cjdelphi:
In that circuit i see a constant 5v regulator and a constant current of 300ma.....So what are you saying?
In the circuit, as drawn, I see a 7805 linear voltage regulator being fed by a 1.5V cell. But assuming that the poster meant that the source voltage was, infact 7V or above, as required by the regulator, the regulator will supply 5V out, as long as the supply can keep input at, or over, 7V; at a MAXIMUM of 1A draw, as long the supply can supply 1A or more. The output is only regulated to 5V at a max of 1A. if the input current (source battery depleation, for example) drops below 1A, so does the output current. One Amp in, through 10 Ohms (disregarding losses in the transistor) is 500mA, not 300mA. What I am saying is read the datasheet.
MarkT,
"But a series resistor is one perfectly good way to get constant current drive..." for a first year E.T. student, or maybe more accurately said, Until the resistor heats up and changes value (remember what lousey temp coefficients resistors have?).
I do a lot of playing around LASER diodes at various wavelengths, and the first rule (after use your goggles) is ALWAYS use a constant driver driver circuit. Then you worry about things like heatsink requirements...
You CAN depend on a resistor for your current control. if you want to (they call it 'Kip Kay'ing'), but you are gonna loose more diodes to over-current-burn-outs than you will to ANY other cause.
You do it however you want. Listen, or not, read the datasheet, or not I did my part. See ya!