Electronics > Beginners
Diode in GND pin of voltage regulator
not1xor1:
--- Quote from: Hero999 on July 05, 2018, 08:48:14 am ---
--- Quote from: not1xor1 on July 04, 2018, 05:27:12 pm ---
--- Quote from: Hero999 on July 04, 2018, 12:42:23 pm ---I think the diode hack will be slightly better than two resistors, because the voltage drop will be more stable, for changes in the bias current.
Of course this isn't something which should be designed in. The purpose of such hacks are to get one out of trouble, when they've make a mistake and ordered the LM7805, rather than the LM7812 and there's no time to get the correct part.
--- End quote ---
Or use an opamp as a buffer to provide the required bias current while programming any voltage (within reasonable parameters) you need.
--- End quote ---
What do you mean?
That sounds silly, when the LM317 or the appropriated LM78xx could be used in the first place, to give the correct voltage. The resistor/diode hack is only useful for correcting a mistake: unsolder the regulator's ground pin and add a diode or series resistor and another resistor from the output to the ground pin.
--- End quote ---
I'm referring to one of the circuit applications published in the ST datasheet (and quite likely in the original one of NSC).
You might even program a different voltage by using a couple of high value resistors as divider and a series of NPN + PNP emitter follower to roughly compensate the Vbe drop.
I like to think about various possible solutions... just for fun. :)
Relayer:
Hello tester43,
Here's just a simple reason you would insert diode/s to ground:
If say you only had a 12V regulator (i.e. 7812), but you wanted
13.4V on the output, you just insert 2 diodes in series and it will
give you the voltage you need...
Regards,
Relayer
wraper:
--- Quote from: not1xor1 on July 04, 2018, 05:09:47 pm ---And in any case, regarding the question of the original poster, an LM78xx is perfectly equivalent to an LM317 with built-in voltage divider.
--- End quote ---
It's not an equivalent at all.
78xx
LM317
not1xor1:
--- Quote from: wraper on July 06, 2018, 10:33:28 pm ---
--- Quote from: not1xor1 on July 04, 2018, 05:09:47 pm ---And in any case, regarding the question of the original poster, an LM78xx is perfectly equivalent to an LM317 with built-in voltage divider.
--- End quote ---
It's not an equivalent at all.
--- End quote ---
:palm:
equivalent adjective
us /ɪˈkwɪv·ə·lənt/
equal to or having the same effect as something else:
https://dictionary.cambridge.org/dictionary/english/equivalent#dataset-american-english
BTW the original (i.e. Latin) meaning is just "works like, has the same value as", not "it is equal to".
Even if you buy the same IC (e.g. LM358) from different manufacturers, you get something that is equivalent but which is not an exact copy. :wtf:
wraper:
--- Quote from: not1xor1 on July 08, 2018, 06:33:36 am ---:palm:
equivalent adjective
us /ɪˈkwɪv·ə·lənt/
equal to or having the same effect as something else:
https://dictionary.cambridge.org/dictionary/english/equivalent#dataset-american-english
BTW the original (i.e. Latin) meaning is just "works like, has the same value as", not "it is equal to".
Even if you buy the same IC (e.g. LM358) from different manufacturers, you get something that is equivalent but which is not an exact copy. :wtf:
--- End quote ---
:palm: Yep, it does not work like LM317 with resistive divider. If we add resistor divider to LM317 and assume it's now became a common pin, then bias current through this pin won't depend on load, temperature, input voltage. It's not equivalent because there is a huge part of the circuit attached to common pin of 78xx besides internal resistive divider (yes it's there). It's basically will be a stable current source, therefore it will be different what you can do with it. Say you could just attach another resistor on GND side to increase the voltage, and it will be stable. If you do this with 78xx, it won't be stable.
BTW don't forget what you said:
--- Quote from: not1xor1 on July 04, 2018, 05:09:47 pm ---And in any case, regarding the question of the original poster, an LM78xx is perfectly equivalent to an LM317 with built-in voltage divider.
--- End quote ---
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