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Tuesday, November 30, 2021

HOW TO TEST LED by digital and Analog multimeter test led with multimeter

  Test   LED   with   Digital  and  analog  multimeter 

Test LED
test  led  with  multimeter


LED    TESTING
LED  is   a    kind  of  diode  and  it is a  basic electronic device on circuit so  it's good to know  how to check LED  good or bad.  The common size available on  the market is  3mm  5mm and  10mm Dia which is popular type  and other size and style  are  produced  too but it  use with some equipment.  . Before checking  LED , it  need  to know the most important information
1). Specification of  LED    2) Test  Function of multimeter    to check  LED.
After learning  these 2  points   then  we  can  move to  step  to LED checking.


Light emitting  Diode  LED
LED  size  and  color


1). Specification of  LED.
Led   is  a  low current consumption  device.  Don't let too much current  flow through it when using  at circuit  it always  need  limiting current resistor to decrease the current.   Limiting resistor calculation will  be discussed  when make a circuit design.   Generally small led need current  around  10-25mA and  big led  around 20-50mA  so  when  check it  by analog multimeter  use the proper current range  to check it and don't hold the test  lead  for a long  minute   , we  will describe on the  next section.



LED
LED  Size  5mm




light emitting diode
LED  Size  10mm


The basic LED  has  2 terminals  , namely , Anode and Cathode.  How to know which one is Anode , the longer  lead  is Anode terminal  and the lead  that  has a  big metal part inside the len is Cathode.
There  is   LED symbol  on circuit diagram and  PCB ,  this is useful  for  LED installing  and explain the circuit operation.

light emitting diode
LED  terminals    notice

LED
LED   symbol




2)  Test  Function of multimeter.  Refer to  Analog multimeter
We  use  Rx1 range  and Rx10  range  resistance test  function to check LED  , see on the multimeter you will find  Rx1 range = 150mA , Rx10 =15mA , Rx100 = 1.5mA , Rx1K =  150uA.  It means when you  turn rotary switch  to resistance test function there is  current and voltage at the test leads.  for example Rx1 = 150mA  3VDC ,   Rx100 = 15mA 3VDC  ect.   3VDC supply  from internal battery size AA  2pcs.   We use these voltage and current from the test leads  to check  LED.  Be notice from diagram below  that  the red test lead connect with negative polarity of  internal battery so the red test lead is  V-  and  the black test lead connect with  positive  polarity of  internal battery   so the black  test lead is  V+  e.    This information is useful when testing  mode as  forward bias  or  reverse bias  to LED  , including test  Diode , Zener  Diode.


TEST   LED   test multimeter
Rx1   range supply  150mA  3VDC at the test leads.



Internal   connection  of  test  leads    when  we set  to  resistance  test  function.
The red test lead  supply   -  Voltage   and    the   black  test  lead  supply    + Voltage.








Step to check  LED  with  digital multimeter.
1.  Turn the rotary switch of multimeter to   Diode test  function ,  it will display diode symbol on screen.
2. Connect the test leads  as shown on photo.   The good  LED  will shown  voltage drop across  LED.
Note  different model of  digital multimeter ,  it has the different voltage polarity at the test leads but it is not a problem ,  you have to make  2  time measurement ,  switch the test leads and make  the second time measurement.    It will be automatically forward bias and reward bias.

light emitting diode
The good LED , there is voltage  drop if  supplying forward bias.



Test    LED   with  digital  multimeter
There is voltage  drop (Forward bias)
Different LED color has different voltage drop.



test  light emitting diode
There is voltage  drop (Forward bias)
Different LED color has different voltage drop.


3.  Switch the test leads  opposite to the first connection ..
 the good LED will  display   " OL " as shown  on  photo.



light emitting diode
 The good  LED will display  " OL"   if  supplying  Reward  Bias

In conclusion...
Good  LED  will display voltage drop 1 time (around 1V-3V it depends  on LED color )  and  will  show  " OL "  1  time.
Bad  LED (open)   will  show  " OL "   2  times.
Bad  LED (shorted)   show  very low voltage drop    2  times.



test   light emitting diode

     Shorted  bad   LED   display   very low  voltage drop  2  time.



Step  to   check  LED   with   Analog  multimeter.
1. Set    rotary switch   to  Rx1 range
2. connect the test leads  and  read  the result  on  meter scale.
3. switch the test leads  opposite to the first measurement and do testing.
Be careful don't hold the test lead with led for  a long  second as 150mA current from Rx1  range may damage the LED  so make the measurement  as  take a  little time as short as  possible.

In conclusion...
Good  LED  will    light  1 time  and   , led does not light  1 time.
Bad  LED (open)  will  show  very high resistance  the pointer point at   ∞   (led does not light)   2  times.
Bad  LED  (shorted)  show  very low resistance    0   ohm   2    times.



test   light emitting diode    test multimeter
Good    LED     light    if  supplying  forward  bias.



Test   light emitting diode
Good  LED  does not light   if  supplying   reward   bias.