VA versus watts

[Bill Wright]

It doesn't seem to work out here, compared to other manufacturers. Anyway, what power factor should be used?

Bill

[John Rumm]

1 near enough... its basically a SMPSU with power factor correction built in.

[harryagain]

All transformers are limited by the (K)va, If you use (K)watts (VxAxCos phi ), zero power could be used (watts) yet the current could theoretically be infiity.) So a tranformer is never sized by watts only by volt-amps.

[meow2222]

It doesnt say what its pf is. If its legal it'll be high. But more to the point, why is it an issue? The load is so small that it shouldnt matter in normal circumstances.

NT

[John Rumm]

What has that go to do with the SMPSU under discussion?

SMPSUs over 25W must now include PFC as a legal requirement if they are to be put on the market in the EEA.

[harryagain]

That only corrects the tranformer, not the load.

All transformers regardless of operating principle are limited by VA not Watts.

[meow2222]

yes, which is immaterial. And halogens are pf 1 anyway

yes... but an smpsu isnt a transformer

NT

[John Rumm]

The PFC applies to the load characteristic of the PSU as presented to the mains.

Given the load is typically a filament lamp, it will present a resistive load to the PSU.

Yup, but not relevant to the discussion in hand since we are not discussing a simple transformer (in spite of the colloquial use of the term for pretty much any LV lighting PSU regardless of design), but a SMPSU. It will have PFC on its input, and a resistive load on its output.

[The Other Mike]

Electrically yes, but they can also be limited by the heat dissipation in Watts that they can lose to atmosphere so as to avoid the creation of hot spots within the windings that will degrade the insulation.

[harryagain]

Where would the heat come from if not from electricty? It's theoretically possible to have an AC current flowing yet no power generated or transmitted.

You could have say 25 VA "load" but zero Watts. (In practice it would be near zero as no components are perfect.).

[Bill Wright]

why is it an issue? The load is so small that it shouldnt matter in normal circumstances.

It says

60VA: Suitable for 3 x 50W / 7 x 20W.

Bill

[meow2222]

Transformer heat diss depends on load current, ie load VA not W.

NT

[The Natural Philosopher]

Its a bit more complex with smps, to the point where power factors doesnt actually mean a lot.

[The Other Mike]

VA is not a recognised unit of thermal power

[The Other Mike]

Yes, but VA is not a recognised unit of thermal power

[The Natural Philosopher]

No conflict in those two statements

[meow2222]

transformers pushed hard can reduce P_diss as load goes up due to reduction of saturation.

NT

[The Natural Philosopher]

Thats just ONE of several losses..

I am ashamed to say I know far more than I would prefer about losses in SMPS circuits...

Let's just sauy that when designing them, at a certan point ypu put the slide rule away aad build the damned thing and start measuring instead.

Its quicker..

[The Other Mike]

A rating plate on a oil filled forced cooled transformer

Electrical 'VA' on the left, Thermal no load and load losses in 'kW' on the right

The electrical VA can be limited by the temperature rise of the oil and windings caused by previous loading or failure of the cooling banks.

[Andy Wade]

Oops, so it does: clearly an error or typo on the part of QVS. It does say 150 VA in the title. The manufacturer's instruction sheet is here:

The input current is given as 0.62 A, so the input power factor must be as close as dammit to unity.