PSU Circuit Review

Discussion in 'The Projects Forum' started by van53, Jul 22, 2012.

  1. van53

    Thread Starter Active Member

    Nov 27, 2011
    Hello Everyone,

    This is my first post on this forum. I took a few courses at the community college level in electronics about nine years ago. I have since worked in a non related field and I would like to increase my knowledge in electronics and decided to first build a linear power supply. Attached is the circuit layout and a photo of where I am currently at.

    The power supply is powered by a DC adapter (likely for an HP printer) which provides 32V (1.1A) and 16V(1.6A) and a common ground. The power supply I am building will have a variable output using the 32V from the dc adapter, providing an output of 1.2V to about 28V using an LM338K to regulate the voltage. I thought it may be useful for future projects to be able to limit the current of the power supply so I have used an LM138K for this purpose. Several resistors can be selected via a rotary switch providing a max current limit of 1A and a minimum of 20mA (if I recall correctly). The output of the LM138K acting as a current limiter is connected to the LM338K which regulates the voltage. The LM138K and LM338K are mounted on two large heatsinks located on the outside of the power supply.

    On the 16V rail of the dc adapter I have used an LM317 to limit current to 500mA and then connected a 7805 to provide a fixed output voltage of 5V.

    Also powered by the 16V rail on dc adapter is a 7812 regulator to power a circuit which will turn on an internal fan and an external fan (each drawing 200mA)
    when the temperature crosses a specific threshold. The VA1212S (also powered by the 16V rail on the dc adapter) will be used to provide power to the digital volt meter display.

    I have a couple questions:

    1) I realized that for the fixed 5V output I did not connect a protection diode on the output. On the attached circuit layout I have drawn in red how I believe this should be connected and I wanted to confirm if this is correct.

    2) I have read that the protection diodes for the voltage regulator will allow current which could be discharged (during a short) from the capacitor at the output to be directed around the voltage regulator. If this happens (for either the variable or fixed 5v output) then what about the two regulators highlighted in red (in the attached file) which are acting as current limiters. Will they be damaged? Do I need additional diodes to protect them?

    3) What happens if there is a short circuit (at either the variable or 5V output) in my attached circuit layout? Will the regulators acting as current limiters only allow current to reach a specified level or can the supply be damaged? Will the voltage regulators go into thermal shutdown?

    4) Is it recommended to connect two fuses or PTCs from the 32V and 16V lines going from the dc adapter to my power supply? (I am thinking this would not be needed as the regulators setup to limit current would ensure [by the resistors I have chosen] that the supplies total draw would not exceed the dc adapters rating)

    5) Any other suggestions or feedback on issues that anyone can see in the attached circuit (or potential problems by short circuits on the variable or 5v output, loads, etc) would be appreciated.

  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    Do you mind bringing this into a more readable format? A schematic look more like the one on page 7 of this LM138 datasheet.
  3. van53

    Thread Starter Active Member

    Nov 27, 2011
    Sorry about that. I have provided a schematic for the part of the circuit in question. I have circled the diode in red (question 1). The voltage regulators setup as current limiters have been circled in red (questions 2 and 3).