dana@lando.la.locus.com (Dana H. Myers) (03/17/90)
In article <844@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Gregory Ebert) writes: >You can 'fool' a 78L05 into working as a "78L07" by putting 3 1N4148 >(or your favorite Si diode) in series with the ground terminal of the >regulator. Basically , the regulator only cares that the "out" terminal >is 5 volts above the "gnd" terminal. The 3 diodes will give about 2.1 >volts drop, thus Vout = 5.0 + 2.1 = 7.1 volts ~ 7.0 volts. Be sure to >put a bypass capacitor from the "gnd" terminal to the "out" terminal, >as well as across the diodes. > > If that doesn't satisfy you, try using an LM317L -- it's fully >adjustable. Using the diodes like that makes the output voltage become quite temperature sensitive, I'd advise against it. The LM317 isn't terribly different from the 7805, it just has a low "output" voltage, very close to the internal 1.2V reference. The output voltage is boosted by using the device quiescent current to raise the "ground" pin to a higher voltage. Most configurations also feed some current from the output to the ground pin, I believe to swamp out the effects of device current variation. Either use the LM317 or the 78L05, in either case use the "variable output" configuration. The LM317 comes in handy when the desired output is below 5V (since floating the ground pin only raises the output). BTW - using diodes as a voltage reference used to seem pretty tricky to me, but then I remembered the -.21%/degree C variation in output voltage. Take a standard 1N4148, measure the forward voltage at 1 mA, it'll be around .6V at room temp. Try heating the diode up with a blow drier or just breathe on it. The voltage starts dropping. Now shoot some freon "Component Cooler" on the poor diode. Voltage suddenly jumps up. Now, the only time I use a diode for the voltage drop is when I'm compensating some other part of a circuit for temperature sensitivity. ***************************************************************** * Dana H. Myers WA6ZGB | Views expressed here are * * (213) 337-5136 | mine and do not necessarily * * dana@locus.com | reflect those of my employer * *****************************************************************
straka@cbnewsc.ATT.COM (richard.j.straka) (03/21/90)
In article <5598@oolong.la.locus.com> dana@locus.com (Dana H. Myers) writes: >In article <844@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Gregory Ebert) writes: >>You can 'fool' a 78L05 into working as a "78L07" by putting 3 1N4148 >>(or your favorite Si diode) in series with the ground terminal of the >>regulator. Basically , the regulator only cares that the "out" terminal >>is 5 volts above the "gnd" terminal. The 3 diodes will give about 2.1 >>volts drop, thus Vout = 5.0 + 2.1 = 7.1 volts ~ 7.0 volts. Be sure to I thoought that the commonly accepted way of doing this was to simply put the ground terminal of the regulator at the middle of a 2-resistor divider between the output and ground. The extra current coming out of the ground, as I recall, is either quite repeatable, or negligible. Anyway, the divider circuit is found all over the place in the old National Semi data books. At worst, it would be a little trial and error with the resistor values. -- Richard Straka AT&T Bell Laboratories, IH-6K311 ------------------------------------------------------------------------ UUCP: att!ihlpf!straka MSDOS: All the wonderfully arcane ARPA: straka@ihlpf.att.com syntax of UNIX(R), but without the power.