OK I'll restate my question, I know what the forumila dose, what I want to know is why is 1.25 volts used, why not 1.35672 volts ?
Because, using an LM317, is the voltage that you have as reference voltage ..... but i think i have understood what you mean, and this explanation can confuse, so, let me try to state it in a more clear way (and forgive me for the bad English
)
The LM317 serie, is a
voltage regulator, where the output voltage is stabilized and regulated from a
voltage controlled loopback ..... the reference voltage generated inside the chip, is
1,25V, and the IC act in this way (in voltage mode): you set the output voltage with a resistor network that
take part of the output and give it back to the ADJ pin, that is connected to the comparator ..... when at the ADJ pin there's
1,25V, the output voltage is kept stable ..... if the output voltage increase, the VADJ (through the resistor network) increase, so the chip decrease the output voltage until the VADJ is 1,25V again, and ofcourse, the opposite when the output voltage decrease (hope it's clear til this point
)
Now, we use this regulator as
CURRENT stabilizer, that is not its main use (but it still work), but the regulation
is still made with a VOLTAGE loopback system ..... this means that, connected in the current regulation mode, the chip increase and decrease the output voltage for keep the
difference of voltage between the output and ADJ pin at 1,25V, and this voltage is the difference of voltage
at the sides of the resistor, that is produced from the current circulating through the resistor itself (and, cause it's a serie circuit, also through the load, in this case our LDs)
So, for calculate the
current that you want to get from the LM317, you must use the 1,25V value, that is the reference voltage, because you need to keep
this voltage (and no others) at the sides of the resistor ..... as example, for 100mA, you need to use 1,25/0,1 (V/R) = 12,5 ohm resistor, cause this resistance value is the one that keep 1,25V at the sides of the resistor, when a current of 100mA flow through it ..... if you use a different value, like as example the one that you said (1.35672), you get for 100mA a value of 13,56 ohm ..... but this value don't give you 100mA, cause the reference voltage IS 1,25V, and with 13,56 ohm, this give you 92mA only .....
Hope this is clear enough (if not, please don't shoot me, ok ?
)
EDIT: LOL, i know i type slowly, but not imagined that i took enough time for start a war (just kidding, Eudaimonium and Jerry, no offenses
)
