I'm trying to build an adjustable voltage regulator to put out ~3 to 14 volts. I purchased a kit with PCB based on LM2577T-ADJ regulator. The original intention of the circuit was to simply regulate +5V to 12V.
I'm guessing this will be an elementary question for many of you... I took a couple electronics courses in high school, so I know enough to follow a schematic, but that's about it.
...so, here I am, modifying the circuit, and frying parts.
This schematic is more or less what the kit is based on:
**broken link removed**
More details on this page:
https://www.hobby-hour.com/electronics/lm2577-switching-regulator.php
It pretty closely. I wanted it to be adjustable. So I modified resistors R1 and R2, according to this formula:
VOUT = 1.23V (1 + R1/R2)
R1 was originally 2K, and R2 an 18K. Testing the breadboard this worked well to produce 12V. I replaced R1 with a 1K resistor, and R2 with a 10k potentiometer and an inline 1K resistor to which gave me 1K-11K ohms. Plugging this into the formula, it should technically regulate 2.23V to 14.76V.
When I finished it all up, it actually worked well, but the range was more like 5.5-14.5 give or take. That is fine for my purposes so I wasn't worried. The input source is 4AA batteries at 6V.
On the multimeter, this all worked well. I'm using this to power a 12V cpu fan. It operated well at first, but when I ran it at a higher voltage it ended up toasting the inductor. I happen to have a spare inductor, so it I soldered that one in, and it toasted it as well as soon as I cranked up the potentiometer.
I'm sure there is some math I'm not doing, and perhaps dropping from 2K to 1K on feedback resistor is creating some problems?
The PCB:
**broken link removed**
Thanks for any input.
Scott