Lm317T psu voltage drop problem

Thread Starter

Juuso A

Joined Jul 10, 2017
6
I have built this circuit with LM317T and LM337T:
psu.jpg
The problem is the output voltage will drop under load when i have C3 and D7 connected. Unloaded the voltage over R1 is 1.1 volts, and if i connect a load (LED and 1k resistor) it drops to 0.9 volts. Without C3 and D7 it works and the voltage over R1 is 2.2 volts. Same problem on the bottom side.
 

crutschow

Joined Mar 14, 2008
26,974
If it's wired properly with the correct capacitor polarity I don't see how C3 and D7 can affect the output voltage.
If it does, there's an error in your circuit.

When the circuit is operating properly the voltage across R1 is maintained at ≈1.25V by the LM317.
If it's not then the LM317 is not regulating.

The LM317 needs at least 2V of headroom to operate so the voltage at its input can drop no lower than 14V for a 12V output.
Can you measure the input voltage with an oscilloscope?
 

Thread Starter

Juuso A

Joined Jul 10, 2017
6
Input in lm317 is 18.2 Volts with LED and 1k resistor, and input in lm337 is 18.8 volts with LED and 1k resistor.
This is when the output caps and diodes are connected.
 

crutschow

Joined Mar 14, 2008
26,974
So then if you have a problem, it would appear to be a wiring error, since you appear to have plenty of input voltage.
Double check the wiring, and the capacitor and diode polarities.
 

Marcus2012

Joined Feb 22, 2015
402
Hey

I don't think you need D1 & D2. Are you just using +18VDC? or using AC (i.e. +18.8 and -18.8)?

EDIT: I've seen the schem you're using in the 337 datasheet. D7 and D8 shouldn't be there those caps are for ripple rejection (10uF ones, unless they're bypass').
Capture.PNG
 
Last edited:

Thread Starter

Juuso A

Joined Jul 10, 2017
6
I added capacitors from output to gnd on both regulators, and now output voltages and voltages across R1 and R2 are stable. How did this fix the problem?
 
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