Ways to measure L7805ACT regulator output
  • Hello, everyone!

    I am trying to measure the output of a L7805ACT voltage regulator (datasheet) with a multimeter, without any additional parts (no caps, etc, just the regulator). I'm using a regulated wall adapter power supply with settable output (5-12VDC). The adapter's connector can be inversed so + becomes -, but I paid attention to connect it the right way.

    The multimeter I'm using is an EXTECH EX330.

    Here's the regulator's pin layout:
    https://www.ghielectronics.com/uploads/forum/img/13877_regulator_large.gif

    Steps:

    set multimeter to VDC setting
    attach COM probe to regulator's GND (middle pin)
    attach red probe to output pin (3rd)
    connect wall adapter to input pins (1 and 2)
    switch the wall adapter on
    Diagram:

    https://www.ghielectronics.com/uploads/forum/img/13878_diagram_large.png

    With the first regulator I tried, the multimeter displays the input voltage and the regulator quickly heats. No voltage drop whatsoever.

    At first I thought 12V input is too much, 7V have to be dissipated as heat and that the overheating is normal, so I set the input to 7.5V, but the results were the same.

    Changed the wall adapter, same thing. Measured the adapter's output power with the same multimeter, 7.5V exactly as set.

    Tried with another regulator, 7.5V input, and now the measured output voltage starts from about 7V and rapidly drops to 0 (didn't really waited to get to 0, don't want to see the regulator burning).

    Although the multimeter shouldn't be the problem, as even without connecting it the regulator still heats, I thought to change the multimeter batteries. Low battery indicator was showing on the multimeter, although the batteries were only half discharged. I changed them anyway. Same result.

    So what am I doing wrong? Does the regulator need a load on the output, like a resistor? This might sound stupid, but does the multimeter draw too much current from the regulator?

    I know this is a basic task, but it got me a little frustrated after possibly destroying 2 brand new regulators bought from Kynix. (http://www.kynix.com/Detail/983055/L7805A.html).

    Any suggestions will be very helpful !!
    Regards,
  • recheck your connections. Regulator does not need any load to provide the correct voltage. Capacitors are good to have, but missing them will not cause the problems you are describing.
  • It is probably wrong connection. Does it heat event without load on output?
    btw, is is more efficient to use buck converter. Something like this:
    http://www.ebay.com/itm/Super-Mini-3A-DC-DC-Converter-Step-Down-Adjustable-Power-Supply-buck-Module-/122155531744?hash=item1c71097de0:g:uVEAAOSw8gVX631x