240v to 12v transformer, how do you calculate current?

Heedtheadvice

Well it is getting late and I am not sure that our digression will be helping the OP. So.....
Your latter statement that the peak of the AC is 1.414 times the rms value is true - for a perfect sine wave (or cosine wave).
Not true if the wave has harmonics, hence one of the reasons for using RMS values.
Diode voltage losses can be zero such as could be obtained with the old mechanical switch diodes and thus the smoothed peak DC voltage you originally mentioned can be the 1.414 times the AC voltage. (Assumption of no current drawn or negligeable resistance/impedance in the supply circuit).

It is not true in most modern rectifier circuits that use semiconductor diodes even if we ignore high frequency effects. For very low output voltages that loss can be very significant (sometimes over a volt for full wave bridge circuit) and maybe designs require low voltage loss diodes. Not as big an issue at a 12v RMS AC input but still possibly significant for accuracy.

Suggest an easy to understand web page at

https://en.m.wikipedia.org/wiki/Rectifier

That has simple to more complex theory illustrated.

I do feel when making advisory statements​ on forums such as this we do need to include qualifiers such as with the earlier posts on this thread such as 'approximate' or 'ignoring' as we do not necessarily know how the information will be used -not always by the OP!

Heedtheadvice

........And that leaves me open to others picking holes (quite rightly too) in my post:rotfl:

keith969

Heedtheadvice wrote: »

........And that leaves me open to others picking holes (quite rightly too) in my post:rotfl:

Ok, you did ask :rotfl:

The OP says "The power supply says Input 100V-240V 50/60htz 1.6A so ok for 2A socket and the ouput says 12V 6A"

Note that input voltage: 100-240. This is no transformer/rectifier power supply, being able to take a wide range of input voltages and give a fixed output voltage implies it's a switch mode psu. Good old transformers aren't used these days as they would be big and heavy, plus smoothing at 50Hz would require equally large electrolytics etc.

"The power supply says Input 100V-240V 50/60htz 1.6A so ok for 2A socket and the ouput says 12V 6A but how do you get from 1.6A to 6A, i.e. how do you calculate this?"

You can't assume you can calculate from DC V and I out (12*6 I.e 72W) and somehow get back to 1.6A in. That input current is probably worst case with 100V AC in and a shorted output. About all you can say is that a 2A socket is going to be fine, which is all the OP needs to know...

Heedtheadvice

Excellent points and a valuable post to actually give a good answer for the OP.
I did ask, as you wrote Keith, and more than happy to see a post relevant to the question that had been asked. :j