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voltage regulator current issue. — Parallax Forums

voltage regulator current issue.

dr_eggman198dr_eggman198 Posts: 15
edited 2005-07-12 00:18 in BASIC Stamp
I am using a 300 Ma 9 vlt transformer to operate a voltage regulator (7805). I used my multimeter to see if the current was enough for the basic stamp to handle, but my multimeter can read up to 200 Ma, and it did not display a value for the current that I was measuring. I used an LED, however, that was rated at 20 Ma to test if the current was enough to render the LED inoperable, and it did not. So i'm assuming it is safe enough to use with the basic stamp? If each pin can accept 20-25 ma?

Comments

  • KenMKenM Posts: 657
    edited 2005-07-10 01:15
    Short answer, your transformer will work.

    Longer answer.....The stamp and associated circuitry will take only the current it needs to operate.

    When selecting a power source you need to consider how much current the source is capable of providing versus how much current the circuit needs to operate.

    In your case, the transformer can supply 300 mA. The stamp alone will not consume anywhere near that amount. If your stamp is controlling relays (via a transistor driver or other suitable method) then it is possible the 300 mA supply is not going to work. Bottom line is you need to know the worst case (maximum) current that will be used by your stamp plus any external components.

    Except for my own equipment, almost every handheld meter I pick up to measure current reads zero because some previous user inadvertantly used it in an incorrent manner and blew the fuse in the meter.

    My guess is that your amp meter fuse is blown, or it is not connected properly to the circuit. It is common for the positive lead to be plugged into a receptacle on the meter different than the receptacle used for measuring ohms or voltage.

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    Ken
  • AmaralAmaral Posts: 176
    edited 2005-07-11 22:11
    The 7805 is capable of 1 AMP so as long as you work below it , it will work fine.

    Don’t forget to use an capacitor AFTER the 7805 otherwise the voltage will be pulsing and not 5 volts , an electrolytic cap. ( I suggest over 10 uF , micro Farads ).

    I have tested uses of the 7805 without capacitor, in the oscilloscope it’s pretty neat, but damages the IC after some time, and it will no longer give you 5 volts.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-11 22:27
    Amaral,

    ·· I my experience the 7805 only gives you 1 amp if you have adequate heat-sinking/cooling and if your input voltage is 12V or less.· Otherwise the device tends to experience thermal overload.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • AmaralAmaral Posts: 176
    edited 2005-07-11 22:48
    Chris

    I supposed that everyone always use the 7805 with heat-sinking/cooling. sorry about that ! Wrong supposition , but about the 1 amp with more than 12V, that I have never tested! I’ll do some tests than I post back , just to confirm it!

    thanks for the new information

    Ricardo Amaral
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-07-11 22:58
    Ricardo,

    ·· The issue with the input voltage is directly related to the comment about heat-sinking/cooling.· The higher the input voltage (It will go up to at least 30 volts), the more heat the regulator has to dissipate to maintain the output voltage.· This brings you closer to the fateful thermal shutdown condition when you're drawing 1 amp from the device, which I have done successfully at 12 volts input.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • AmaralAmaral Posts: 176
    edited 2005-07-11 23:29
    GOT IT !


    great ! , Thanks

    Ricardo Amaral
  • dr_eggman198dr_eggman198 Posts: 15
    edited 2005-07-12 00:18
    yep, I used an electrolytic capacitor (100mf) with the 7805's input, and one with it's output. I tested it, and there didn't seem to be any problems.
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