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Thread: Help Adjusting Dave's Dual Drivers

  1. #1
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    Default Help Adjusting Dave's Dual Drivers

    Having a little bit of difficulty while trying to set up one of Dave's drivers. Must be just a brain fart, but...

    Per the instructions I applied 5V power and 5V modulation signal to the channel being adjusted which was "driving" a fluke DMM on 10A scale.

    Adjusting the 'offset' pot never produced any current reading whatsoever, even after the red light came on.

    I was able to adjust the "drive" pot to obtain the desired current, but its behavior seemed to vary based on where the "offset" pot was left.

    I also observed that if the modulation was brought to zero, and then back to 5v, the driver would not turn the drive current back up; it sat at zero.

    What am I doing wrong?

    Thanks,
    Mike

  2. #2
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    try a minimum load of three 1n4001 diodes in series with a 1 ohm resistor.

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  3. #3
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    To adjust the offset pot, you shouldn't have 5V on the mod lines. That is your 'idle' setting.
    Can't recall if Dave's instructions cover setup, but you normally set offset first with no modulation, then do the drive current, as the offset affects the max current (e.g. if you have 2A output with 5V mod, turning the offset down by 200mA will leave you with only 1.8A output with 5V applied)
    So, if you have to adjust offset after setting current, you should go back and re-do the current.

    I do all mine based on the laser diode's response. So I set the offset with the laser diode attached and view the threshold by eye, and I set my current by using a power meter as I'm usually matching pairs of modules for power, so setting current doesn't really get me where I want to be.
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    Thanks for the tips, guys. Norty, I was wondering about setting offset with max input on the modulation line, but the manual Dave sent me says explicitly to do that.

    "Carry out offset and drive current setting with 5v on the modulation input."
    I will try again with your method and the dummy load.

    Steve, I have a diode tree I built for the purpose of being a driver dummy load; I will try this method again with the 0V applied to the modulation input for offset setting.

    FWIW, the beam suppression feature appears to be active below about 0.25V, a tad higher than the 0.1V mentioned in the manual.

    Best,
    Mike

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    Ah yes, the beam suppression.

    What you need to do is back off both pots totally. Then when you apply 5V you should get no current. Add offset to the desired level first, then do the main current.

    Steve is absolutely correct, you need a dummy load on the output to measure across.

    My apologies, brain fart...
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    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  6. #6
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    I appreciate the good ideas guys, but I'm still not getting reliable results.

    I put the dummy load on and adjusted things to where the currents looked reasonable. I used 10 diodes in series to get the voltage drop up to something approaching the blue and green diodes' voltage drops. I powered the driver with 8.0V limited to 200mA and connected the green PL520 diode. (It turns out that 7.5V was insufficient headroom to get 150mA through the PL520.)

    With 0V on the modulation input, adjusting either or both pots does nothing.

    After powering on the system with about 0.3V on the modulation input, adjusting the bias pot gives a good current adjustment. I set the current for about 50ma, just below where the diode flipped from "LED" to "laser". I then raised the modulation input to 5V and adjusted the drive current for about 140ma. Adjusting the modulation voltage gives a continuous change in the measured current and observed brightness. So far so good.

    My problem is this: Once the modulation voltage dips below 0.27V, the driver goes in to beam suppression mode and does not come out of it. If the modulation ever falls below about 0.27V, the diode will not be turned on again. This is true both if I apply power initially with the modulation input below 0.27, or if I dip below 0.27 after the diode has been on. If power is applied with the modulation voltage held below 0.27, the diode will never come on.

    I tried a variety of offset current adjustments but was unable to find a setting where I could "go dark and come back."

    I raised the system voltage to 9 and readjusted gain and offset in case more headroom was required, but this additional voltage did not change the observed behaviors described above.

    I'm kinda stumped. Help? Other thoughts?

  7. #7
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    With 0V I would expect to see nothing. This is the beam suppression working correctly.
    No idea why it won't modulate once you've reduced it below suppression threshold though.
    Have you tried setting your drive current a bit higher? 140 seems very low even for testing
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    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  8. #8
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    OK, but I would expect it to STOP suppressing once the modulation input rose above the threshold voltage, no?

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    Yep. Have you linked Dave up to the thread? He was great when I did my best to kill a couple of mine.
    Frikkin Lasers
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    You are using Bonetti's defense against me, ah?

    I thought it fitting, considering the rocky terrain.

  10. #10
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    Are the grounds from both your mod power supply and drive supply tied together somewhere?

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