So I have a little power supply that is great for little projects. its
I have been feeding it 12VDC from a computer power supply and it seems to work. Is this bad for it?
What I like most is the variable voltage so I'm putting it in a case with a heat sink.
Questions:
1. I have the following parts to improve it:
100k pot 50k pot and 10k pot (comes with 4.7k pot)
Two 2.2k 10watt power resistors,10ohm 10 watt resistor, a whole bunch of 1/4 watt resistors. I want to ideally make a fine and coarse voltage adjustment or at least use one pot if thats not possible with these parts.
2. Two 3300 uf eletrolytic 25V rating. Eight 220uf 50V rating, eight 100uf 35V. Can I parallel this with the output to give it more instantaneous current? It already has a 2200 uf 35v and a 1uf 50V on the board. Or would adding a capacitor make a trivial amount of difference?
3. What does ohms law do with one fixed and one variable cap across the wiper or the ends of the pot?
I think people have different levels of "beginner".
Also I took the LM off the board and attached ~1.5 inch 20ga solid copper wires so I could mount it on the heat sink. Are these wires going to cause loss or heat that will make a difference? I havent turned it on since I have done this.
raspberrypi,
Powering your power supply from 12 VDC coming from a computer power supply is not a problem at all. The schematic you shown also show that 12 VDC is acceptable.
Download the TI LM317 datasheet from here
www.ti.com/lit/ds/symlink/lm317.pdfOn the TI LM317 datasheet, the circuit on page 10, section 8.2, figure 9, is very similar to the circuit you have.
The formula james_s made reference to is shown at section 8.2.2.
I understand that you want a fine and coarse adjustment. This is done using two potentimeters in series. You could add a 470 ohm to the 4.7k potentiometer you already have the 470 ohm pot would be the fine adjustment. You want Linear potentiometer, not logarithmic.
In theory, you could replace R1 with 1.2k and then add a 50k potentiometer in series with 4.7k potentiometer, and then the 4.7k pot would be the fine adjustment. I say in theory because all the examples in the datasheet are using a fairly low value for R1.
C3 on your circuit, is the same a Cadj on figure 9 on the TI datasheet. The datasheet reads "Cadj is recommended to improve ripple rejection. It prevents amplification of the ripple as the output voltage is adjusted higher."
IanB is right, if your kit didn't came with a knob for the 4.7k potentiometer, you should get one as this will help you adjust the potentiometer.
Mounting the LM317 remotely from the pcb using wires should not cause any problem.
Be careful when you desolder components from a pcb, too much heat can lift a pad.