lm317 transistor circuit help

Discussion in 'The Projects Forum' started by bassplayer142, Jan 2, 2007.

  1. bassplayer142

    Thread Starter Active Member

    Jan 2, 2007
    89
    0
    I am currently in the process of building a voltage regulator. I am going to use an external source of power from a wall wart and then tune it down to 3 volts. I am going to be hooking up a laser pointer diode to it and any AC voltage can easily blow the diode. When the circuit is running the AC voltage reads 0.00 volts but when I hook up a motor or any other circuit the AC voltage spikes as the current draws. A few seconds later it is back down to 0.00 volts but by then the damage would be done. Am I under the impression that this circuit does not reduce AC voltage or is something wrong. Thanks.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    We'll need to see your schematic. We'll also need to know where you are taking youre measurement, and where you are hooking up the motor.
     
  3. wireaddict

    Senior Member

    Nov 1, 2006
    133
    0
    Are you planning to connect any load in parallel with the laser diode? If not, you should be OK. It sounds like you're seeing AC ripple or pulsating DC when you first connect a motor or other significant load since the comparatively heavy load or inrush current is momentarily discharging the filter cap, although a sustained heavy load would keep the ripple present.

    A few other points to consider: first, if you are going to use a motor or similar load, if possible, connect it ahead of the regulator & don't exceed the wall wart's current rating; second, diodes can handle some reverse voltage without harm; if possible, find out the PIV rating of this laser diode; it will likely be at least 5 V so some ripple won't be "curtains", just a nuisance; finally, to reduce AC ripple, connect a small [5-10 uF] electrolytic cap across the laser diode.
     
  4. bassplayer142

    Thread Starter Active Member

    Jan 2, 2007
    89
    0
    The laser is a DPSS 532nm laser. It is run off of two AAA batteries. So that is about 3 volts. I just hooked up the motor to the output of the circuit and when it starts it spikes the AC voltage. Building another circuit that is more specially designed to stop AC current would be a plus. Either that or making this one do it. Here is some schematics, they are basically the same to my knowledge.

    http://www.national.com/pf/LM/LM317.html

    http://casemods.pointofnoreturn.org/vregtut/tutorial-full.html


    thanks everyone.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Will the laser diode be hooked up by itself, or will there be a motor in use as well?

    Let me put it another way - what happens when you test your circuit with a resistor instead of a motor?
     
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