Transformer current / power rating.

[lord loh.]

I have a 12-0-12 transformer which is rated 5A.

So the power I shall get is 60W.

Suppose I try to get 24v from this transformer, should I derate the maximum current to 2.5A ? to keep the power constant at 60W ? There is no power specification given for the transformer.

I belive that I can still draw 5A as the thickness of the secondary coil wire is still the same...and is capable of handeling of 5A.

 

[2PAC Mafia]

Hello,

I think the 5A are the maximum output the complete secondary coil can supply you, so if you take the 24Vac you will get them.

 

[Nigel Goodwin]

The transformer will be 120W, each winding will be 5A.

 

[JimB]

The transformer will be 120W, each winding will be 5A.

I take the opposite view.

I think the transformer was designed to provide 12 volts DC using a Bi-Phase rectifier. ie take the -ve supply from the centre tap and a diode from each 12v to the +ve.
This way each half of the winding is only conducting to half of the time.

Before you use it for your project, why not run a test.
Use resistors, car light bulbs or whatever you have to load it to 5 amps, if it gets too hot or the voltage drops too low, try it with 2.5 amps and see if it is OK.

Another thing you could try, compare the size of the transformer with ones in catalogues, transformers of similar ratings will be a similar size.

JimB

 

[Exo]

Doesn't it have a VA rating?

 

[tavib]

:lol:
If you want to use this transformer to feed a rectifier block with holding cap at output, then the rectifier diodes will sink current pulses from transformer, and for tought the average level of continous current sink from cap, these pulses must have a high current amplitude, and can produce a droped voltage on transformer bigger than you expected. Trying to increase the cap value will not improve anything, in some cases can be even worse, because when cap increase the conduction time of diodes decrese and current pulses from transformer becomes thicker and higher. This effect is much affected by the stray inductance of the transformer, but a transformer without stray inductance not exist, so try to compute, at least approximativelly, the amplitude of the current pulses.

 

[lord loh.]

No the transformer does not have any VA rating. The local products I buy do not make any complete specifications.

And how hot is "too hot"? I tried to check one of my regulated power supply made using a LM317 (1.5A). I drew 800mA. The IC was 50°C while the heatsink was at 55°C and the resistor was at 70°C(on top surface and 90°C on the borttom surface.

And since a particular thickness of wire is needed to handle 5A(by thickness I mean length-thickness-resistivity combination) and the thickness is the same throughout from end to tap to end. I guess Nigel Goodwin is right.

 

[ljcox]

Transformers are normally rated in Volt Amp (VA) rather than in Watt since the load could be reactive.

Len

 

[eblc1388]

The IC was 50°C while the heatsink was at 55°C...

Congratulation, you have a Superconducting heatsink. People are looking for these for years but cannot find them until today... :lol:

 

[lord loh.]

I know! It sound wired....Heat sink must have a lower temperature than the IC. But I measured it using a laboratory mercury thermometer. And this was what I saw....Authentic observation-I certify!

 

[audioguru]

The IC was 50°C while the heatsink was at 55°C.

Shows that the plastic part of the IC doesn't conduct heat very well.
Maybe the huge thermometer is a better heatsink than the real heatsink.
An IR thermometer doesn't even touch it to get an accurate temperature reading. :lol: