Voltage Reading/ Voltage Drop

  1. When we take down voltmeter readings from the terminals of a Rectifier (rated 12V), we get the voltmeter reading as 4.5V. Does that mean the rest of the voltage is lost in the form of voltage drop? or is it because of the current drawn by the load? I would be glad if someone could please post the answer.

    Thanks.
    Numair
     
  2. jcsd
  3. If you measure the voltage on the other side of the rectifier, with the voltmeter set to AC, what voltage do you read?
     
  4. its coming out as 4.5Volts...so i want to know if the rest of the voltage is lost in voltage drop? or is it because of the current that the load on the secondary that is withdrawing. thanks
     
  5. Voltmeters put a very, very light load on the voltage being measured so no I don't think the voltmeter is loading the power supply. It's much more likely there's some other problem causing low voltage.
     
  6. If you don't have a filter cap, the output of the rectifiers is still AC( well, call it varying DC like you turn the negative half of the sine wave around and become like lumpy waveform), you are measuring the DC component of the rectified lumpy voltage. Put a 22 to 100 uF cap across the output and you should get the voltage.

    If not, then read the AC voltages to make sure you have everything hooked up correctly.
     
  7. Thanks...but i want to know the loss in voltage is because of voltage drop? or is it because of the current drawn by the load?
     
  8. berkeman

    Staff: Mentor

    Post a sketch of your circuit, with the AC and DC DVM readings you measured...
     
  9. If you measure with just a multimeter, there is very little current drawn. If you don't have a filter cap, it is not accurate. To verify what I said, put your multimeter in AC Volt and measure, you should see AC voltage across the output of the rectifier. This is the AC I am talking about and this is the part that you miss when you measure with a DC Volt setting. As I said, without the filter, the output has a lot of ripple and it mess up your reading.
     
    Last edited: Aug 10, 2011
  10. Do not make measurements in a live power supply with an ohmmeter - even a Fluke.
     
    Last edited: Aug 10, 2011
  11. No, I should say multi meter, not a meter set in ohm!!!! That would be bad!!!! I changed the post already.
     
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