# Simple 240v - 12v power supply

Discussion in 'Electronic Basics' started by Peter, Sep 22, 2003.

1. ### PeterGuest

Can someone please explain the workings and pros/cons of the following
simple power supply. It is from a 240v security sensor
light(Australia). In case the diagram doesn't display properly the
240v AC Active runs via a R68 2W resistor to the "top" of a diode
bridge. The 240v Neutral runs via a 0.068uF 400V poly cap in parallel
with a 100k 1/4w resistor, to the "bottom" of the diode bridge.

After the rectifier is the standard 1000uF cap and 12v 1W zener. That
supplies power to the relay that switches the 240v supply to the
light. After the zener is is a 7408 regulator and 470uF cap that
provides the supply for the heat sensor circuitry. It just seems too
simple.

1. Why would anyone bother with a transformer?
2. How much current can the circuit supply and what formula connects
the current to the 0.068uF cap and 68 ohm resistor?
3. How "safe" is the "low voltage" side? (Not that I plan to go
playing with it with the power on.)

It's just that I recently built a power supply for a small circuit
using a normal transformer and cursed because it would not fit in the
box I was planning to use. Then I saw this circuit and am thinking of
re-engineering the other project using a similar circuit.

R68 2W
------/\/\/\-----.
/ \
240vAC / \_____________________
\ / | |
0.068u 400v \ / | |
----,----||----,-. =1000u |
| | | Z 12vZ
.--/\/\/\--' | |
100k GND GND

2. ### CFoley1064Guest

Can someone please explain the workings and pros/cons of the following
You're looking at a non-isolated power supply. It isn't safe, because it
isn't isolated from the line -- you could easily get an electrical shock by
coming in contact with either side of the 12VDC power supply.

Figure out the impedance of the components at 50 or 60 Hz (depending on your
country), and you can go from there to figure out the ripple on the cap, and
how much current (minimum and maximum) is going to be going to the node where
the zener and cap are. From there, it's just a standard shunt power supply.

The only "pro" to this circuit is that it doesn't have a transformer -- meaning
it weighs less and takes up less space. The negatives are practically endless.
Don't use this -- it's a cheap, foolish design that will probably smoke and
pop a fuse (at best) the first time you get a good voltage spike on the line.
Anyone who comes into contact with the "low voltage" may be at risk of severe
electrical shock if they are in contact with earth/ground.

This power supply is not suitable for anything you would want to make.