Using my new multi meter and a voltage test

Discussion in 'The Projects Forum' started by meestahelectro, Aug 13, 2006.

  1. meestahelectro

    Thread Starter Member

    Aug 10, 2006
    Hi, I just got my new fluke 83 V DMM... I am new to this and was trying to run some tests on a breadboard I got at Radio Shack.

    I connected two D batteries and am passing in 3v into the circuit. the circuite then uses some jumpers and passes through an LED (2.6vDC, 28ma, 10mcd). the LED has just about 3volts going into it, but there is almost no voltage coming out .002A for the rest of the ciruit back to the battery packs.

    How would I power the rest of the circuit? Do I need bigger more volts?

    the resistance measure is something like 1.7M ohms, which I really dont understand either. thats seems incredibly high?

    Sorry if my terminology is wrong, I am just starting out.
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
    Ok the batteries should be in series, the plus goes to the LED then that goes to the batteries minus. Without a current limiting resistor the LED is blown and would easily read a high ohms. You need approximately a 330 ohm resistor in series to protect the LED.

    If you hooked the LED up backwards you may have not destroyed it depending on it's limits.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    Your led must be pretty bright. The tiny residual voltage is to be expected, though. The 2.6 volt figure is the amount of voltage necessary to make the electrons inside the led jump the potential barrier and expend enough energy to emit photons. You might have more voltage remaining, but the current draw is probably limited by the battery capacity. If you had more current capacity in the batteries, the led might burn up.

    Led's always take some voltage to go into operation. It is more convential to use higher voltages, and limit current with a resistor in series with the led.

    Since your led has a large operating potential, it will appear as a large resistance. If the meter has a diode test function, it will appear a bit more reasonable. Most PN junctions in signal diodes or transistors show operating potentials between .5 and .75 volts. Yours should be more like the 2.6 volts, if the meter will read that high.

    Instead of trying to tailor the supply voltage to the load, it is common to use something more like 12 volts and to limit current with resistors, if necessary. There are experiments in the chapters on electronics elsewhere in this site. You might look them over for another way to do your stuff.