# Calculation Of Mains Wattage Of Transformer

#### eirman

Joined Aug 18, 2017
14
Mains Input Rating of Transformer = 230 Volts AC
Low Voltage Output Rating of Transformer = 12 Volts DC / 1 Amp

If the transformer is running at full capacity, the wattage at the low voltage end is obviously 12 Watts.

How does one calculate the high voltage mains wattage, if the transformer is running at full capacity?

#### bertus

Joined Apr 5, 2008
22,292
Hello,

An ideal transformer would also be 12 watts at the mains side.
As most transformers are not ideal, the input power will be higher.
How much higher this power would be depending on the losses in the transformer.

Bertus

#### recklessrog

Joined May 23, 2013
985
Watts is only really valid if the load is purely resistive and the current and voltage through the load are in phase. For any reactive load, then the power rating is calculated as Volt-Amperes V/A.
Transformers have losses which are a combination of many factors, Resistive losses etc are dissipated as heat in the transformer and there is an lot to know about the core material to calculate the magnetic factors. As a general rule you can expect probably 10-20% loss in an iron cored mains transformer.
Also you need to know how good the regulation is, Output voltage vs current, some cheap transformers are appalling in this respect, whilst a really top quality torroid can be excellent.
You should read up on transformer design, which is almost a black art in itself.

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#### recklessrog

Joined May 23, 2013
985
I used to have an excellent book called "designing coils and transformers" by M.C Sharma, published by BPB publications. You may be able to get a copy from Amazon, he covers all you really need to know in a very clear and concise way.
Unfortunately, one of my ex club members borrowed it and never returned it.
I may try and obtain another copy now it has come to mind.

#### recklessrog

Joined May 23, 2013
985
Hi Bertus, Yes that is the one, I can highly recommend it to anyone who is interested in studying or making there own coils and transformers.
Mr Sharma explains everything with a good balance between practical information backed up by the maths involved without getting bogged down with the subject.

#### eirman

Joined Aug 18, 2017
14
I had closer look power supply unit .....
It was made for Maplins and much bigger than average and quite heavy. One would initally guess that it could deliver 3 Amps.
It is regulated and says 12V 1000mA12VA (I'm not sure what the VA means in this case).

Joined Jul 18, 2013
28,782
I had closer look power supply unit .....
It was made for Maplins and much bigger than average and quite heavy. One would initally guess that it could deliver 3 Amps.
It is regulated and says 12V 1000mA12VA (I'm not sure what the VA means in this case).
1000MA (1A) X 12V = 12VA, If large and heavy it is going to be high VA than that by the sound of it.
Usually core size and secondary wire Ga is an indication of (K)va.
Max.

#### eirman

Joined Aug 18, 2017
14
I'll measure the AC current and calculte the wattage. (I'll be careful!)

#### recklessrog

Joined May 23, 2013
985
Some of the Maplins power supplies were very crude, a barely sufficient transformer, four diodes and an inadequate capacitor, whilst others were very good with oversized components and included a regulator I.C.
Also, some of their switch mode psu's were very good, whilst others were C**P.
I have a lot of details on some of their power supplies, if you give me their part number for it I will see what I have.

#### eirman

Joined Aug 18, 2017
14
if you give me their part number for it I will see what I have.
Thanks recklessrog - It's Model: CC10

(I think it was used with PC Speakers)

#### recklessrog

Joined May 23, 2013
985
Thanks recklessrog - It's Model: CC10

(I think it was used with PC Speakers)
I have some info on around 70 of their power supplies and that number does not relate to any of them. If it was part of a system, then I would need to know what that was.
A couple of photo's showing the label and the input/output plugs or sockets may be helpful.
What are the dimensions? Sometimes they used their universal psu's as after market options.

#### eirman

Joined Aug 18, 2017
14
This is the only label on the PSU. (13cm is the longest dimension)
The first Icon icon on the bottom means Double Insulated - What do the other 4 mean?

#### recklessrog

Joined May 23, 2013
985
Hi, the last one means indoor use only, the fourth one probably indicates it is an AC to DC power supply, the third one is an internal thermal fuse, if the power supply has a transformer in it, then they are usually internal in the primary winding. I Think, although I'm not sure that the second denotes no common connection between the input and the output but I may well be wrong on that.
It must be quite old, there is no CE marking or several others like ROHS etc.
To be absolutely sure, try to google "power supply safety symbols" or similar.
There are so many in use now, many meaning more or less the same thing but for use in different consumer markets.

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#### noweare

Joined Jun 30, 2017
115
I would think you would have to look at the current and voltage waveforms on a scope since the mains side is inductive. Transformers have pretty high efficiencies so you could assume 95% or 5% is wasted as heat in the transformer. The secondary is probably 15 volts or so to get 12vdc at the output. So you could just take a stab at it. 15v @ 1 amp = 15 watts. 15 watts * .05 = 0.75 watts So this number would include all losses in the transformer.