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quick question about transformer VA ratings

I

IDM

pfm Member
I am playing with a valve based linestage which uses 2 12B4A triodes for gain. I am using a 9V 20VA transformer for the heaters. The 9V is rectified then voltage regulated to give me the 6.3V I need for the heaters. All seems fine so far and indeed the linestage sounds really quite superb, so now I am plotting to build a valve phonostage, so that's 2 more valves. For the heaters I now have 2 X 0.6A for the 12B4A's + 0.3A +0.365A, a total of 1.865A for all four valves.

Now the transformer as I said is 9V 20VA, so I presume 20/9 = 2.2A max. If that calc is correct I think I have 0.335A spare capacity on the transformer which I think/hope is sufficient.

However as said the voltage is only 6.3V so can you calculate the VA as 6.3 X 1.865 = 11.75VA.

Which is correct, I am confused!

Ian
 
Ian,

The trafo is not really aware of what happens to the power it supplies on the other side of the regulators. In fact the rectifier and regulator waste the power associated with the voltage drop as heat. (9V-6.3V)x1.865A. Moreover(i.e. also) VA is not useable watts, because the trafo has its own power overheads, mainly lost as heat, maybe 10-15% in a 20VA trafo.

So your 20VA trafo is probably just enough. It will get warm however.

David
 
Hi Thanks for that David,

Reading your response and rereading my original post I am wondering is it easiest to think of all this is watts (which I suppose is the same as VA?), i.e. one valve might be 6.3V at 0.3A = 1.9W. Then add these up for all the power consuming components, allow for heat in regulators and rectifiers and then see how that looks. Is it that the best way to calculate the specification for the transformer.

Cheers,
Ian
 
I am wondering is it easiest to think of all this is watts
Click to expand...

You could do this as another check. For a new design you start with voltages and current. Without knowing these you can't calculate watts.

VA is used for transformers, because current is required to magnetize the core as well as supply the load. This magnetizing current is at its maximum when the output voltage is zero, but is still contributes to heating the transformer. The A in VA is the (vector) sum of the current supplied to the load and the magnetizing current. So out of 20VA some of it unavailable to the load. Add to that the power dissipated in the non zero resistance windings, which is higher for smaller trafos, and you might lose 10-15% of your VA rating in the transformer. The output voltage drops and the temperature rises.

The VA rating is actually telling you at what input voltage and current the transformer might catch fire! Well almost, but beyond that its considered dangerous and will be damaged.

In the UK transformer output voltages are specified at full load. From the regulation spec, if quoted, you can work out the no-load voltage. I have a feeling that trafo output voltages in the US are rated at no-load. You use the regulation to work out you full load voltage. I'm not sure of this.
 
Here's a really useful page for this kind of thing:

http://www.atc-frost.com/products/design/va.htm

Note the amount of current you can actually draw within a given VA rating actually varies with how you hook-up the rectifier. Your existing 20VA transformer is right on the limit already, as Bivalve notes - just look at the FWB example.

Oh, and transformer voltages are usually quoted at full load - so the apparent voltage rise when unloaded can be alarming for small transformers!
 
Hi Ian,

Don't know if this would be of use - I've just scanned in a couple of pages (page 1, page 2) from an old copy of Radio & Electronics World. I still refer to this when doing initial psu calcs. The only downside is that you have to choose a ripple value to plug in to the calcs, which, due to my limited knowledge, can be a bit arbitary! I do have some PSU books but the simple ones don't seem to cover off questions like yours, and the complicated ones I don't understand!
 
Hi many thanks for pointing me to the various articles. As ever these things always require more consideration than I originally thought.

It certainly seems my 20VA transformer is insufficent. I have reverted to using a bridge rectifier and cap on the 6.3V 4A winding of the main transformer. By using Schottky diodes I can reduce the voltage drop across the diodes to give me nearly 9V so I think I have enough head room to use a 7805 regulator with a couple of diodes in the ground leg of the regulator to lift the voltage up closer to the 6.3V required.

Cheers,
Ian
 
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