lm317power supply
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?
A home work question! But you declare it as such, and you already understand the concept - in this scenario (a voltage source) current isn't "pushed", it's "pulled" by the load. So the short answer here is that you'll be OK buying the 25 V, 2 A transformer.
A different way of thinking about this is for both voltage and current, the listed rating is what is/can be supplied. Therefore a 25 V, 2 A transformer will have 25 VAC on the output and the maximum current which can be drawn is 2 A. If the load draws less than 2 A, the no problems. More current is beyond what can be supplied, which will mean the transformer will overheat, with the follow-on consequences you might imagine.
You can't use the 12 V transformer because - assuming you're using a linear regulator - you need to supply a higher voltage to the regulator than what it regulates to. Exactly how much higher is a question for reviewing the data sheet for the device.
No, this won't work to give you +/- supplies, because of the common negative rail.
You can, however, use the transformer you have to give you +/- adjustable supplies by using two half-wave rectifiers (2 diodes), double up on the capacitor values to keep the ripple at ~2Vp-p, and an LM317 for the positive and an LM337 for the negative supplies. With suitable (large) heat sinks capable of dissipating about 10W each it should be good for about +/-0.6A. You could also use 10,000uF caps (say 25V rated) to get a bit more voltage range at high current.
simulate this circuit – Schematic created using CircuitLab
Best Answer
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.