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Thread: Perhaps dumb? Scanners and PS's

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    Bradfo69's Avatar
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    Default Perhaps dumb? Scanners and PS's

    Question....
    .
    This stemmed from a thread on FB by Joost and got me wondering. I don't think so but thought I'd ask for clarification. Does the amperage of a power supply have any bearing on the size a given set of scanners can produce?
    .
    Say for example you have a set of DT-40's. (And I'm going to throw a hypothetical voltage/amperage out there.) You attach a 24v power supply capable of 6 amps and display a test pattern. Then you disconnect it and attach a power supply of 24v capable of 12 amps and put up the same test pattern. Would there be any difference in the size of the pattern ... even minimally?
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    Yes it does. Lower amperage results in lower large angle (signal?) performance + worse performance when the scanner needs to make jumps! There is a term for this but I cannot recall right now.
    However the difference between 6 amps and 12 amps no.. eventually it doesn't make a difference anymore but it would with 1 and 3 amps!

    Small scanner power supplies usually only produce like 1.2 amps or so and that's at 12 volts even!.. so yeah

    The voltage will let the scanners eat more current and allow you to use get higher gains at increased heat generation but if it's not there.. well
    Do note that this doesn't mean magically pumping more voltage and current will make galvos better.. with heat comes more resistance so eventually it goes backwards and the components are not ment to go higher then either 18 or 24~26??v depending on the amps. Another thing is resonance.. if you keep cranking resonance, flexing of shaft and scanner mirror will eventually ruin your graphics so that's where it's no use.
    Last edited by masterpj; 06-10-2016 at 05:54.

  3. #3
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    Brad, Too little is bad, and too much is bad. There is a range of voltage and current where things work well. If you have too much or two little voltage, you have a problem. If you are below the supply current minimum, you will have problems with your images or with the scanner amp dying. Like the Story of Goldilocks, there is a range of "Just Right", and outside of that range is bad..
    ~
    ALWAYS CHECK WITH THE SCANNER AMP MANUFACTURER'S DATA SHEET BEFORE BUYING A POWER SUPPLY!
    There are some oddballs out there!

    ~
    !
    For TYPICAL Cambridge Style scanner amps OR Clones without J1 and J2 jumpers installed. (98% of what is for sale in the world, EBAY used units may have Jumpers installed):
    !
    FOR ONE SCAN PAIR:
    !
    Voltage Minimum, 20 Volts per rail, Voltage Maximum, 24 Volts per rail.
    Current Minimum, Four Amps per rail.
    Peak Surge Current, six to eight amps for a few hundred microseconds, requires good filtering capacitors in a four Amp PSU.

    ~

    There is a minimum sized power supply that will allow the galvos to function but not achieve good speed, and the image suffers on large fast jumps... I've had many cases over the last five years where newbies purchase scan sets with only 1-2 Amp per rail PSUs. They then wonder why their images look like crap, the scan amps or PSU melt, or the overcurrent limit trips on the PSUs that have it.
    ~
    Often times the voltage is too low, and a TL084 OPAMP needs a minimum rail voltage on each rail of ~ 9 volts to function effectively, they just become stable at 8 volts. Some people try to use a pair of laptop computer power supplies, and the 18 volt rail often is not large enough to fully turn on the cheap 15 volt regulators in the scanner amp... I cant tell you the number of scan amps I've repaired over the years from under power, but its is a large number, 10 or 20 pairs...
    ~
    So if the PSU is undersized, the software commands a large angle or very fast jump, the supply hiccups, and you get a glitch in the image. Or the newbie insists on scanning the full 30 or 40 Degrees optical, and the Cheap PSU overheats...
    ~
    A 6800 clone pair can draw 6 to 8 amps peak on big jumps, and about 2.25 amps per rail average, depending on amplifier and tuning. Something faster might need a bit more then that. So the minimum power supply should be at least four amps per rail in a conservative design.
    ~
    Does a poorly performing power system impact scanning, yes... Does boosting the voltage and current ever higher help you, NO! A +/- 24V rail is enough unless you have one of the very rare industrial scan amps designed for more voltage.
    ~
    I'll spec a PSU pair that is a minimum of two and a maximum of four times my measured test bench current when SAFELY scanning a Grid or ILDA test pattern wide angle. However, some high amperage switching PSUs have a minimum load, and if your below that, the PSU will generally not come up to full voltage.
    ~
    Cheap PSUs with undersized output capacitors hurt the image the most... I generally add a pair of big Electrolytic capacitors to my projectors... Typically a pair of 8000 to 10,000 microfarads @ 36 or 48 volt rated. That lets me deal with the surge demand on big jumps... Lets not forget the wiring, number 22 wire common in Chinese projectors is very much undersized for long power supply wiring runs. Small diameter wire adds series resistance in the PSU leads.
    ~
    When I sold the "Red Board" clone amps for 506s, I found 20 volts to be about the minimum... 24 volts is about the maximum you'll ever need... I tuned the boards at 21 volts, so anything over that was gravy...
    ~
    Caveat, a few older 6850/67XX amp boards are out there that have the +15 and -15 volt regulators jumped out (Jumpers J1, j2 installed) and those will run just fine on regulated supplies rated for +/- 15 volts.
    ~
    Explanation for Engineering oriented types. Scanner speed is ultimately limited by the Torque to Inertia ratio, if you can not produce the design torque, your servo loop will be unhappy, and in some cases oscillate or spike the galvo. The Galvo is designed to produce the design torque at a certain level of coil current. So the PSU must be able to source the current, in order to produce maximum Torque. Also, the coil is inductive, so you can get 6-8 amp surges of a few hundred microseconds duration in the ground lead on fast jumps. Fast Galvos are current driven devices, not voltage driven devices.
    !
    If the ground wiring and supply wiring is not adequate, the high peak current can "lift" the amp input common mode voltage, and "lift" the signal ground resulting in a distorted image.. I like a minimum of #14 stranded wire in my scanner amp wiring.
    ~
    PJ is right, things go backwards if you overdrive the Galvo. If you have the classic design, or the clone of the classic design, The overdriven coils heat up, the copper in the coils expands, and the sharp edged portion of the rotor will scrape the expanded drive coil... This shorts out the coil!
    ~
    ~
    I have a pile of twenty dead 6210s on my desk that were sent to me for analysis, half of them have damaged coils from overdrive, and the other half have one side of the position sensor burned out. The position sensor failed because the so called "expert technician" used a miss-wired test cable on ten tests... The coils failed because he used unregulated power supplies on his test bench.. Too much voltage.. All of them are un-repairable scrap..
    ~~
    By all means, down load Pangolin's excellent white paper on scanner amp wiring and follow it...
    ~~
    ~~

    Steve
    Last edited by mixedgas; 06-10-2016 at 07:22.
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    ... sometimes this sort of 'changes' will be more depending on the cables/plugs and internal wiring to the output (wire length, capacitance, ...) than on the given voltage and current.

    I'm actually battling some quirks with fast switching high currents (up to 10Amps in 500ns), where I have dramatic changes with different PSU's of the same nominal values ... but this is too depending on the internal wiring and capacitance of the drivers :-/

    Viktor

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    Steve. As always, you are amazing with more information than I ever needed to know but, it still makes for a great educational read.
    PM Sent...

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