DIY USB Power

Lee Hosty pointed out Jason Striegel’s clever USB power kludge. It gets its juice from a Zener diode that conducts in the reverse direction when it reaches its breakdown voltage of 5.1 volts. Voltage across the diode will be that same 5.1 volts, so it acts to limit the 9 volt battery’s power supply down to roughly 5 volts, which is the USB power rating. The resistor in Striegel’s setup prevents the circuit from shorting when the load (your iPod or your phone) is removed.