# Beginner regulated power supply circuit help

Feb 14, 2018
40
Hello so im pretty new in electronics and i need to make a regulated 100mA 12v output power supply.
The capacitor formula is =
Capacitor=(load current*ripple voltage time of one cycle)/peak to peak ripple voltage
The zener diode use 20mA so the load current is 120mA.
But how do i know the ripple voltage time and the peak to peak ripple voltage.

#### SOLARWIND

Jul 18, 2014
5
Hello so im pretty new in electronics and i need to make a regulated 100mA 12v output power supply.
The capacitor formula is =
Capacitor=(load current*ripple voltage time of one cycle)/peak to peak ripple voltage
The zener diode use 20mA so the load current is 120mA.
But how do i know the ripple voltage time and the peak to peak ripple voltage.

The only reliable way, is using an oscilloscope.

#### duke37

Jan 9, 2011
5,364
I presume that you are using the mains for the power. The time between charging pulses will depend on whether you have full wave or half wave rectification and the frequency of the supply.
In the UK we have 50Hz so the time for one cycle is 1/50sec or 20ms. With full wave rectification the time between pulses will be 10ms.
dV/dT = I/C

Rule of thumb, the reservoir capacitor can be sized at 1000μF/amp.

Make a supply to provide 15V then use a 7812 to give a stabilized 12V output, you may need a heat sink for 120mA.

Feb 14, 2018
40
I presume that you are using the mains for the power. The time between charging pulses will depend on whether you have full wave or half wave rectification and the frequency of the supply.
In the UK we have 50Hz so the time for one cycle is 1/50sec or 20ms. With full wave rectification the time between pulses will be 10ms.
dV/dT = I/C

Rule of thumb, the reservoir capacitor can be sized at 1000μF/amp.

Make a supply to provide 15V then use a 7812 to give a stabilized 12V output, you may need a heat sink for 120mA.
Yes thanks i had the info about the 10ms but i didnt understand how it was found.I am using a full wave rectifier it is a project so i need to use a zener diode already found one with reference.I need to explain everything so what about the ripple voltage

#### duke37

Jan 9, 2011
5,364
As shown, dV/dT = I/C
Rearranging, dV = dT*I/C
Taking dT=10ms, I=100mA, C=1000μF
dV = 0.01*0.1/0.001 = 1

This assumes the pulse charges the capacitor in a very short time and that the discharge current is constant.

A 7812 regulator chip will be simpler and better than a Zener diode and will only drain about 5mA.

#### BobK

Jan 5, 2010
7,682
The formula you are using is for an unregulated supply. Any form of regulation will reduce the ripple. A decent one will reduce it to near zero.

Bob

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