Troubleshooting and repairing Switch Mode Power Supplies

Adjustable Power Supply

Adjustable Power Supply Using LM317

For the LM317 electronic regulator :lm317

  • R2 = (192 x Vout) – 240, where R2 in ohms, Vout is in volts and must be at between 1.2 V and 35 V.
  • Vin should be at least 2.5V greater than Vout. Select a wall adapter with a voltage at least 2.5 V greater than your regulated output at full load
  • Maximum output current is 1 A. Use proper heatsink for LM317 if it has to dissipate more than 1W.
  • The tab of the LM317 is connected to the center pin.
                    I +-------+ O
    Vin (+) o-----+---| LM317 |---+--------------+-----o Vout (+)
                  |   +-------+   |              |
                  |       | A     /              |
                  |       |       \ R1 = 240     |
                  |       |       /              |           ___
                 _|_ C1   |       |            +_|_ C2      |_0_|  LM317
                 ___ .01  +-------+             ___ 1 uF    |   |  1 - Adjust
                  |  uF   |                    - |          |___|  2 - Output
                  |       \                      |           |||   3 - Input
                  |       / R2                   |           123
                  |       \                      |
                  |       |                      |
    Vin(-) o------+-------+----------------------+-----o Vout (-)

Equation for calculating the output voltage when R1 and R2 are known:

 Vout = 1.25V * (1 + R2/R1) + Iadj * R2

Where Iadj is typically in the range of 50 microamperes.

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