Help every time I build a power supply the output voltage is 2X the input
and I don't want it that way. I connect a transformer to the mains line and
then put a bridge on the output and follow that with a 2200 microfarad cap
to soften the DC. The voltage will be twice what it is suppose to be e.g..
transformer is rated at 12V output I get around 20V why?

IS THERE ANY WAY TO STOP THIS?
"Robert Morein" wrote in message
...

"Patrick Cobb" wrote in message
. ..
Help every time I build a power supply the output voltage is 2X the
input
and I don't want it that way. I connect a transformer to the mains line
and
then put a bridge on the output and follow that with a 2200 microfarad
cap
to soften the DC. The voltage will be twice what it is suppose to be
e.g..
transformer is rated at 12V output I get around 20V why?

Because the 12V is the "RMS", ie., "root-mean-square" value.
In the absence of a load, the capacitors charge up to the peak value.
This accounts for the discrepancy you see.

Or you could build a simple switcher and not waste the heat. Check out
National ( www.national.com ) they make many, easy to implement, with great
apps notes.

JF
"Dan Barlow" wrote in message
...
Patrick Cobb wrote:
IS THERE ANY WAY TO STOP THIS?

Yes! You need a load resistor to absorb the extra peak voltage.
If you are going for 12V, use a replacement lamp from the auto
section of your local department store. You can get a tiny
dash light for a little current drain, or a headlight for around
6 Amps. It might be fun to get a few different small to medium
ones and chart how much voltage drop you get for a certain amount
of current through the bulb.

If you want a very close 12V you need a 7812 voltage regulator.
Radio Shack may(!) still carry them. They waste the extra volts
as heat so you need a metal tab on them so the air can carry the
heat away [called a "heat sink" also at R.S.].

Combining both can be very effective.
-Dan Barlow

"wave_man" wrote in
:


Well, if you keep old electronics nearby, most devices (VCRs, TVs, PC
Monitors, etc) will have AT LEAST ONE regulator in circuit that you can
use, as are the heatsinks. and yes, RadioShack does still carry the 7812
and Heatsinks. tho heatsinks are easy to pick off of an old parts PCB.








Or you could build a simple switcher and not waste the heat. Check
out National ( www.national.com ) they make many, easy to implement,
with great apps notes.

JF
"Dan Barlow" wrote in message
...
Patrick Cobb wrote:
IS THERE ANY WAY TO STOP THIS?
Yes! You need a load resistor to absorb the extra peak voltage.
If you are going for 12V, use a replacement lamp from the auto
section of your local department store. You can get a tiny
dash light for a little current drain, or a headlight for around
6 Amps. It might be fun to get a few different small to medium
ones and chart how much voltage drop you get for a certain amount
of current through the bulb.

If you want a very close 12V you need a 7812 voltage regulator.
Radio Shack may(!) still carry them. They waste the extra volts
as heat so you need a metal tab on them so the air can carry the
heat away [called a "heat sink" also at R.S.].

Combining both can be very effective.
-Dan Barlow