Need a 240V to 120V 100W transformer?

Not W, but VA for some reason.  I think because the transformer is a passive device.

Don Hersey wrote:

 

Not W, but VA for some reason.  I think because the transformer is a passive device.

Perhaps read up on real power, apparent power and power factor in AC circuits.

As I understand it, generally W is used for calculations involving heat dissipation and consumer electrical tariffs whereas VA is used for calculating conductor sizes (transformer windings) and fuse ratings. Keep in mind that in AC circuits voltage and current are not always in phase which also makes W difficult to measure with the likes of a standard multimeter whereas VA is the product of the voltmeter and ammeter readings.

Perhaps this subject is a contender for a Element14 bite-size tutorial ?

Hi,

It's exactly that. Easier described as out of phase as you say, although in practice it happens due to non-sinusoidal waveforms too.

The power the device (computer monitor in this case) consumes will be given off as heat, and that is marked on the monitor in Watts.

However, the apparent power could be larger. Plus there is a certain efficiency for the transformer (85% in this case I think - not looked

again at the spec sheet).

I've no idea what the power factor is (was), but I found this on google books:

It would suggest that there is quite a lot of variance, but maybe 0.6 or 0.7 would be a good number to choose.

So, that means 120-140VA but at 85% efficiency 140-160VA is more appropriate.

Hi Shabaz are you suggesting that the  TIM100TIM100 would not be sufficiently powerful to power the monitor Even though 85W is stated on the chassis Surely it should be fine

Sorry if my question is in simplistic terms but I'm sure you guys have guessed this is going a little over my head!

Always difficult to tell without having it on the bench to actually measure.

I seem to recall that over here at least electrical products drawing in excess of 500mA need to have their PF substantially corrected. Not sure when that rule came in or if it was internationally adopted. However from previous tests with my budget power meter it appears to be true.

Also the 85W referred to was specified as a maximum rating which probably means peak power draw at on switch-on and not continuous.

There is the Block  TIM200TIM200 which is rated at 200VA but price jumps up again

TIM200 - BLOCK - Isolation Transformer, Class II, 200 VA, 1 x 230V, 2 x 115V, 860 mA | Farnell element14

Always difficult to tell without having it on the bench to actually measure.

I seem to recall that over here at least electrical products drawing in excess of 500mA need to have their PF substantially corrected. Not sure when that rule came in or if it was internationally adopted. However from previous tests with my budget power meter it appears to be true.

Also the 85W referred to was specified as a maximum rating which probably means peak power draw at on switch-on and not continuous.

There is the Block  TIM200TIM200 which is rated at 200VA but price jumps up again

TIM200 - BLOCK - Isolation Transformer, Class II, 200 VA, 1 x 230V, 2 x 115V, 860 mA | Farnell element14