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Thread: Diodes in series voltage question

  1. #1
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    Default Diodes in series voltage question

    When running diodes wired in series, doesn't it cut in half the voltage going to each diode? So if you are using 0-5 volts to modulate 1 diode, if you put 2 diodes in series wouldn't you need 0-10 volts? Or, do I have that backwards and 2 diodes in series would only need 0-2.5 volts input to give each diode 0-5 volts?

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    Quote Originally Posted by Phredy1 View Post
    When running diodes wired in series, doesn't it cut in half the voltage going to each diode? So if you are using 0-5 volts to modulate 1 diode, if you put 2 diodes in series wouldn't you need 0-10 volts? Or, do I have that backwards and 2 diodes in series would only need 0-2.5 volts input to give each diode 0-5 volts?
    No, laser diode drivers are current limiting, so as long as your supply voltage is 2 times the diode voltage plus a little for the driver, the driver will push the required current through the diodes.

    Modulation voltage is still 0-5v
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    Quote Originally Posted by Doc View Post
    No, laser diode drivers are current limiting, so as long as your supply voltage is 2 times the diode voltage plus a little for the driver, the driver will push the required current through the diodes.

    Modulation voltage is still 0-5v
    Ok, I will buy that. So if you were using say a flexmod to power 4 diodes in series, would you need to supply the input voltage to the flexmod at a little over 20 volts?

    If a person was to just use a PSU to test diodes, for 1 diode you could use up to 5 volts. If you had 2 diodes in series you would use up to 10 volts to power both diodes, correct? Or am I missing the boat here?

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    It depends on the forward voltage of the diodes, if they're reds that will probably be around 2v, if they're the 445 diode it would be around 5v.

    I run my dual 445 in series using a flexmod 2, I use a 12v supply.

    The only reason I decided on series was because the Flexmod 2 offered more diode protection when configured for its low (up to 2 Amps) current range.

    When running in series it doesn't matter how many you have in the string; the current doesn't increase, you just need enough voltage to push the current. I don't know about the Flexmod 3, but the 2 only dropped a little over a volt itself.

    Ian
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    would a combination work?

    i.e. i have 12 LOCs

    to drive them in series i would need more than 12*2 = 24volts (24 + Vdrop at the psu). to drive them in parallel i would need more than 12*250mA = 3amps. both solutions look quite demanding in terms of psu (a flexmod p3 can take it easily but i don;t want to stress things so much)



    i was thinking about an intermediate solution, to drive them in parallel in groups of 4 and then have all groups wired in series, so i would need overall 1 amp (4*250mA) and 6 volts (2+2+2+Vdrop at the psu).
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  6. #6
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    For 12 diodes I would go with parallel and balancing resistors, the flexmod will be fine with a decent heatsink, also you are not running them in CW, they will be run at a percentage duty cycle.
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