December 2014
I picked up a 9V switching power supply wart from one of the online suppliers that advertise in Nuts & Volts. Unfortunately, it melted two Arduino's before I realized what was happening. Apparently, the no-load voltage on a switching supply can be up to twice the rated voltage — in my case, 18V. Should I add a load resistor to the internal circuit of the wart so there's always a load? Any other suggestions?
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Fritz, all you need is an appropriate 3-terminal voltage regulator to protect your Arduino boards. I don't know what voltage your board needs, so I suggest integrating an LM317T programmable regulator between the board's power jack and the board. The regulator will take the wall-wart's high voltage and reduce it to what the board requires. To program the regulator, connect it between the wall-wart and your board's DC input using this diagram as a guide:
Wiring isn't too critical (point-to-point), but keep lead lengths as short as practical. Be sure to include the 2 capacitors as they'll help "keep things stable" and prevent unwanted high-frequency oscillations. NOTE: the voltage rating of C2 should be above 24V as it's typically an electrolytic. Variable resistor R2 will let you dial in the output to what your board requires. BE AWARE: Keep the maximum current draw ** BELOW 1 AMP **, use a heat sink on the regulator (a TO-220 type clamp-on will be sufficient) and keep the regulator's input voltage BELOW 35 VDC!
If you don't want to bother with a variable resistor for R2, visit www.reuk.co.uk/LM317-Voltage-Calculator.htm as it has a nice table of standard resistor values to use for R1 and R2 for programming various output voltages (and other good info on the regulator).
