# DC Power Supply

#### Ahmed Zafar Khan

Joined Feb 18, 2016
12
I have designed a 5V ,350mA fixed power supply with a 12 Vrms step down transformer. the voltage across filtering capicitor should be almost peak value i.e 15V or 16V . but I am getting 12 volts acrossthe capacitor why is it so.

#### AnalogKid

Joined Aug 1, 2013
10,308
Schematic.

Is the supply loaded at 350 mA when you are measuring the voltage?

Schematic.

What is the size of the filter capacitor?

Schematic.

ak

#### Ahmed Zafar Khan

Joined Feb 18, 2016
12
YES IT IS WHEN THE CIRCUIT IS LOADED .

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#### Dodgydave

Joined Jun 22, 2012
10,829

#### Ahmed Zafar Khan

Joined Feb 18, 2016
12
image

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#### crutschow

Joined Mar 14, 2008
32,062
We mean the schematic.

#### Alec_t

Joined Sep 17, 2013
13,441

#### Ahmed Zafar Khan

Joined Feb 18, 2016
12
MAY IT WORK

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#### Dodgydave

Joined Jun 22, 2012
10,829
NO, you have resistors in series with the output supply, and the zener is pulling too much current through the resistors, i would use a lm7805 regulator instead of the resistors and zener.

#### Ahmed Zafar Khan

Joined Feb 18, 2016
12
SIR THIS IS OUR LAB PROJECT USING ZENER MY QUESTION IS WHY VOLTAGE DROPS ACROSS CAPACITOR THEORY BASED ANSWER NEEDED

#### ronv

Joined Nov 12, 2008
3,770
I have designed a 5V ,350mA fixed power supply with a 12 Vrms step down transformer. the voltage across filtering capicitor should be almost peak value i.e 15V or 16V . but I am getting 12 volts acrossthe capacitor why is it so.
The capacitor gets charged by the peaks only so the charge current is quite high. This makes the drop across the 1 ohm and the diodes higher than you think.

#### WBahn

Joined Mar 31, 2012
28,491
What is the average current through the 15 Ω resistor between the cap and the zener?

What fraction of the time do the diodes spend conducting?

What is the average current through the 1 Ω resistor between the bridge and the cap?

What is the average voltage drop across that 1 Ω resistor?

#### WBahn

Joined Mar 31, 2012
28,491
SIR THIS IS OUR LAB PROJECT USING ZENER MY QUESTION IS WHY VOLTAGE DROPS ACROSS CAPACITOR THEORY BASED ANSWER NEEDED
Typing in all caps on a forum (or e-mail) is considered the equivalent of shouting and it is also considered quite rude. Please refrain from the practice (except, of course, when your intent is to shout and be rude).