~4Watts DC power from a 6VA transformer ?

 

ok, finally i need a little confirmation, back to my old 6-0-6 500mA transformer, with reference to your post no 2 in the above link :

AC component of bridge rectifier :
--------------------------------------------
Vpk = 12V AC
Irms = 0.5A
= 6VA

DC output with 1000uF cap & bridge rectifier:
---------------------
Vdcpk = 12*1.4 - Vdiode= 16V
Idc = 0.5/1.63=0.3A
Vavg = Vdcpk -(Idc/4*0.001*50) = 16 - 0.3/0.2 = 14.5V
=14.5*0.3=4.35Watts

so, i can expect ~4Watts DC power from a 6VA transformer ?

That's about right. A 6 VA transformer is small. Small transformers don't draw such a peaky waveform (compared to a larger rated transformer) when feeding a bridge rectifier.

The completely proper way to find out what your transformer can do safely is to put a variable load on the DC output ( I use a wirewound rheostat; you could use a selection of power resistors) while monitoring the True RMS current out of the secondary. Adjust the DC load until the secondary current is exactly equal to the transformer's rated current and that's the allowable DC output.

 

That's about right. A 6 VA transformer is small. Small transformers don't draw such a peaky waveform (compared to a larger rated transformer) when feeding a bridge rectifier.

The completely proper way to find out what your transformer can do safely is to put a variable load on the DC output ( I use a wirewound rheostat; you could use a selection of power resistors) while monitoring the True RMS current out of the secondary. Adjust the DC load until the secondary current is exactly equal to the transformer's rated current and that's the allowable DC output.

thanks, what chances are there, that transformer will burn up or catch fire if i put it on the rated rms current or around for some time ?

well i will try this, but i dont have those think i can get rheostat, i have some power resistors, will check with them. and about true rms dmms ...... how accurate are these chinese dmms ? i dont really have access to such sophisticated instruments. :/

 

thanks, what chances are there, that transformer will burn up or catch fire if i put it on the rated rms current or around for some time ?

well i will try this, but i dont have those think i can get rheostat, i have some power resistors, will check with them. and about true rms dmms ...... how accurate are these chinese dmms ? i dont really have access to such sophisticated instruments. :/

The whole purpose of ratings is to let the user know how much current you can draw from the transformer indefinitely without burning it up, so, yes, you should be able to run it for a long time within its ratings.

I can't tell you anything about those meters beyond what they say in their ads. You really must have a meter than can measure True RMS to make a proper measurements.

There have been a lot of reviews of low cost DMMs on various forums, so try a general search for DMM reviews.

 

The maximum rating is a heat thing so you can load the transformer rectified output with your desired load and see how it much it warms up.
If it's too hot to touch after a few hours (or less) then it's probably being overloaded.

 

so, i went to market and bought myself a 12-0 / 500mA transformer without a centre tap, for testing & playing purposes.

as there are both of these kind of transformers available, with or without centre tap, lets say 12-0-12 & 12-0 both having secondary rated @ 500mA, does it mean their VA will be different ?

i mean, 12-0 with 500mA is simply 6VA, but is 12-0-12 like 24x0.5=12VA ? or is it like ,the current rating is given with respect to centre tap & 12-12 terminals have like 250mA current flowing through them, resulting 24x0.25=6VA as well ?

 

......................
i mean, 12-0 with 500mA is simply 6VA, but is 12-0-12 like 24x0.5=12VA ? or is it like ,the current rating is given with respect to centre tap & 12-12 terminals have like 250mA current flowing through them, resulting 24x0.25=6VA as well ?

The VA rating is independent of the number of taps and refers to the winding without any taps.
So a 6VA rating with a 12-0-12 CT means that each half can output 3VA or 250mA maximum.
It does not mean one winding can output 500mA by itself since the winding dissipation goes up as the square of the current.

 

The VA rating is independent of the number of taps and refers to the winding without any taps.
So a 6VA rating with a 12-0-12 CT means that each half can output 3VA or 250mA maximum.
It does not mean one winding can output 500mA by itself since the winding dissipation goes up as the square of the current.

thanks, but actually i was asking the opposite way, given only the current spec, what will be the VA rating of a centre tapped transformer ?

it seems double of a non centre tapped one ......

12-0 /500mA => 12x0.5=6VA
12-0-12 / 500mA => 24x0.5=12VA ?

 

thanks, but actually i was asking the opposite way, given only the current spec, what will be the VA rating of a centre tapped transformer ?

it seems double of a non centre tapped one ......

12-0 /500mA => 12x0.5=6VA
12-0-12 / 500mA => 24x0.5=12VA ?

No. You misunderstood me. The VA rating is for the whole transformer (both sides of the tap summed together). It is still 6VA total.

 

okay, thanks !