troubleshooting a voltage regulator

Discussion in 'General Electronics Chat' started by deefactorial, Nov 18, 2008.

  1. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    Hi all,

    I'm trouble shooting a voltage regulator that is not functioning as expected. I have a 12v 6a switching power supply hooked up to an Analog Devices 5v 250ma voltage regulator.

    I have wired the voltage regulator as normal operation in the spec sheet for the device, with a 10uF cap after the regulator. I have also tried adding a 100uF cap before the voltage regulator but the problem still persists.

    Here is what happens: I hook up the power supply and the voltage out of the voltage regulator goes to 5.02v @ .75a This is the behavior I am looking for; but after about 30 seconds to 1min the voltage will switch to 12v @ 5a. which is the behavior I am looking to get rid of.

    What could be a potential reason for this happening? Is it that the input power is not regulated enough to keep a constant supply to the voltage regulator. Or is it that the Power dissipation is too much for the voltage regulator?

    Any help with this would be greatly appreciated.
    PS: here are the specs of the Power supply and the Voltage Regulator
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=EPS337-ND
    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=ADP667ANZ-ND
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469

    I don't know if you have a typo in your question, but you said the regulator is rated for 250 mA, but you are trying to draw 750 mA. Obviously, (if your numbers are correct) you are overheating the device.
     
  3. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    so when I put the voltage tester on the voltage regulator, does it draw 750ma or perhaps I have a shorted the power and ground plane after the voltage regulator.

    The actual power consumption of the device I am going to power is 5v at 70ma, so theoretically I should not be overheating the voltage regulator. I will recheck my soldered joints.

    Thanks
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Here is the datasheet of the regulator:

    http://www.analog.com/static/imported-files/data_sheets/ADP667.pdf

    You will find how to connect it for proper functionality. I think that your regulator is broken and thats the reason of its strange behaviour. If it passes 5 amps through it it will burn, so its not possible for it to pass 5 amps as you said and still work properly.
     
  5. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    What would the expected voltage and amperage difference between the circuit be when there is a load on voltage regulator and when there isn't.

    I noticed in the spec sheet that it said that if I added a 40ohm resistor that it would drop the input voltage by 4v. When I added a 47ohm resistor to the input pin (without a load) it measured 12.27v on either side of the 47ohm resistor. I asked a colleague of mine about it and he said that it only drops when there is a load on it. So I connected everything together, 47ohm resistor, voltage regulator, 10uF cap and added a 47ohm resistor load across the cap.

    I was getting 5.05v after the input resistor. and 5.05v after the regulator. It worked for a while then it reverted to 12v 5a and toasted the load resistor.

    I think that mik3 is right and the voltage regulator is broken.
    luckily I ordered two.

    my question is this: How does adding a load affect the measured voltage and amperage?

    Thanks,
    Dominique
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    In the datasheet i dont see anywhere to say that this regulator is thermal protected or has an overload protection, so you have to be careful as not to draw more than the rated output current and not exceed the power dissipation of the chip (read the part 'power dissipation' in the datasheet). I guess you connected something wrong in the circuit or your load was not 47R but it was less, the chip overheated and the inside transistors became short circuited. Post a schematic of your circuit to check it.
     
  7. deefactorial

    Thread Starter Active Member

    Jun 11, 2008
    33
    0
    yes I guess I connected it together without a load and toasted the voltage regulator. Or accidentally created a short. I wanted to test the circuit out.

    I have read the part about power dissipation, the device I intend on powering is only using 22ma @ 5v Pd=157. Pd(Max) = 333, so I am well under the max power dissipation.

    It may be probable that I created a short once, in my testing/soldering.

    With a 47ohm resistor as a load on the circuit it would create a 106mA load @ 5v on the circuit. Which it should be able to handle. I would much rather fry a resistor than any of the devices I intend on connecting to this circuit.

    I've added a schematic of my circuit.
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I think you created accidentally a short a destroyed it. I suggest you use a LM7805 regulator which has overload and overheating protection.
     
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The datasheet shows a capacitor to ground on the input from the 40 ohm series resistor. The capacitor keeps the regulator from oscillating which might kill it.
    Your circuit does not have the capacitor.
     
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