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

Hi Jim,

As Dave says, the 85W might not be sustained. Also, the user docs for the monitor specify multiple screen sizes, but lists 85W as a general figure. A larger screen might be closer to 85W perhaps, a smaller one could be less, but I can't say for sure.

It is all guesswork, this 85W is a number off a sheet of paper for a guideline to customers, it isn't a full detailed description - they possibly never expected a typical customer it to run on 240V and so they didn't provide more detail.

Damn, that would be annoying. As Dave says, the price does jump a lot at 200W. Would seeing the manual for the monitor help? There's an online version here: http://www.wellsgardner.com/pdf/Service/K7000.pdf Does it include any info which might confirm one way or another? Also as it's 85W max, wouldn't that mean it doesn't need to be sustained?

Hi Jim,

The manual doesn't help (I'd already checked it out).

I can't assume anything, the 85W is going to be some vague number is about all we know.

No definitive answer on this one I'm afraid.

We can take guesses, like that it may be a brief maximum.

It is really down to you, what you feel comfortable with.

I feared that might be the case. I guess my only option is to try and find other users with experience of the monitor and see if they know concrete facts on the matter. And if that fails just drop the cash for the 200W version.

I'd like to say that I really appreciate the help and advice I've received here. You guys have been really patient with my questions and I feel I've learnt a lot. Many thanks! And don't be surprised if you see me post again asking for help on connecting dual outputs in parallel.

Hi Jim,

No worries, we can address the connections question when you get to that.

Sorry there wasn't a clear answer between which of the two to go for : ( I hope your project works out well. Is it an arcade machine?

If you can borrow a site transformer (or hire it <£10 from Jewsons) you could take an actual current measurement with a multimeter before deciding on purchasing a transformer for permanent use.

You will also need the appropriate adapter cable though to convert from a 110v 16A commando plug though for the purpose of the test.

You could also perhaps take the monitor to a local electrician and get them to measure the current for you.

However, with these methods you will quickly have paid out the difference in cost between the two transformers.

Another option might be to visit a local FabLab / Maker group who may have such test facilities that you could use.

Just a quick question though - the manual mentions that the monitor can be furnished with an isolation transformer. I take it that you have checked that there isn't already a 1:1 isolation transformer in circuit ?

The circuit diagram also shows a AC supply fuse rating of 1.5A so that gives an indication that the current will be significantly less than that.

If you can borrow a site transformer (or hire it <£10 from Jewsons) you could take an actual current measurement with a multimeter before deciding on purchasing a transformer for permanent use.

You will also need the appropriate adapter cable though to convert from a 110v 16A commando plug though for the purpose of the test.

You could also perhaps take the monitor to a local electrician and get them to measure the current for you.

However, with these methods you will quickly have paid out the difference in cost between the two transformers.

Another option might be to visit a local FabLab / Maker group who may have such test facilities that you could use.

Just a quick question though - the manual mentions that the monitor can be furnished with an isolation transformer. I take it that you have checked that there isn't already a 1:1 isolation transformer in circuit ?

The circuit diagram also shows a AC supply fuse rating of 1.5A so that gives an indication that the current will be significantly less than that.

Thanks for the suggestions Dave. Aye the trouble is I could very well end up spending the difference quite quickly. I'm posting in a monitor enthusiasts thread so perhaps someone there will know a definitive verdict?

No sadly there's no transformer with the monitor. I aquired the monitor seperately and am putting it in my own casing. Afaik, originally whatever cabinet the monitor was in would have held its isolation transformer too.