making dc power supply(Issues)

Discussion in 'The Projects Forum' started by abdullah8391, Sep 16, 2015.

  1. abdullah8391

    Thread Starter Member

    Sep 6, 2015
    53
    1
    Again under your care.... :)
    I designed a dc power supply, it was working fine on simulator, but when i i put it on breadboard, R1 resistor(337) ohm burnt down. i as curious if i have to use resistor of high watt (because currently i am using quarter watt resistor). Though according to my concept a few mA current should pass through the R1 and R2.(Please correct me if my concepts are wrong because i am a beginner).
    Components that i am using are:

    1 transformer (step down 220 to 12*2V(i am taking 24V output))
    capacitors of same value as shown in figure of 50V.
    resistors of same value.
    image file:
    circuit.jpg
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    You have a wiring (assembly) mistake.
     
  3. abdullah8391

    Thread Starter Member

    Sep 6, 2015
    53
    1
    hm... you might be right, because i tried it once.
    Actually when i asked about this from my lab teacher, she said "You are using 3A transformer, that's why the resistor burnt. You have to use resistors with high power(of more watt)". This caused me confusion because according to my concepts:
    1- If there is no load then minimum current should pass.
    2- Even if i use high load(like of 1A or 1.25A), then even then there shouldn't be any effect on resistor because R1 and R2 are just being used to control ADJ terminal.

    I just want to clear up my concepts.
     
  4. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    Teacher is wrong. R1 has 1.25 volts across it unless you made a mistake in wiring the circuit.
    Your 500 ohm load is almost a whole watt if you get the 20 volts you intended.
    Transformers do not force amps to flow, resistors allow current to flow.
     
    InspectorGadget likes this.
  5. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    What #12 said. Wiring mistake. Are you sure you have the right pinout for the LM317K? Is it really an LM317K TO-3 old-style power-transistor case, or is it a 317T power-tab transistor case?

    [​IMG]

    If it's a 317K on the left, you may have confused which way was "up" in the package orientation. If you invert the orientation, you switch Vin and Adjustment and things would go terribly wrong. Look carefully at the bottom of the case. The pins are slightly offset to one end, and this end is "up" as pictured in the pinout above.

    If it's a 317T on the right, perhaps you incorrectly assumed ADJ was the center pin (I did that once!) because of the schematic symbol...

    Double check your parts orientation and your wiring and replace the LM317 and see what happens.
     
  6. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    By the way, make sure you're not grounding the 220V AC line to your low-voltage circuit ground the way your simulation schematic shows it is. You could electrocute yourself or blow out your low-voltage circuit by shorting it to something that's earth-grounded (like a plug-in piece of test equipment or soldering iron).
     
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