# Beginner transformer question

Discussion in 'Electronic Basics' started by Ant_Magma, Feb 26, 2006.

1. ### Ant_MagmaGuest

I'm looking for a transformer that is capable of stepping down 240V
from the wall outlet to maybe less than 20V depending on the type of
voltage regulator. My objective is to produce 3.3V from the wall outlet
to power my ICs.

1) Most of the times i see 2 same values for the secondary voltage, eg:
0-9,0-9 (9V). What does it mean? Does it mean i still get 9V output?

2) If i want to calculate the output current, do i take the power
rating lets say 3VA and divide with the output voltage, for example 3VA
/ 9V=0.33A?

2. ### JamieGuest

power is power, if the device can only deliver 100 Watts for example,
that is what you get .
when you scale down the voltage you simply get higher current at the
lower voltage which can still work out to the same max watts.
if you know the wattage of the supply source via the label on the
unit? then you can calculate the amps you will expect to get .
Watts/Volts = Amps.
so if lets say your little step down supply is 100 watt handling and
you have 9volts, the current should be a little less than 11 amps/.
i say a little less is because of other factors that come in when using
something like a xformer. you have losses and efficiency factors.
that is about as basic as i can get it.

3. ### Ant_MagmaGuest

But i do not know how much is the power supplied by the power outlet.
How do i calculate the output current then?

4. ### Rich GriseGuest

Is the assignment "make a power supply", or is the assignment "these ICs
need 3.3V @ 500 mA"?

If all you need is volts, then go here:
http://www.globalsources.com/gsol/I/AC-DC/p/2000000003844/3000000149681/sm/1000560192.htm
[mind the word wrap]
and look at:
GFP051-A315: 3.3V/1.5A

They don't show prices, but it can't be as much as you'd spend trying to
duplicate it.

If the assignment is "learn how to build a power supply", then any of the
basics tutorials should help here, or "Art of Electronics", by Horowitz
and Hill.

Good Luck!
Rich