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Thread: Heads up- scanner power supplies.

  1. #1
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    Default Heads up- scanner power supplies.

    OK, A word from our Sponsors(TV)... Now Hear This!(Military) Stat!(Medical)

    I've had two Recent cases of users trying to reuse/recycle undersized PSUs that came from existing low cost projectors.

    In one of those cases this resulted in "fried" components on the scanner amp.

    In the other case it resulted in severe oscillation of the scanners on power up. The power supply was oscillating as the inductive load kicked back when the amp started up and overloaded the PSU. The PSU shut down in short circuit protect, waited 50-100 milliseconds or so and switched back on. This repeated ad-infinitum and eventually something would smoke.

    Guys, 24V at One amp or less per rail just does not cut it with fast scanners. Back in the 12K days, sure, one to two amps might work. Now days you have scan pairs that may draw 6.5 Amps peak on a long jump and two amps or more AVERAGE CURRENT per rail at 30K full angle.

    So the Moral of the story. One Hundred Watt per rail PSUs (4 Amps each. ) are starting to be a VERY good idea. 200 watts Total in the projector per scan head is a very good idea.

    Yes, 100 watts per rail might be 55-60$ for a Meanwell. But new in the box surplus they are often 26$ per unit.

    For example. C506 idle at about 100 mA for the negative rail and 140 mA for the positive rail. The extra 40 mA is for the position sensor. On a 1 Amp PSU, 10% of your "Headroom" for faster scanning and smooth large jumps just disappeared at IDLE alone.

    Why should I care:

    The master reason: Your shows may look distorted or fail. That makes you look bad.

    A. Starved Servo amplifiers tend to do very interesting things when the current is clamped and limited below what they need.

    B. Crowbar circuits on undersized switcher PSUs tend to oscillate. This can result in start surge after start surge slamming your scanners to one side of their travel.

    C. LM3886 Power Amplifiers used on most Galvo amps draw huge start surges. I've seen as high as three amps per rail as the loop closes for the first time. On both rails.

    D. Low cost PSUs often do not clamp the output for overshoot when overloaded.

    F. Club line voltage is usually crap and filled with sags and surges.

    -------------------------------------------------------------------------------------

    Ways to cure:

    1> Use enough Gun, ie buy PSUs at least one size up from what you need. Have some Headroom and Reserve. A Larger PSU can be running at barely warm to the touch.

    2> Install Electrolytic Filter caps very close to the power supply. 5000-10,000 Microfarads rated at 48 Volts or more is a GOOD idea. Put the caps close to each Amp board. Use two pairs of two Caps. (Overkill? Maybe Not!)

    3> Place two small 24V relays with 600 Ohm or greater coils on the PSUs. These are cross fed. This means the Positive Side relay is fed by the Negative rail. The Negative rail is thus gated by the Positive rail. The Idea is both PSUs will be well on their way to being stable before power is connected to the scanner amps. Simply because mechanical relays are SLOW. This offers a small, inherent, time delay. If one PSU fails, the Amps will not set there "hung" on one active rail. Alternative is to use a time delay relay set for 2-3 seconds.

    4> Wire your projector carefully, making sure large gauge wires are ran from the PSUs to the amps. Bill Benner's guide to correct projector wiring is offered on the Pangolin Website, its worth a read.

    5. Use dedicated scanner amp PSUs. Unless the PSU is very oversized, avoid hanging the Diode Drivers, TECS, and Inductive loads things like actuators off the Scanner PSUs.

    6. Make sure both PSUs have the same Maker, have the same model number, and have the same voltage and current rating. Electrical Symmetry is important. Avoid hanging sideloads such as actuators and diode drivers off the Galvo PSUS.

    7. A fan and proper PSU heatsinking is cheap insurance.

    8. Fuses in the PSU rail and in the line power prevent mishaps when the PSU reaches end of life.

    9. A proper line filter In your projector is often very good at keeping RF CRAP out of the projector. (Hint, The surge protector /filter in your power strip won't help you) Moving light ballasts often generate such CRAP.

    --------------------------------------------------

    Link to Pangolin's Document:

    http://www.google.com/url?sa=t&rct=j...82001339,d.aWw

    ---------------------------------------------------

    This is not just true with my C506 Amps, but is very true with any fast galvo. Is it really worth saving 20 Dollars to loose a show down the road? If your excuse is making world's awesome, tiny, Micro projector, ask yourself if you really have enough PSU heat sinking in the seven inch cube. Then buy the miniature Lambda PSUs designed for the job that can run hot.
    -------------------------------------------------------

    In defense of the one Gentleman, his goal was to test the scanners while waiting on the shipment of the good PSU. His low cost PSU had a poorly designed feedback loop. It probably surged up way higher then 24 volts.

    If you think I'm crazy, most of this is not my idea. Its what I was taught was the Minimum years ago. Ask yourself, "Self, What is the Minimum Average Time to System Fail I'm willing to Accept". Usually the small PSUs are corner cutting to lower manufacturing cost.

    Steve
    Last edited by mixedgas; 12-21-2014 at 12:19.
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    most chinese amps I have seen have a 7815 and 7915 right after the input anyway, so they really can't draw more than 1.5A per rail anyway

    but I definitely agree most of the chinese switchers that come with galvos are complete garbage

    but older setups were also pretty bad, most 6800 power supplies I have run across are nothing more than a toroid, a bridge rectifier and a pair of large caps

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    Quote:

    "most chinese amps I have seen have a 7815 and 7915 right after the input anyway, so they really can't draw more than 1.5A per rail anyway"

    The 7815 and 7915 just power the OpAmps and Position Reference. The +/- 24 GOES RIGHT TO THE LM3886.

    Hence the larger draw!

    Steve
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    well I think the LM3886 can put out up to 11A on the output no? I have no idea how it works in the galvo world since I have only used it in audio designs personally, and I don't have any chinese amps with 3886s

    never really understood how these were running off .8A /rail power supplies

    would it make sense to run these amps with higher input voltages? I think the 7815's can take 35V and the LM3886 can take like +/-60

  5. #5
    mixedgas's Avatar
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    No, There is no reason with C506 to enhance the voltage above the 24V rail. I easily meet the specs with 21V rails on the bench. In fact I can meet them with 19V rails.

    As for LM78xx, the general rule to stay within SOA curve limits is add 9-10 Volts to the rated output voltage. This derates with increasing current. So a LM7815 would be good to 24V input at moderate currents.

    Steve
    Last edited by mixedgas; 12-21-2014 at 19:10.
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    With SMPSUs its always a good idea to have ample headroom, that is inevidably exhausted with peak demands. Just like with regular audio.

    Even the humble DT30s can shutdown their supplied PSU at 30k set a tad too wide on wild cues... more, always need more....
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    Quote Originally Posted by mixedgas View Post
    If your excuse is making world's awesome, tiny, Micro projector, ask yourself if you really have enough PSU heat sinking in the seven inch cube. Then buy the miniature Lambda PSUs designed for the job that can run hot.
    Hey Steve!

    Can you point me to which series you are referring to?

    I just used the two sets of 506's with your amps in mini builds. In one I used a new, rhobust PSU with about 2.4amps per rail (running at 25v) and another one with a recycled 1Amp per rail chi PSU. I don't see any performance difference as yet, but would be happy to change out at least the recycled Chi amp. Note that the large amps already take up a great deal of space for a mini build, so small form-factor PSUs are appreciated.

    -David
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    I've had two Recent cases of users trying to reuse/recycle undersized PSUs that came from existing low cost projectors.

    In one of those cases this resulted in "fried" components on the scanner amp.

    In the other case it resulted in severe oscillation of the scanners on power up. The power supply was oscillating as the inductive load kicked back when the amp started up and overloaded the PSU. The PSU shut down in short circuit protect, waited 50-100 milliseconds or so and switched back on. This repeated ad-infinitum and eventually something would smoke.

    Haha funny......Its called Laserkarma

    That,s what you get when taking the El cheapo route. Buy some traco,s and be done with it. They never fail
    Last edited by edison; 12-22-2014 at 05:59.


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    i know not laser related but a medical company "terumo cvs" tried using switchers made by deltron that on paper worked, but the bypass machine has an isolation transformer before the switchers and caused an oscillation that most times blew up 2 600watt power supplies, i was a tech there and the engineers did not believe me until they started failing at hospitals on power up since i was lower on the pecking order i was ignored when i told them to use lambda "now tdk i think" power supplies, they ended up adding a large thermister bank and switched to lambda and what did ya know, the problem went away. when the deltrons blew up they sent a nice flame out one end

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    cool thread bro, I will be looking this one up when I build my next projector.
    I remember seeing some old projectors here on pl. They appeared to be all about the scanners and there power supplies.
    The laser appeared to be a small part of the projector.
    It would be cool to perhaps use one of edisons colour modules and go the full monty with the scanners and associated infrastructure .

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