Old educational power supply upgrade.

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, especially 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 circuit to 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. Now we uses LED, transistors, buzzers and "delicate" components. So these power supply are no more suitable for my teaching...

Solutions?

As you pointed out, in the old days that kind of regulation was sufficient. I don't think adding additional capacitors will be sufficient to give yo the clean voltages needed for modern circuits. However, you have the basic and expensive parts of the power supply there already, what you rellay need to do is change the voltage control from being the AC input to being the DC ouptut. You can keep the same transformer, recitfier and capacitors but you can replace the variac with a small potentiometer and use an electronic regulator at the output instead. This will give you excellent stability, almost no ripple and also allow you to incorporate things useful in classrooms like short circuit protection!

Brian.

Google LM317 - Its a variable output regulator chip. needs to be mounted oa a heat sink (piece of ALI).
Frank