# AC Mains & Power

#### jzq

Joined Jul 28, 2008
6
hi,

im having a hard time understanding current and power. i want to connect a step-up transformer directly to the mains. the uk mains is 240 Volts and the sockets are rated at 13 Amps.
Now my transformer would be marked something like: output is 1,000 Volts and outputs 20mA. <- is that all my circuit will be able to draw, or is that how much the transformer can take before over heating?

Appliances like kettles and tvs have makings on them that say how much power they take... But how do i find out how much power my circuit will take.

I know the power equation things but how do i use them to find how much my circuit will want to draw?

Does my circuit, connected to the secondary of the transformer affect how much the primary coil will take?

how much can the primary take, etc...

thanks!

#### sparkey2000

Joined Mar 9, 2008
8
For your transformer V.I = VA so;
230x13 = 2990VA

Therefore you can only draw a maximum of 2990VA on the primary side of your transformer.

To work out the secondary current divide the VA by the secondary voltage:

2990VA / 1000 = 2.99 Amps

You transformer can be rated at 3000VA (3kVA) however the max current you will be able to draw on the secondary will be 2.99A (without blowing the primary fuse).

You will also need to consider the transformer inrush current; it may be worth talking to the transformer supplier and informing him you will be fusing the primary with a domestic BS1362 plugtop fuse.

I am worried about the secondary voltage you require (1000VAC); while this is classed as low voltage 600 - 1000VAC in terms of BS7671 the transformer voltage will be higher off load.

IF YOU DONT KNOW WHAT YOUR DOING PLEASE DONT PLAY WITH THESE VOLTAGES; REMEMBER 50VAC CAN KILL YOU.

#### jzq

Joined Jul 28, 2008
6
I still dont think that answers my question. my fault, im finding it hard to explain.

I understand what your saying, but the mains sockets are rated at 13 Amps before they melt.

What i want to know is how do i find out how many amps my circuit will take from it. It obviously wont take the full 13 Amps!

- The transformer markings say that it will give an output of 1000V at 20ma

so what fuse would i need, and how many amps will this draw?

By the way Im just using hypothetical values!

#### thingmaker3

Joined May 16, 2005
5,073
1000V * 20mA = 20VA. 20VA / 240V = 83mA. If it were me, I would allow some overhead, and limit current to 80% of maximum rated value. To the best of my (admittedly limited) knowledge, that would be a 63mA fuse. I would choose a fast-acting fuse.

I would also remember at all times that high-voltage experiments have killed people who knew what they are doing, and even more people who did not know what they were doing. Always follow all safety measures. Never ever cut corners or say "just this once, its a tiny chance."

#### theamber

Joined Jun 13, 2008
321
hi,

im having a hard time understanding current and power. i want to connect a step-up transformer directly to the mains. the uk mains is 240 Volts and the sockets are rated at 13 Amps.
Now my transformer would be marked something like: output is 1,000 Volts and outputs 20mA. <- is that all my circuit will be able to draw, or is that how much the transformer can take before over heating?
Both but usually the maximun rate is done for safety.
Appliances like kettles and tvs have makings on them that say how much power they take... But how do i find out how much power my circuit will take.
I know the power equation things but how do i use them to find how much my circuit will want to draw?
Usually the appliances say how much power they will consume at maximun load. I think you need to look at the power factor chapter this may help you understand more.
Does my circuit, connected to the secondary of the transformer affect how much the primary coil will take?
how much can the primary take, etc...

thanks!
You need to look at Kirchoff law.
I do suggest you to use caution but certainly experimenting with the real thing is what is going to make you undestand it better.

#### studiot

Joined Nov 9, 2007
5,003
I don't know where you obtained your transformer, but step up transformers are not so common these days.

If you are using a junkbox look carefully at the label for other information. In particular make sure it is rated at 50/60 cycles not some higher frequency. 400Hz used to be common in the aircraft industry. These will overheat if placed on UK mains.

#### jzq

Joined Jul 28, 2008
6
thanks thingmaker3. You came up with the answer i wanted.

to sum it all up: basically you find the power of the secondary, and then divide that by the input voltage of the primary! - It took me ages to find this!

I thought it would be better to use a fuse 10% higher than the maximum draw (to compensate for transformer loss). And also Because if i go lower, it would be likely to blow the second i switch on!

#### theamber

Joined Jun 13, 2008
321
Yes but make sure that don't want to use a fuse higher than the maximun current rating of the transformer wires then if a short happens it will melt the wires instead of blowing the fuse.

#### thingmaker3

Joined May 16, 2005
5,073
And also Because if i go lower, it would be likely to blow the second i switch on!
You expect inrush current? You have me curios now... what manner of beast is the load?