# Help

#### Nirvana

Joined Jan 18, 2005
58
Hi there could anyone tell me what a voltage regulator actually does, i know that it helps to smooth DC but can it amplify the voltage in any way?

#### pebe

Joined Oct 11, 2004
626
Originally posted by Nirvana@Jan 18 2005, 08:30 PM
Hi there could anyone tell me what a voltage regulator actually does, i know that it helps to smooth DC but can it amplify the voltage in any way?
[post=4639]Quoted post[/post]​
It has the property of holding its output voltage at a fixed value with virtually no ripple - even though its input voltage may vary considerably. Some regulators are adjustable.

There are provisos. The minimum input voltage must exceed the output voltage by a certain amount (usually around about 2 volts), and the maximum input voltage and current drawn are not exceeded.

#### Nirvana

Joined Jan 18, 2005
58
Originally posted by pebe@Jan 18 2005, 08:35 PM
It has the property of holding its output voltage at a fixed value with virtually no ripple - even though its input voltage may vary considerably. Some regulators are adjustable.

There are provisos. The minimum input voltage must exceed the output voltage by a certain amount (usually around about 2 volts), and the maximum input voltage and current drawn are not exceeded.
[post=4641]Quoted post[/post]​
Hi i have recently made a dual power supply for my couse but i am wondering how i got the output voltage. You see i have a transformer which outputs 12V and from that using the made up circuit board along with the variable resistor I can adjust it to give out 17.5V. How can i do that when i am given only 12 volts , and there are no amplifiers in the circuit.??

#### pebe

Joined Oct 11, 2004
626
Yes, there are amplifiers in the regulator and they can become unstable unless they have capacitors on input and output rails as prescribed in the data sheet. These are usually .1uF or .22uF, but *not* aluminium electrolytics. If you leave these out the circuit will oscillate. I have no experience of output exceeding the input volts, but I have seen 5V increase to 8V when the caps have been left out.

Have you measured the rectified/smoothed input to the regulator? It could be around 17V with no load. Could your regulator be short circuited?

#### n9xv

Joined Jan 18, 2005
329
Hi,

Your 12-VAC transformer is most likely 12.6-VAC, which is a common rated value. The voltage regulator is not amplifying anything, its just regulating a voltage to the rated value of the regulator. You ask where does the 17.5 volts come from when the transformers output is only a 12-VAC - - -

The transformer rating is an RMS value. In a typical transformer-type regulated supply, the DC voltage is derived from the peek value of the AC voltage. Lets walk through the circuit:

The transformer delivers 12.6-VAC, that is input to a full-wave bridge rectifier. The bridge rectifier throws the negative going half of the AC sine wave up on top with the positive going half of the AC sine wave. The output of the bridge rectifier is unfiltered DC at a value equal to the peek value of the AC sine wave or mathematically;

VDC = 1.414 * VAC (transformer output) NOTE: square root of 2 = 1.414

1.4 * 12.6 = 17.6

This being the maximum value available. It sounds like your circuit uses an adjustable voltage regulator so as to vary the output from a minimal value up to the 17.5-VDC.

For a pure AC sinewave voltage, the peek-to-peek value is 2.828 * VAC-rms.

The 120-VAC from the wall outlet is actually 340-VAC (peek-to-peek).
But 120-VAC-rms is the effective value that will produce the equivalent heating effects as 120-VDC. The peek-to-peek value is there and can be measured, but the rms value is the part that does any real work in the circuit.

did that help at all?

n9xv

#### Nirvana

Joined Jan 18, 2005
58
Yes thanks you very much, that has helped me a lot, much appreciated.

#### vineethbs

Joined Nov 14, 2004
56
but n9xv , can a voltage regulator give an o/p which is very near to the supply voltage ,& nirvana , also cud u tell me what regulator ics r u using ?

#### Nirvana

Joined Jan 18, 2005
58
Hi there THEY ARE
: 337 P+ AND 317

#### magik

Joined Jan 31, 2005
2
use 7812 IC