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I built a very basic circuit to get around 5.5V from a +15V source

I have a 220 Ohm between Adj and Vout and one 470 Ohm, one 220 Ohm and two 100 Ohm in parallel between Gnd and Adj summing up to about 740 Ohm that should give me about 5.45V between Vout and Gnd.

With a 1.5k Ohm Load between Vout and Gnd I get 5.5V. However with a 470 Ohm Load I only get about 3.7V.

Putting in 100nF capacitors between Vin and Gnd and Vout Gnd did not help

schematic

simulate this circuit – Schematic created using CircuitLab

Any Ideas what I'm doing wrong?

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    \$\begingroup\$ Chech the input voltage to the LM317 while the 470 Ohm load is connected. It must be higher than about 7.5 volts for the LM317 to maintain regulation. \$\endgroup\$ – Peter Bennett Aug 15 '18 at 15:11
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    \$\begingroup\$ Are you sure your lab supply is not set to current limit at a very low current? Did you actually measure the input voltage? You said you put 100nF on Vin but show it on Vadj. \$\endgroup\$ – Spehro Pefhany Aug 15 '18 at 15:26
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    \$\begingroup\$ This issue sounds similar to this one where the regulator was installed backwards, may or may not be the issue here: electronics.stackexchange.com/questions/24944/… \$\endgroup\$ – CoreyF Aug 15 '18 at 16:38
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    \$\begingroup\$ Poor layout can cause oscillation, which will lower the output voltage. \$\endgroup\$ – Leon Heller Aug 15 '18 at 16:41
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    \$\begingroup\$ @xuma202 Fair enough, I ran the calculations front the schematic you provided and I'm out of ideas other than a bad IC which can be checked by comparing results from using a different one. \$\endgroup\$ – CoreyF Aug 15 '18 at 17:08
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I'm guessing you have swapped the input and adj pins by having the LM317L reversed.

enter image description here

Note that the view is from the top looking down through the plastic (dashed lines for the pins).

Connected backwards, it perhaps acts as a sort of zener breaking down at 9V or so with some resistance in series.

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R1 must have 1.25 V between Vout-Vadj to be functional. If not, then it is not working per spec.

By ratios 220/Rtot*Vout =1.25V. and your Rtot looks correct for 5.5V.

So the criteria for 1.25 is not correct, either bad IC or routing.

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  • \$\begingroup\$ Indeed I only have around 200mV between Vout and VAdj \$\endgroup\$ – xuma202 Aug 15 '18 at 17:07

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