Transformers with multiple secondaries - calculaing VA rating

[lordearl]

I'm using a transformer rated at "0-9v, 0-9v, 0.85A" to power an AD1865 dac chip (which I assume consumes just milliamps) - how do I calculate the VA rating of the transformer?

(I'm assuming 50mA max is the most it will consume, which would be 50mA/240v AC = 12VA transformer required).

[Tony]

^ 2 x 9 x 0.85 = 15.3watts
your primary va = 15.3/0.7 = 21.8va

[lordearl]

aha! my missing link was the "power factor"
Thanks Tony.

The reason I am seeking the VA rating is to upgrade the smoothing caps. They are currently 2,200uF, but I have some 22,000uF ones laying around that should give better performance, however need to determine whether the transformer can handle the extra stress associated with the increased capacitance (if there is any). 20va should be fine, would you think?

[Tony]

yes, the impedance of such small transformer will probably be enough to limit start-up currents...take note that with such high value filters, it will take a while for the voltages to ramp up to working voltages.....

[lordearl]

the i/v stage of the dac uses a separate transformer (0-220,0-9) - so the tube section takes a while to warm up anyway

I might change out the caps (will increase the 220v from 150uF to 470uF) and then see just how the transformers perform under load for a few hours, check they don't run too hot.

I recall reading that after certain level (roughly 10,000uF) there is not much return value on increasing smoothing capacitance - I wonder why some preamps and dacs have > 100,000uF - is there any way of justifying this expense?

[Michael Bean]

The quick and sort of correct answer is that the VA rating is the same as output current X volts, however (and this is a big however) that is only true if you put a pure sine wave in and get a pure sine wave out. In this age of digital devices, switch-mode power supplies, light dimmers, brown outs and other power line anomalies, that is rarely the case. There are others here who are certainly a lot smarter then me about this, but in your case that transformer should have plenty of capacity for that purpose. As for the actual method to calculate VA, maybe one of our better schooled members can chime in, I just don't know how to do that.

Mike

[Tony]

you can look here about capacitance multipliers : Capacitor multiplier supply for ALEPH 3

[abraxalito]

Originally Posted by lordearl

I recall reading that after certain level (roughly 10,000uF) there is not much return value on increasing smoothing capacitance - I wonder why some preamps and dacs have > 100,000uF - is there any way of justifying this expense?

It really depends on the ripple voltage - a big power amp might need 100,000uF to keep the ripple voltage low enough. But in the case of dacs and preamps where the draw is normally very low I'd say 100,000uF is a waste of money yeah. If low ripple voltage is a target better to put the money into regulation circuitry to reduce it rather than huge caps which only decrease the power factor and hence increase transformer heating.

[!]

LOL, no you won't need 12VA or more transformer.

yes 10,000uF is absurd for a chip dac using 50mA. Transformer -> rectifier bridge -> 2 x 220uF capacitors (one per rail) -> positive & negative regulator -> 10 uF ceramic capacitor (one per rail) is all that is needed, plus decoupling near the chip pins.

The initial calculation was wrong, it's not 50mA @ 240VAC, it's 22mA/+5V rail, -23mA/-5V rail... 45mA * 5V = 225mW, which is also listed in the AD1865 spec sheet as the typical power consumption.

So, you have 45mA sum across both rails, so 0.045A * 9V instead of 5V = 0.405W
0.405W/0.7 = 0.6VA transformer needed.

[abraxalito]

Yeah but do we really want to be using the trafo at its maximum operating temperature? I suggest we don't - the copper losses are higher when the wire's hotter. So how about a 1.2VA one?