Obviously, it is another post in the venerable LM317. I am reviewing a stabilizing module that I've done in the past for my ULTRA WOW LOW RIPPLE DUPER SUPER STABILIZED BENCH PSU but the lack of experience teaches me that a fixed voltage can't be kept fixed under load how I dream.
As you can see from the attachments the stabilization module is a dual-rail version of the scheme in Fig.9 at pag. 11 of the LM317 datasheet BUT with the pot replaced by a set of fixed resistors, because I wanted the PSU with fixed voltages selected by a rotary switch, with a 120R resistor to keep more stable the IC and with a ripple filter at the beginning that I copied from the Dave's video. Under no load and at ambient temperature everything works fine.
Now let me show the issue.
From the Electrical Characteristics table at pag. 6 we can read that under V_0 > 5V the max load regulation is 1.5%V_0 (supposing at high temperature) and that the Output-voltage
temperature stability is 0.7%V_0 so we SHOULD (and here could be the issue) sum them to obtain a maximum voltage drop of (1.5+0.7)%V_0 so, using a 21V input voltage and an open circuit output voltage of 9V, the drop should be at maximum 9V*2.2/100 = 0.198V leaving 8.8V clean at the output.
This is not the case.
As two 17W 4.7R resistors put in series are connected, the clean 9.00V drops to 8.76V at 38° 0.95A and with a stabilized temp of 80° the voltage reaches 8.43V at 0.90A. It is too low for the specs.
Probably I didn't read the right way datasheet and maybe summing everything that could be the thermal error like the base 1.5% load regulation + 0.7% temperature stability + 10W*0.07%/W = 0.7% Thermal regulation + I don't know what else to sum we can obtain a 2.9%V_o drop = 0.261V that corresponds to an output of 8.74V. However, I'm too out.
I tried to change the output capacitor from 100u to 1u and to avoid the fixed resistors using an external pot to manually regulate the open voltage but nothing.
What I'm not considering?
You can find the schemes attached below.
Please don't hit me too hard for my mistakes.
Thank you.