[ECM]

I started building a power supply with a 10-0-10 center tapped transformer. when i measured the actual output without a load it was 11-0-11 VAC. i built a bridge rectifier and added some caps (1100uf). when i tested the DC output of the rectifier and caps it was +/-15V. how can that be? will the voltage decrease once i add a load?

 

[peranders]

That is totally right!

(11 x 1.414)-0.7 = 14.86 V

This "0.7" is losses in the rectifier. The number can be 0.7-1.2 in your case.

1.414 = SQR(2) which is the peak value of the sinus curve.

The voltage will decrease somewhat when you add load but this is dependent of smooting caps and how big the transformer is.

 

[tangent]

It's normal for transformers to put out more voltage than they are rated for with no load. Their rated voltage is what they put out with a full load.

Regarding peranders' equation, it should be 2*Vf if you're using a diode bridge (4 diodes). It's only 1*Vf when you use a full-wave rectification scheme (2 diodes), but that's overall less efficient.

(Vf = the forward voltage drop of the diode)

 

[peranders]

If you have a real centertap it's no difference voltage wice if you have a bridge or two diodes but if you have two separate windings and two bridges you will get 2x0.7 in losses.

Anyway 1-2 volt losses is valid most of the time.

 

[tangent]

I just measured a single-voltage power supply that uses a transformer with a center-tapped secondary. The AC voltage times 1.414 was 2.4V higher than the DC voltage across the main filter cap, which comes after a diode bridge.

The current drops across two of the diodes in the bridge when the AC wave is positive, and the other two when the AC wave is negative. This is where you get 2*Vf.

 

[peranders]

Did you have two caps forming + - voltage? Did you measure between the center-tap and the smoothed voltage.

 

[ECM]

thanks a lot for the explanations!!

i still have a couple of questions though. the transformer is rated at 40VA. so each 10V tap is capabale of 2A max. how much capacitance is required for keep the voltage clean at 2A? i'm currently using 1000uf. should i bump that up some? maybe 4700uf or more?

 

[peranders]

Note that this rating is valid if you connect for instance a resistor. If you rectify AND smoothe the voltage the rating goes down. Very heavy smoothing give you half the VA rating. Normal cap values here are I think, 1000-4700 uF. Note that this is continuous load.

 

[ECM]

so if i smooth the voltage out and regulate it will 1 amp per side be enough for a Szekeres class "a" amp?

Class "A" amp

 

[peranders]

1 A is OK for this transformer.

 

[ECM]

so what's the best way to filter the output of the transformer? lots of capacitace before or after the voltage regulator?

 

[tangent]

Quote:

Did you have two caps forming + - voltage? Did you measure between the center-tap and the smoothed voltage.

I see what you're talking about. You talked of center-tapped transformers, and I thought you were saying that the transformer itself caused the difference in the voltage drop. What you were actually saying is that with a center-tapped transformer, you only need one bridge to provide +/- voltage.

In my case, I was using a center-tapped transformer, but ignoring the center tap and using it to make just a single-voltage supply.

 

[JahJahBinks]

Quote:

Originally posted by ECM so what's the best way to filter the output of the transformer? lots of capacitace before or after the voltage regulator?

You would need both to get the best result, the bottom line is you cannot do without big caps after rectification.

 

[peranders]

Quote:

Originally posted by ECM so what's the best way to filter the output of the transformer? lots of capacitace before or after the voltage regulator?

You can check my QSXPS. The only thing you have to is to connect a transformer. All filters are on the pcb.