LM317T and output capacitor?

hi. is it ok to use the 100uF/35v electrolytic capacitor as the output capacitor in a variable power supply based on LM317T? the actual circuit is in the given link below.

https://www.electronics-lab.com/projects/power/010/index.html

Yes, it is OK. You can use caps up to 1000uF. 1uF is recommended to increase the transient response. Take a look at the datasheet at www.nsc.com

ok. how about the minimum voltage rating of the output capacitor?

National's Guru Bob Pease recommends: 0.1 to 20uF should be avoided in low noise appl, especially those with low ESR. The most effective noise reduction is with values of 50uF and up and at least 1uF on the adj pin.

Also another tip! Replace the 240R with 120R or regulation will suffer. 240R is for use with premium devices and not commercial grade devices. This is NOT mentioned in National application data. I has to do with the program pin current that is higher in the low cost devices. Most published circuits seem to have this wrong as the attached example pulled from the web.

btw. i use 270R instead of 240R. and i replaced the 1uF output capacitor with 100uF 35v electrolytic capacitor. and i observed upon switching off the pointer of the voltmeter takes how many seconds before it goes to zero reading. any idea?

By using a value higher than 240R you are making the regulation even worse.

Here is the reasoning: It is regarding a LM338 device but the principle is the same for the 317.
----------------------------------------------------
The adjustment pin current may vary anything from 45uA to 100uA
according to the datasheet. This output current combines with the
current through the voltage divider on the output formed by the 240
Ohm resistor and the adjustable resistor. This causes a error term in
the Vout formula .....+( IadjR2). To minimize this error the quiesent
operating current ( 1 to 3mA ) of the device is directed to the output
requiring a min. load current tot be present for the device to operate
within specs. If the load current is too small the output voltage will
increase.( Error term comes into play ). The required min load current
for the LM138 is 3.5mA to 5mA and for the LM338 3.5mA to 10mA.

At the min. voltage output of 1.25V the bottom resistor equals zero.
The top resistor is now responsible for the min. load current. Taking
the worst case, to get 10mA flowing through R1, R=V/I then
R1=1.25/10mA = 125 Ohm ( Closest standard value - 120 Ohm ). By using
a 330 Ohm the min load current of 3.8mA is lower than the worst case
of 10mA for the 338. For the 5mA of the 138/238 a value of 250 Ohm is
required ( Closest standard value - 240 Ohm ).
---------------------------------------------------------


The output cap will discharge slowly with no or little load.

Yeah, the output cap discharge rate is completely dependent on the load, including the load of the voltmeter.