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7805 Regulator Circuit Problem

Discussion in 'General Electronics Discussion' started by assail, Oct 12, 2010.

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  1. assail

    assail

    2
    0
    Oct 12, 2010
    I have a number of circuit cards that are failing and this could possibly be attributed to a design problem.

    The circuit consists of a +24VDC supply, connected to a 100 Ohm 1/4W resistor which is connected in series on the input side of a 7805 voltage regulator. Measurements indicate approximately 13-14V dropped across the 100 Ohm resistor and 10VDC input to the 7805 and 5VDC output from the 7805. The load draws approximately 170mA from the output 7805.

    The problem is: the resistor that is connected in series with the 7805 appears to be showing signs of heat stress or blown altogether. Is the resistors power rating too low, or is the resistance value incorrect for the 170mA current draw.
     
  2. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    You have about 2W of dissipation across the 1/4 W resistor. Why do you feel you need the resistor at all?
     
  3. davenn

    davenn Moderator

    13,560
    1,856
    Sep 5, 2009
    The resistor is dropping the 24V to a value that the 7805 can handle ... it wouldnt last long with 24V going into it ~ 15V is a prefered max for a 7805 (data sheet would be specific)

    yes at 1/4 Watt the resistor is way under rated I would be replacing it with one rated at least 2W which will still get warm

    Watts = Volts x Amps to find amps I (amps) = Volts / Resistance

    14volts dropped across 100Ohms = 14/100 = 0.14Amps

    14V x 0.14 Amps = 1.96Watts dissapation in the resistor

    cheers
    Dave
     
  4. assail

    assail

    2
    0
    Oct 12, 2010
    Ok, i have found a circuit card that appears to work correctly and taken some measurements. The good card has 24VDC supply with about 1.4V dropped across the 100 Ohm resistor and 22.6V at the input to the 7805, 5V at the output of the 7805 with the load drawing approximately 170mA.

    My question now is, what would cause the voltage drop to increase across the 100 ohm resistor in the faulty circuit.

    I also forgot to mention that there is a 0.1uF cap located at the input and output of the 7805 (to ground).
     
  5. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    1.4V drop with 170mA indicates a resistance of 8.2 Ohms. The 7805 will dissipate 3W and will need a moderate heatsink. Capacitors are a common good practice here.
    Are you sure the 100 Ohm resistance value you are referring to is correct?
     
  6. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    Admittedly - I switched over to LM217 / LM317 adjustable regulators years ago - and rarely find a use for fixed 3TRs. But my Fairchild data sheet on the 7805 calls out a max input of 35V. I don't recollect a problem inputting around 24V in the past - especially at only 170 mA. Is there an issue I'm missing? :confused:
     
  7. davenn

    davenn Moderator

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    1,856
    Sep 5, 2009
    Yes 35V is the ABSOLUTE max and almost garanteed to kill it

    for a 5V reg the safe working max for good regulation is 20V (7.5 ≤ VIN ≤ 20) 10V typical
    for a 12V reg the safe working max for good regulation is 27V (14.8 ≤ VIN ≤ 27) 19V typical
    for a 15V reg the safe working max for good regulation is 30V (17.9 ≤ VIN ≤ 30) 23V typical

    Taken from the National semiconductor datasheet

    this is why you will often see a 5V reg taken off a lower voltage rail than what the 12V or 15V regs are (often from the 12V or 15V reg rail assuming that regulator can handle the current thats gonna be pulled through the 5V reg)

    Cheers
    Dave
     
  8. Militoy

    Militoy

    180
    0
    Aug 24, 2010
    I'm sort of running between jobs right now - and too busy (or maybe too lazy) to dig into the National datasheet. Are these limits based on max input/output differential voltage - or max dissipation at full load?
     
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