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Combine LM317 and LM7805 together

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EDA_hg81

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lm317 15v

Want I want to do is to use LM7805 to generate 5V and use this 5V as the ground of LM317, so the output of LM317 is 5V + the output of LM317.

Can I do this ?

Thank you.
 

lm 317 for +5v

Hi EDA_hg81,

Yes you can but you have to make sure that the load current of
the 7805 exceeds the quiecent current of the LM317. (The current
trough the resistive divider.) If there's an additional load connected
between the output of the LM317 and the output of the 7805 you have
to take this extra current in to account as well.
Do you need a higher output voltage than the maximum of the LM317 ?
You can do that without stacking it on top of another regulator on
condition that you don't short circuit or overload it, otherwise the
maximum permissable voltage will be exceeded.
You also could use a TL783 to replace the LM317.

https://focus.ti.com/lit/ds/symlink/tl783.pdf

on1aag.
 

    E

    EDA_hg81

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stacking lm317

Thank you so much for your advice.

How I shold connect the output of 7805 to LM317?

In my design I connect the output of 7805 to the variable resistor for setting the LM317.

Shoule I only connect the output 7805 to output Lm317?

I used scope to check and found out the output 7805 is variable why?

very appreciate for your any suggestions.
 

lm317 lm7805

Hi EDA_hg81,

I hooked it up the same way you did, it worked fine. I couldn't
find any instability, but in absence of any load the 750 Ω resistor
should be present, otherwise it's not necessary.

31_1179462352.gif


You can also use the LM317 as a preregulator for the 7805.

75_1179462517.gif


on1aag.
 

    E

    EDA_hg81

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lm7805 variable regulators

Of course, you can do it, but unless you have any hidden agenda behind this, it doesn't make much sense ..

Regards,
IanP
 

    E

    EDA_hg81

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lm317 lm7805 better

Since in this design we want to get two outputs( but do not want to use Switching regulator).

The output LM317 ( 15V ) is used for driving OLED.

and 5V is only used for setting the bias volatge of transistors for deiving OLED.

this is so simple, but do not know why 5V is variable.

but when use external supply, it is fine.

The current is so low for 5V 3mA for LM317 is 58mA. ( maybe here is the problem and may be I should present that 750 ohm resistor.).

So sad.:cry:
 

lm317 a lm7805

It is always a good idea to have a small load connected to the regulator's output ..
Instead of a resistor connect a resitor and an LED - this will give you an indication that a voltage is present ..

However, I still think that you should separete both regulators and reference them individually to 0V ..

Regards,
IanP
 

    E

    EDA_hg81

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combine lm317 and lm7805 together

your suggestion is a good idea.

but the sad thing is it is not me to decide how to realize it.
:cry:
 

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