Electronics > Beginners

Diode in GND pin of voltage regulator

<< < (4/5) > >>

Zero999:
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.

not1xor1:

--- Quote from: wraper on July 03, 2018, 09:31:31 pm ---
--- Quote from: not1xor1 on July 03, 2018, 04:11:20 pm ---
--- Quote from: wraper on July 03, 2018, 10:13:17 am ---78xx have relatively high bias current of a few mA flowing through GND pin. Thus it creates forward voltage drop across the diode. That's why they suck as adjustable voltage regulator and LM317 with resistive divider should be used instead.

--- End quote ---

There is not much difference. The quiescent current of 78xx is around 4.3mA (typical - ST datasheet) while LM117 is granted to be stable with a load of 5mA and LM317 with 10mAs (although the few I checked work even with as little as 3-4mAs).

If somebody needs an higher voltage and is not concerned by the voltage drift can just put a LED in series with the ground pin to get an increase of few Volts (depending on the forward voltage of the led).

--- End quote ---
You mix minimum load with bias current through GND (common)/ADJ pin. For LM317 ADJUST terminal current typ 50μA, max 100 μA. LM78xx passes bias current through the common pin but LM317 through the load.

--- End quote ---

You are right... and wrong... as I wrote "quiescent current" and "minimum load current"... so no mix-up  :).
And in any case, regarding the question of the original poster, an LM78xx is perfectly equivalent to an LM317 with built-in voltage divider.

not1xor1:

--- 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.

C:
Step one could be putting better labels on the chips.

For a 7805 you have V+, 5 Vdc & 0 Vdc

You could label a 317 as V+, 1.25 Vdc & 0 Vdc

If current does not change at the 0 Vdc pin then a simple resistor will work. If it does change then you need more voltage regulation.

The 317 uses a voltage divider, You can also do this with a 7805.

You could use a zener, diode or LED to get less voltage change due to current change.
A bypass resistor will allow a smaller current  zener, diode or LED and adjusting where on current knee it will operate.

This is all just working with current change and there are many ways to do the same, some better then others.
Extreme is just a voltage regulator to raise 0 volt pin.

C

Zero999:

--- 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.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod