Voltage regulator question

Discussion in 'General Electronics Chat' started by mbird, Feb 7, 2009.

  1. mbird

    Thread Starter Member

    Jan 29, 2009
    24
    2
    Hi --

    I made a simple 555 Timer Astable Multivibrator Circuit that pulses every 1 second.

    When I run this with two 1.5VDC batteries it works fine.

    I wanted to experiment with voltage regulators so I got a UA78M33CKCSE3 (Jameco Datasheet) Voltage regulator. I also had one of those common wall plug power supplies (it says it is 6VDC 300mA). I connected the power supply to the voltage regulator (input and common) and I connected the output to a 22 Ohm resistor (so I expect 3.3VDC 150mA and that is what I see).

    Now I use the output (with the 22 Ohm) and common to power my 555 circuit. The result is that pin 3 of the 555 is always 3 volts! If I switch back to my battery it goes 0 - 3 volts over over 2 seconds as expected.

    Every where I check the voltage regulator seems fine (it has 3.3 volts and the current is around 150mA).

    I must be missing a key idea about using a voltage regulator??? If you can help with any ideas it would be appreciated!

    Thank you!

     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    Did you put the two capacitors, 0.1 μF on the input to ground and 0.33 μF on the output to ground.
    These capacitors are needed to prevent the regulator from oscillating.

    Greetings,
    Bertus
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Adding to what Bertus said, you will also need a larger capacitor on the output of the regulator, in the range of 100uF to 330uF. 555 timers create a momentary short from Vcc to ground when they are changing output states. Unless you have a fairly large "bypass" capacitor, the timer can appear to get "hung up".
     
  4. mbird

    Thread Starter Member

    Jan 29, 2009
    24
    2
    Thanks -- your info fixed the problem (I did not have any capacitors in my original experimenting!).
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Glad that Bertus and I were able to help :)
     
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