Created by
Created February 26, 2012
Last modified May 17, 2012
Tags power-supply   transformer   voltage-regulator  


A 7805 linear voltage regulator can't quite maintain its output voltage. Can you fix it?


V1, XFMR1, and BR1 model an unregulated wall-connected AC-to-DC adapter ("wall wart").

R1 models a load circuit, such as a microcontroller and LCD and a few sensors, that requires about 50mA.

U1 is a 7805 linear voltage regulator. It does a great job maintaining 5 volts on its output... until its input voltage (node "cap" here) drops below 7.5 volts.

Use the time-domain simulation to experiment.

There are several ways to fix this circuit, all of which you can try and see in CircuitLab:

  1. change XFMR1 and specifically the turns ratio. (What's the potential downside to this approach?)
  2. change C1's capacitance. (What's the potential downside to this approach?)
  3. change C2's capacitance. (What's the potential downside to this approach? Does it take more or less capacitance than putting it on the input side where C1 is?)
  4. change U1 to be a low-dropout-voltage model voltage regulator. (What's the potential downside to this approach?)
See also

AC Power Supply (Transformer and Bridge Rectifier)


Great tutorial. Keep 'em coming!

by krel
March 06, 2012

This is an excellent way to make students think more creatively

by JackPreacher
April 07, 2013

I'm a novice at circuit design and would be interested in the answers to questions #1 - 4 on this example.


by enusbaum
November 04, 2013

Here are my guesses at the answers:

  1. Decrease turns ratio = higher secondary AC voltage which will require higher voltage rated components and also dissipate more power
  2. Increase in C1 costs more, will stay charged longer after power shutoff. seems like best option is this one
  3. Maybe same downsides as 2, also less effective as increasing C1
  4. Not sure what downside to LDO reg would be here
by a5194326
August 29, 2014

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