29-08-2019, 02:34 PM
One last note I would add about these regulators is:
Although the regulator worked fine in my test setup, when it was fitted in the unit it was intended for powering, the voltage stabilisation was not very good.
I discovered that the extra length of cable to the ammeter and back was essential to make the regulator work correctly. The regulator seemed to need a resistance of at least .4 ohms in the supply to the load.
The problem was solved by changing the 1uf ceramic capacitor ( I initially used ceramic because of my hatred of Tantalum) on the output pins to ground for a 2.2uF tantalum capacitor.
The notes on the National Semiconductor data sheet say;
"Although the LM138 is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF aluminium electrolytic) on the output swamps this effect and insures stability."
Mike
Although the regulator worked fine in my test setup, when it was fitted in the unit it was intended for powering, the voltage stabilisation was not very good.
I discovered that the extra length of cable to the ammeter and back was essential to make the regulator work correctly. The regulator seemed to need a resistance of at least .4 ohms in the supply to the load.
The problem was solved by changing the 1uf ceramic capacitor ( I initially used ceramic because of my hatred of Tantalum) on the output pins to ground for a 2.2uF tantalum capacitor.
The notes on the National Semiconductor data sheet say;
"Although the LM138 is stable with no output capacitors, like any feedback circuit, certain values of external capacitance can cause excessive ringing. This occurs with values between 500 pF and 5000 pF. A 1 μF solid tantalum (or 25 μF aluminium electrolytic) on the output swamps this effect and insures stability."
Mike
