Monday, December 7, 2020

Test a Diode with Analog Multimeter check foward voltage drop forward current and reverse current.

Test  a   Diode   with  Analog    Multimeter

We  use LV and LI scale to test a   diode condition.   How to set  those scale ,  it use Ohm range then reading the  measured value  from  LV scale to test  forward voltage drop (Vf)  and   use  LI  scale to test  forward current  and reverse current.  This  method can  test  LED and  zener  diode  too.  See photo below ,  LV scale range   0 - 3  full scale  and  LI  scale  range  0-15  full scale.


LI scale reading  0-15  full scale.

x1     Full Scale  150mA    ,  multiplied by  10   with 0-15 reading 

x10   Full Scale  15mA      ,  multiplied by  1     with 0-15 reading 

x100  Full Scale  1.5mA    ,  devided by  10       with 0-15 reading 

x1K   Full Scale  150uA    ,  multiplied by  10    with 0-15 reading 


Diode  test

                                 LV scale range   0 - 3    and   LI  scale  range  0-15



Step  to  test   Diode   with  analog    multimeter .

1.   Set the multimeter to Ohm range.  Select the right range and turn rotary switch to  suitable  range. Multimeter will supply voltage and current to diode from the  test leads.  The voltage and current supply from internal AA  battery which installed inside the multimeter.

Rx1 ( 150mA 3VDC )      for  big  diode. 

Rx10 (15mA  3VDC )      for medium diode.

Rx100 (1.5mA  3VDC )   for small diode

Rx1K ( 150uA 3VDC)     for test leaked current.


Test   a  Diode


2.   Plug the red test lead to +  terminal and  black test lead to COM terminal.

3.  Zero   Ohm adjustment before testing so it will get accurate measured value.  Short 2 test leads together and adjust  0 Ω ADJ  knob till the pointer point at  0  Ohm mean it is  ready  to  test.


Test   a  Diode
                                                              Zero   Ohm adjustment 


4.   Forward  bias  to  diode by connect  the black test lead  Anode terminal and the red test lead to Kathode  terminal  as  photo.   The black test lead will supply  positive voltage and the red test lead will supply negative voltage ( it  come from internal AA battery polarity ) so the diode will get forward bias.

Diode  test
                                                      Forward  bias  to  diode


5.    Read the measured value  ,  forward  bias case.    

LV scale ( 0-3) as above photo diode has  forward voltage drop   =  0.7V  

LI scale (0-15) as above photo diode has  forward current    =  12.5mA 


6.   Reverse   bias  to  diode by connect  the black test lead  Kathode  terminal and the red test lead to Anode  terminal  as  photo below.

Diode   check
                                                Reverse   bias  connection  to  diode 

7.    Read the measured value  ,  reverse  bias case.    

LI scale (0-15) as above photo diode has  reverse  current    =  0 mA 

As  reverse current of small and medium diode is very small amount  micro amp ( such as  5.0uA) so the multimeter can  not sense  that  micro amp current   by setting  X 10 range.

8.   Check diode condition   and   summarize.

Good diode will get  forward  voltage drop  0.3-08V  when  forward bias testing.

Good diode will get 0mA ( very small amount uA) when  reverse bias testing.

Bad diode ( shorted )  will get  0 Ohm and  switch the test  leads still get 0 Ohm.

Bad diode ( Open )    will get  ∞  Ohm and  switch the test  leads  still get  ∞   Ohm.


Test   a  Diode
Bad diode ( shorted )  , the pointer will point at  0 Ohm and switch the test  lead , it still get 0 Ohm.

Test   a  Diode

Bad diode ( open )  , the pointer will point at  ∞   Ohm and switch the test  lead , it still get ∞   Ohm.