Using a 12VAC Transformer For a LM317T Power Supply

Question

I have found a circuit diagram for a regulated power supply using a LM317T regulator which I wish to build as my first electronics project.

The only problem is that instead of a 24 VAC transformer I have a 12 VAC transformer and I do not have a 3300uF 35V capacitor, the highest rated one I have is 2200uF 16V.

Is it safe for me to replace these parts in the circuit with the only effect being the output will be a maximum of 12V?

Answer 1

Not quite safe.

First, the transformer is fine. However...

12V AC would deliver 16.8V DC minus the diode bridge drop of 1.2V, so 15.6V nominal. This is pushing the limits of the 16V capacitor, but looks OK at first glance.

But if your AC mains is above nominal, or if the transformer is poorly regulated*, then the voltage at no load will rise above the nominal 15.6V by anything up to: 10% for the mains voltage, and 15 or 20% for the transformer's regulation; I'll use a total of 25% - that would give a worst case voltage of 19.5V across it.

So be prepared for the capacitor to have a short, messy, and possibly spectacular life. You might get away with it for a short while, but better to replace it with a 25V one.

• Transformer regulation : to deliver 12V AC at its rated current, a transformer is designed to give 10%, 15% or 20% more when unloaded. Big transformers (> 2amps) may have 10% regulation; I'm assuming 15% for yours, but its datasheet may tell you for sure.

Answer 2

With a 12V ac input, running @ 11.5V load ,LM317T wont be needing to much

heatsink to use . Except when using 9V output and below. Your 16V capacitor is OK,

to use if it is 12V ac input or lower.