The output voltage of LM317 normally starts from around 1.25V and the output voltage of LM337 normally starts from around -1.25V. Here D5 and D6 create positive reference voltage of around +1.3V which is used to create offset for IC2. Also D7 and D8 create negative reference voltage of around -1.3V which is used to create offset for IC1. That is why the output voltages V3 and V4 can start from almost ground levels. If we are in need of better stability we may use reference diodes for 1.2V as LM385-1.2 instead of the couples of ordinary diodes D5, D6, D7 and D8. The diodes D1, D2, D3 and D4 protect the regulators from reverse voltages.
IC1 and IC2 should be mounted on appropriate heat sinks, e.g. with thermal resistance less than 4C/W. The maximal power dissipation can go up to 10W if we require output currents above 0.5A at lowest output voltages. When calculating the required heat sink we should take into consideration that the maximal power dissipation of LM317T and LM337T in case TO-220 is 20W, the thermal resistance junction-case is 4C/W and the maximal junction temperature is only +125C.
For the testing the power supply is connected to the mains (preferably with protective isolation transformer for the initial testing).
- Outputs are loaded with resistors of 33 to 51Ohm with power dissipation at least 10W (preferably above 20W)
- The potentiometers P2 and P3 are put approximate in the middle position
- The output voltage is adjusted with the stereo potentiometer P1A+P1B approximate to the required voltages, e.g. to around +-10V.
- In case of need some readjustment of the output voltages is made with P2 and/or P3.
- Now if you connect and disconnect the loads the output voltages V2 and V4 should change slightly around the initial value of +-10V.
That short article presents bipolar adjustable power supply with LM317T and LM337T.
The power supply has three main features (particularities):
- The simultaneous adjustment of the output voltages is made with stereo potentiometer P1A+P1B.
- We have the possibility to adjust more precisely the output voltages with individual potentiometers
- Both output voltages can be adjusted from near ground level not from the typical +-1.25V.
These three features make this bipolar power supply useful for hobby and more professional usage.