Old educational power supply upgrade.

Thread Starter


Joined Mar 25, 2009
I'm a high school teacher and have about 16 power supply that i want to upgrade. I repair them all but there is still a problem : they present, on a oscilloscope, ripples with a load and they don't present constant voltage when varying the load.

They are very simple construction : 120V to a VARIAC, VARIAC to a transformer (27v AC max), tansformer to a bridge rectifier that have a 1000uF capacitor in parallel with the "DC" output. They are built like tanks!

I tried to add parallel caps up to 10000uF, it helps for the ripples but that is not perfect, expecially with high current.

These power supply are rated 30v 5A VARIABLE "DC"

What can i do to remove the ripples and to regulate the variable output. I'm looking for an upgrade circuitto add in series that will do the job that i can put in the exixting box without too much mods.

In the old days we only power light bulbs, ceramic resistors and switches. Not we uses LED, transistors, buzzers and "delicate" components. So these power supply are no more suitable for my teaching...



Joined Oct 2, 2009
Can you post a photo so we can see if it is worth salvaging?
What is the range of voltages and currents do you plan on using in the present and future?


Joined Nov 23, 2012
Really, the ripple is not so bad, add a voltage regulator to the output and you will be set. Los note that the modern LEDs, chips and transistors require much less current than light bulbs, motors. You can teach a lot of different circuits, microcontrollers and LEDs without needing more than one amp. Check how much ripple these units have with only one amp load.

Some ripple will actually be an added learning experience for your students. They will be able to directly see the benefits of a voltage regulator. A regulator can be used in each circuit breadboard, they are dirt cheap (lm317, lm7805, lm338,...) and required for many circuits anyhow.


Joined Apr 24, 2011
Simple is good! But simple is, well...simple. While it sounds like these supplies are rock solid and dependable they do lack the subtle and sophisticated advantage of a semiconductor regulator.

A post linear regulator would give about zero ripple, and also provide some better current limiting.

What sort of projects does your curriculum cover, and what are their power needs? Knowing that we could suggest some choices.

The good ole 78xx (high current <=1.5A) and 70Lxx (low current <=0.15A) series IC regulators would be a good starting point; replace the xx with the voltage you need (05V, 12V, 15V). Radio Shack even has them. You should have some cap on either end too. And maybe a heatsink... making these could be a good student project in itself.


Joined Mar 3, 2012
The downside of going for a full voltage regulator is that you then either have to leave the variac at max the whole time and sweat off lots of power at low output voltages, or somehow gang the regulators control pot with the variac control so the variac always puts out just a few volts more than the regulator is set to.

If you want to remove ripple but aren't too worried about absolute voltage regulation try adding a capacitance multiplier. http://sound.westhost.com/project15.htm

Keep your extra capacitance in the circuit because it will still reduce the ripple, and also the amount of voltage that must be dropped by the capacitance multiplier. In the capacitance multiplier circuit a moderately sized capacitor (100 to 100 uF) is multiplied by the transistor gain, maybe 1000 if you use a darlington pair.

This is very simple and reduces the ripple dramatically whilst still allowing the variac to control the output voltage. It's not regulated so the voltage will still drop under load as the power supplies do now. You will also get a slightly lower output voltage than previously by a near constant offset, you could rotate the label on the variac slightly to correct this