Diode in Parallel

Discussion in 'General Electronics Chat' started by lynnfaiz, Dec 17, 2012.

  1. lynnfaiz

    Thread Starter New Member

    Dec 16, 2012
    29
    1
    diode in parallel.jpg

    Referring to the attached image, please help to advise are both diodes turned ON. And I appreciate if you can also explain why.

    I am not sure as both are having same diodes and resistance.

    Thanks.
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,979
    744
    both will conduct, but depending on their resistance will drop different voltages, normally 0.7V each, so the Vo will be 10- 0.7=[9.3] /2 = 4.65V
     
    lynnfaiz likes this.
  3. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    I beg to differ, Vo will be aproximately (10-0.7)*2k/3k=6.2V, and simlation agrees with that.
    As long as the current through the diodes is of similar magnitude, you can rearrange the circuit to have a single diode and then the two 2k resistors in parallel, then the 2k load resistor.
     
    lynnfaiz likes this.
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Both diodes will be on ONLY if they are exactly the same, otherwise only one would have most of the current through it and the other very little if any current.
     
    Reloadron and lynnfaiz like this.
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,016
    3,789
    Kubeek is correct.

    The voltage drops 0.7v through each diode, V=-9.3 at each doide-resistor node.

    The two parallel 2k resistors is like 1 x 1k resistance. The 2k resistor to ground means V0= 9.3*2k/(1k + 2k) = 6.2v
     
    Reloadron and lynnfaiz like this.
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,016
    3,789
    Even if the two diodes are off by 0.1v, you can still calculate the current flow through each route (Assume V0 is what we estimated from the previous = 6.2V, then you have 1.55 mA through one route and 1.50 mA through the other route. Current cannot flow from one 9.3v to the other 9.2v since the diodes block current. Therefore, current will still flow through both routes since the 2k resistor acts as a shock absorber in each bridge. Some curcuit designs prevent current flowing from one leg but not this one.
     
    cmartinez, Reloadron and lynnfaiz like this.
  7. lynnfaiz

    Thread Starter New Member

    Dec 16, 2012
    29
    1
    Thanks everyone- understand now that both will turn ON.
    I manage to get Vo = 6.2V , Id = 1.5mA.
    Thanks so much.
    :cool:
     
  8. tinkerman

    Member

    Jul 22, 2012
    136
    34
    Isn't it a practice when parralleling diodes or SCR's that a small resistance in each circuit forces the diodes to share and not allow one diode to hog all the current?
     
    lynnfaiz likes this.
  9. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    Yes, tinkerman. The 2k resistors will make the diodes share. If those 2 resistors weren't there, the diode with the lowest turn on voltage would try to pass all the current, and as it heated up, it would pass very nearly all the current.
     
    cmartinez and lynnfaiz like this.
  10. lynnfaiz

    Thread Starter New Member

    Dec 16, 2012
    29
    1
    delete.jpg

    referring to this circuit, would both diode turn ON?
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,531
    1,248
    I disagree with Kubeek. Even if the Vf of the two diodes are drastically different, there will be current in both series resistors.

    ak
     
    OBW0549 likes this.
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,308
    884
    ...and the two currents will be nearly the same, about 1.55 mA.
     
  13. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,308
    884
    Both diodes will conduct current, but D1 will conduct much more than D2.
     
  14. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Here's an LTspice simulation of both circuits.

    I added a 0.3V variable voltage in series with D1 to simulate diodes with drastically different forward bias voltages. This only causes a small change in the current division between the two diodes, as expected.

    The bottom circuit has a very large difference in the two diode's current, also as would be expected.

    Parallel Diodes.gif
     
    lynnfaiz and cmartinez like this.
  15. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    Wow! Pulling my posts up from more than 3 years ago? Impressive!

    About Post #10: Assuming a 1N4148 signal diode at room temperature
    In the real world, D1 would show a voltage of 0.65 volts at 0.9 ma of current.
    If you apply that 0.65 volts to an exactly same diode in series with 680 ohms, some smaller current will flow.
    If 100 ua flows through D2, then the voltage across D2 is 0.58 volts and the voltage across 680 ohms is 0.07 volts. Double check that as 0.07 volts across 680 ohms and we have 0.1029 ma. Close enough for government work. :p

    That's the way I see it. D2 is going to carry about a tenth of the total current because diodes have performance curves.
     
  16. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    Pretty close agreement with my sim where the two diode currents are 125μA and 817μA. :cool:
     
  17. lynnfaiz

    Thread Starter New Member

    Dec 16, 2012
    29
    1
    Diodes current are different , but the voltage across both diodes will be the same?
     
  18. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    You have to read the answers.
     
  19. crutschow

    Expert

    Mar 14, 2008
    13,001
    3,229
    No. The voltage across a diode is a logarithmic function of the current.
    Here's a typical curve of diode forward voltage versus current.
    [​IMG]

    My simulation gave 490mV and 575mV drop respectively for the two diodes.
     
    cmartinez likes this.
  20. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    A similar graph is in the datasheet provided in post #15
    Page 2, top left corner of the page.
    The diode I chose is a different part number than what crutschow used, but the proportions are about the same.
     
Loading...