Need help to double check LM317 circuit

Thread Starter

floxia

Joined Mar 7, 2023
18
Hi there!

I have made the following schematic based on the datasheet from ST attached below.
Everything seems pretty straight forwards expect for the wiper of the pot R2, as it is unclear to me where it needs to be connected.

Is the schematic below correct?

Also in regards to the capacitors, C3 can be electrolytic, what about C1 and C2 can they be ceramic and electrolytic respectively given that C1 is non polarised while C2 is?

If it can help Vin will be 15V and Vout 7V, to power a circuit that draws 100mA.

Thanks!
Screenshot 2024-04-09 at 7.50.12 pm.png

Datasheet:
Screenshot 2024-04-09 at 8.30.38 pm.png
 

sarahMCML

Joined May 11, 2019
401
You need to connect the wiper of R2 to the top terminal of the pot, so that the pot just becomes a variable resistor. That's how the symbol shows it in the datasheet!
 

Thread Starter

floxia

Joined Mar 7, 2023
18
Hi flox,
Wire the pot so that when fully clockwise it will be the maximum resistance [ max Vout]
C1 is ceramic and C2 is electrolytic.
The higher values are usually electrolytic,

E
You need to connect the wiper of R2 to the top terminal of the pot, so that the pot just becomes a variable resistor. That's how the symbol shows it in the datasheet!
Like so?

Screenshot 2024-04-09 at 9.05.41 pm.png
 

crutschow

Joined Mar 14, 2008
34,704
You need to connect the wiper of R2 to the top terminal of the pot, so that the pot just becomes a variable resistor. That's how the symbol shows it in the datasheet!
The reason is, that connection makes the output voltage linear with pot rotation, whereas the connection shown in the first post makes it non-linear.

See the difference below in the sim for the two configurations showing output voltage versus pot rotation .

1712670084446.png
 

Thread Starter

floxia

Joined Mar 7, 2023
18
Hi,
This may help.
E
The reason is, that connection makes the output voltage linear with pot rotation, whereas the connection shown in the first post makes it non-linear.

See the difference below in the sim for the two configurations showing output voltage versus pot rotation .
Thank you guys for showing me the connections and simulations of the circuit, extremely helpful and appreciated!
 

Ian0

Joined Aug 7, 2020
10,076
The reason is, that connection makes the output voltage linear with pot rotation, whereas the connection shown in the first post makes it non-linear.

See the difference below in the sim for the two configurations showing output voltage versus pot rotation .

View attachment 319546
That's not quite how he had it. The wiper went to R2 and the track end to ADJ.
It would have still been mainly linear, but the pot resistance would have been in series with ADJ which would have produced an error due to the 50uA ADJ current.
 

crutschow

Joined Mar 14, 2008
34,704
That's not quite how he had it. The wiper went to R2 and the track end to ADJ.
It would have still been mainly linear, but the pot resistance would have been in series with ADJ which would have produced an error due to the 50uA ADJ current.
Correct.
I misread his first schematic.
 
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