Newby question

[Trzalica]

Hi guys,

It is my first attempt to build everything from the ground up so I need some explanations and help in regards of power supply, especially toroidal transformer.

For the power amp I need dual 48V and would like to have at least 200W. I checked some (Plitron, Hammond e.t.c) and found specs confusing.

Plitron have their transformers ranked in VA. I guess that it directly translates to W, please confirm.

Situation with Hammond is even more complicated, there is a value for VA but also Series and Parallel under VAC Secondary (RMS). Values are, for example:

VA: 225
Series: 36V C.T. @ 6.25A
Parallel: 18V @ 12.5A

So please explain how to "decipher" it.


Thank you,
Marin

[richie00boy]

VA is the AC voltage equivalent of watts (Volts x Amps). Remember that nothing is 100% efficient so you will need a bit higher VA rating than your amp power rating.

You need to look at the series voltage as that's how you will need to wire the transformer, the example you describe would be a 2x36V transformer giving you approx 49-0-49 amp rails.

[wakibaki]

Transformers are rated in Volt-Amps. This is because some of the loads that transformers can drive are not resistive. VA=Watts when the load is a resistance.

An amplifier is close to a resistive load, so VA are treated as equivalent to Watts when designing an amplifier power supply.

A capacitor or inductor can have a big voltage across it and a big current flowing in it and still draw no real power (i.e. heat up). This is called reactive or imaginary power. There is no heating because the voltage and current are not in phase. The voltage and current in a resistor are in phase, and hence create heat - this is called real power. The words real and imaginary or reactive have a special technical meaning in this sense.

See here:- AC power - Wikipedia, the free encyclopedia

and here:- http://www.eznec.com/Amateur/RMS_Power.pdf

for an explanation of real, reactive, apparent and RMS power. Note that 'RMS power' is a misnomer, but is an expression commonly (mis)used in electrical engineering.

w

[Trzalica]

Thank you clarification Richie. The output of my power amp to be is about 60W so 200W transformer should be enough, right?

[Trzalica]

Thanx both guys!

If I am reading it correctly, in my example I will get 2 x 36V producing with about 225W. This transformer is huge so can I use little bit smaller one but still being able to produce enough power for 60W amp (I am trying to make is as best as possible, without paying an arm and leg)

[DF96]

A perfect Class B amp is 78.5% efficient, but no amp is perfect so assume 50%? Then 60W audio out needs 120W DC in. A capacitor-input power supply takes its current from the transformer secondary in narrow pulses, which means that the heating effect is much greater. 120W DC might need a 600VA transformer. However, unless you are using your amp as part of an industrial servo system (or sine wave testing) it will spend most of its time at low power. 200VA might be fine, as long as you don't do full power sine wave tests for more than 10-20 seconds at a time.

You can use Duncan's PSU Designer to investigate, but you will still need to estimate your duty ratio.

[BobEllis]

Originally Posted by Trzalica

...
Situation with Hammond is even more complicated, there is a value for VA but also Series and Parallel under VAC Secondary (RMS). Values are, for example:

VA: 225
Series: 36V C.T. @ 6.25A
Parallel: 18V @ 12.5A

Originally Posted by richie00boy

You need to look at the series voltage as that's how you will need to wire the transformer, the example you describe would be a 2x36V transformer giving you approx 49-0-49 amp rails.

Richie, I think the transformer described is 2 x 0-18 meaning that you'd get 24.5V rails in series. A 2 x 0-35V transformer would be a decent choice for approximately 48V rails.

Unless you are planning PA duty, you probably don't NEED a VA rating significantly more than expected output power. But this is DIY, and the cost of upgrading is low. It's only $23 to go from 400 to 600 VA at Antek

VA translates directly to watts only with a resistive load. A reactive load is different. Strap a capacitor across the secondary and no real power is delivered, but there is current through the cap that is 90 degrees out of phase with the voltage. This means you have no current when the voltage swings to its peak and maximum current as the voltage crosses zero.

One reason to go with higher VA rated transformers is their regulation is usually better, meaning the rails sag less at high power with a larger transformer.

[sawreyrw]

Marin,

I'm surprised that no one told you that the real issue with VA is that the A part is determined by the RMS current in the winding. This means that if you put the secondaries in series, the rated secondary current is 6.25 amps RMS. It turns out that the RMS current may considerably larger than the average current. This is because the current is large relatively narrow 60 Hz pulses. So a 200 VA transformer cannot be used to provide 200 watts in a DC power supply circuit.

Rick

[AndrewT]

Originally Posted by sawreyrw

a 200 VA transformer cannot be used to provide 200 watts in a DC power supply circuit.

yes, absolutely.
The manufacturer tells us the de-rating factor that must be used when the transformer feeds a capacitor input PSU.
That factor is ~0.7

After taking account of the higher voltage on the smoothing caps cf. the Vac then the maximum continuous DC current from a capacitor input PSU is roughly half the rated Vac current.
i.e use a 6Aac rated transformer and draw a continuous 3Adc from the PSU and the transformer is running at full rated VA. It will run hot !
and not what you feel at the surface, inside the temperatures are very much higher !