LM338K and 317k power supply

Discussion in 'General Electronics Chat' started by Ranjith Peiris, Apr 28, 2011.

  1. Ranjith Peiris

    Thread Starter New Member

    Apr 27, 2011
    Dear friends,
    I made a variable power supply using LM338k , but it was not working properly the problem is when I put a car bulb as a lord of 21w 12v double filament type for12v supply one filament is lighting up and the other one is not lighting up then I got another single filament bulb, but the problem was same (21w 12v). I noticed one thing when I connect the power LM 338 will shut down the voltage, then I connected another Ic LM317K but the problem was same, two ICs are good and some bulbs are working for12v some are not working , then I put another IC LM 317T package 3 pin type all the bulbs are working beautifully but it gets very hot and won't shut down the voltage. So Please tell me what is the cause of this problem. thank you.
  2. bertus


    Apr 5, 2008

    Can you post the schematic?
    What is the input voltage of the LM ic's?
    Are you using heatsinks on the LM ic's?
    Are you using the decoupling capacitors?

    Ranjith Peiris likes this.
  3. SgtWookie


    Jul 17, 2007
    21 Watts at 12v = 1.75 Amperes current. This is too much for an LM317 regulator; they are designed for 1.5A maximum current.

    The regulator will dissipate power as heat; if the voltage across the regulator (from the input voltage supply to the output) is large, the regulator will overheat - even if a large heat sink is attached to the regulator.

    Power_in_Watts = Voltage_across_regulator * output_current.
  4. designnut


    Apr 21, 2011
    a cold lamp draws almost 10 times ratings as the filament resistaance is lower cold. It's a terrible inrush problem.