I want to power a GHI Panda II single board computer that has a GHI FEZ Connect shield attached for Ethernet access. GHI Documentation specifies 6.5 V to 7.5 V input for the Panda and shield that can use a total of 238mA. I have a nice regulated power supply providing 12 Volts. What is a decent way to change that 12V to 7V (give or take 0.5V)? Is a simple resistive voltage divider adequate? If so, what size of resistors? It seems to me that the resistors need to be in a ratio of 5:7, but not sure what to use to provide the required current. Or maybe i should be using something totally different. Thanks.
You could use a resistor divider but that doesn't regulate the output voltage at all - the more current you take, the lower the output voltage is.
If maximum current is 238mA you could use a resistor from the 12V feeding into a zener diode connected to ground. 1N5342 is a 6.8V zener diode rated at 5W. If this is connected to 12V via an 18 ohm resistor you should get a reasonable 6.8V output across the range of currents. In fact, at the point where 6.8V is on the verge of dropping, the current output will be about 288mA. If you put a schottky diode in series with the zener you could lift the voltage up to about 7.2v. This is an optional idea should tolerances on the 6.8V zener work against you and it ends up more like 6.46V (down 5%).
Power ratings are 5W for the specified zener although a 3W device would be OK. The resistor power rating needs to be greater than 2W to avoid it frying too much.
You could use a linear voltage regulator like the 7805 - these can easily be adjusted to produce 7V with a small pot in series with the ground pin. How much power will this self-generate - it's dropping 5V whilst supplying up to 238mA so it'll dissipate over 1.2W so a heat sink is going to be needed - just a small one.
Or maybe even consider a power efficient buck regulator like the LM2575 (simple switcher). These are probably about 85% power efficient so it'll dissipate about 0.3W and not need a heat-sink.
Personally I'd go for a switcher unless your application was particularly prone to high-frequency ripple noise on the regulated output.