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Thread: LD driver configuring questions

  1. #1
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    Default LD driver configuring questions

    This is the first time I'm using diodes and drivers instead of premade modules so here is an amateur question. I expected the trimmers on the driver to be for settign the Voltage and Current, but they are for "Bias" and "Gain". The driver is Simpledrive.

    Knowing the voltage and current I need my diode to run at, how can I determine what my Bias and Gain has to be from that?

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    Quote Originally Posted by Maltes View Post
    This is the first time I'm using diodes and drivers instead of premade modules so here is an amateur question. I expected the trimmers on the driver to be for settign the Voltage and Current, but they are for "Bias" and "Gain". The driver is Simpledrive.

    Knowing the voltage and current I need my diode to run at, how can I determine what my Bias and Gain has to be from that?
    First turn down bias and gain by rotating the pots counterclockwise until they click (or 20 turns).

    Bias is then set by connecting the diode and modulation voltage of 5V (as well as power of course). Slowly turn up the bias pot (clockwise) until the diode has a low non-coherent LED glow, then back it back down so that the glow stops and then a tiny bit more (e.g. a quarter turn).

    The Simpledrive should have come with a one page set of instructions that tells you how to measure current with a multimeter. There are three methods. The method recommended in the instructions is the easiest and does not require a test load or your laser diode to be connected while you set current (with the gain pot). Note you measure current, not voltage. If your simpledrive did not include the one page instructions, let me know and I'll post if for you.

    Have fun.

    -David
    "Help, help, I'm being repressed!"

  3. #3
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    Thanks David. It did come with one page instruction, but if I read it correctly it mentions how to measure current by having a ammeter connected "in series" which I think means you need a test load or diode connected. What i the other method?

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    Sure thing, Maltes. The recommended method (on my older copies at least) was to measure the current across an open power supply which would have a +10mA error (which you simply subtract I assume). You can also measure as you describe or with a short circuited load (the later may have some degree of error) while the former has a slight risk of ESD damage. You can even measure across the voltage in mV across the circuit monitor pins and then multiply by 20 to get mA.

    So many options to choose from, but I don't generally do any of these. After setting bias, I ramp up gain slowly until I get the desired output as read by a laser power meter. You just need to make sure you stay short of the peak of the power curve so you don't blow the diode. This is definitely not considered "best practice", but I've done this for hundreds of diodes without issues.

    -David

    P.S. For future use, people on this forum request that you add your location to your profile. It helps us identify regional solutions / persons for assistance, indicate when not to use slang, etc. Thanks.
    "Help, help, I'm being repressed!"

  5. #5
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    ... for adjusting the current I'm inserting a 5W-resistor with 100 Milliohms instead of the LD and measuring with a multimeter (or mostly with my DSO) the voltage across the resistor and then multiply the (milli-) Volts with x10 to get the Amperes.

    My "problem" is the sheer power of the diodes, which will damage my indicator cards or the power-meter, if driven with full power - even if not focussed :-/

    Viktor

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    Quote Originally Posted by dkumpula View Post
    First turn down bias and gain by rotating the pots counterclockwise until they click (or 20 turns).

    Bias is then set by connecting the diode and modulation voltage of 5V (as well as power of course). Slowly turn up the bias pot (clockwise) until the diode has a low non-coherent LED glow, then back it back down so that the glow stops and then a tiny bit more (e.g. a quarter turn).
    Is it the same procedure if I try powering few red diodes connected in series from one 4.5A Simpledrive?

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    Hello? I'm sorry if the last question sounded self evident to the more experienced.

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    Quote Originally Posted by Maltes View Post
    Is it the same procedure if I try powering few red diodes connected in series from one 4.5A Simpledrive?
    Yes the procedure is the same, however be aware that diodes connected in *parallel* run the risk of all being blown if a single diode fails. Its best to not overdrive the diodes in this situation. In addition, be aware that the diodes won't all ramp up or have the same peak output, so be sure that none of them are being pushed beyond their rated max current.

    Assuming you have adequate voltage, you can connect in *series* instead which won't kill all diodes when one dies. Reds typically have low voltage requirements compared to greens and blues, so you may be able to run 3 in parallel with as little as 12V input to the driver.

    -David
    Last edited by dkumpula; 10-10-2016 at 05:14. Reason: Fixed stupid error
    "Help, help, I'm being repressed!"

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    I use a 1ohm resistor is series with 4 1n4001 diodes as a simulated load and measure the voltage across the 1 ohm resistor.

    E=IR. since r is one voltage is current. Reads directly on a volt meter. 1v=1amp. That came from my friend matt Polak. Works well.

  10. #10
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    Running in parallel is super risky. He stated it exactly backwards. If one pops in parallel the other diodes have to share the current and risk over current. If in series the string simply goes out if the pop is open. If closed short the other diodes continue to run at the same current they did. Usually the diode pops open and you loose the whole string till you change the diode. It also makes it easy to figure which one went bad as all you do is short the leads on each one till they all light up except the blown one. If it happened during a show you could if you can access the projector do a quick fix that way and just run on reduced power.

    In series you add the forward voltages. So as he stated you could run 3-4 reds on 12v but only 1-2 green/blue.

    Series good parallel bad. Ok.

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