Seeking help about limiting current

Discussion in 'General Electronics Chat' started by lukemunnell, Mar 11, 2014.

  1. lukemunnell

    Thread Starter New Member

    Mar 11, 2014
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    Hi everyone.

    First post, glad to be here. Looking for some quick guidance with a project of mine:

    I've got a piece of photography equipment, a studio strobe pack (reference: http://www.speedotron.com/products/details/black_line/black_power_supplies/177/desc ) that I'm trying to power via a 125Ah 12VDC deep-cycle battery and a 1000w (2000w peak) true sine wave inverter.

    The studio strobe pack is basically a box of capacitors that amass power from a 120VAC source before sending it to a number of flash heads, up to 2400 watt-seconds at a time. The "recycle time" it takes the pack to do this is about 4 seconds, the first few miliseconds of which produce a peak draw of 15 amps (30 amps with the pack's built-in "fast recycle" option).

    I've found that my 1000w/2000w inverter falls short of that initial load, but I'm wondering if there's a way to limit current from the inverter to the pack to "lighten" the initial load, at the trade-off of stretching the recycle time out longer than 4 seconds.

    Any guidance?

    Thanks, guys.

    --Luke
     
    Last edited: Mar 11, 2014
    abhaymv likes this.
  2. lukemunnell

    Thread Starter New Member

    Mar 11, 2014
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  3. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Perfect..........
     
  4. lukemunnell

    Thread Starter New Member

    Mar 11, 2014
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    Thanks, Ron.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You would probably want to use the 240V version which limits the current to 6A. The 120V version limits it to 12A and that may still be too much for your inverter.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
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    Before peeling off a large wedge of folding money - the essential element of an inrush limiter is nothing more than an NTC thermistor.

    Whenever I scrap electronic stuff, I always open it to see if there's an inrush thermistor to salvage - you can of course order new ones from the likes of Farnell etc.
     
  7. lukemunnell

    Thread Starter New Member

    Mar 11, 2014
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    Thanks, Ian. The strobe pack has a built-in "slow recycle" feature which make me think it already has an NTS thermistor onboard. I'm just not knowledgeable enough with this sort of thing to tear into a box full of big caps.

    What I really want to find is a true sine wave inverter that limits current in situations like these. Wagan used to make some smaller inverters that did, but these days I can't find a single one.

    Any thoughts on how an inverter like mine could be modified to limit current?
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
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    No idea about the professional gear - but in the little disposable cameras, they have a self boosting modification on the blocking oscillator to charge the photoflash cap as quickly as possible.

    The HV winding is returned to the transistor base so the charging pulses augment the positive feedback, the rectifier gives a negative output to keep the phasing at the other end of the winding correct - that's why the electrolytic is usually "upside down".

    When the cap has just been discharged, the current pulses in the HV winding are large, so the boost to positive feedback is large, as the capacitor reaches full charge, the feedback augmentation diminishes - eventually the blocking oscillator is only ticking over to compensate for leakage.

    Its fairly obvious the blocking oscillator draws a large current to get the cap up quickly and then tails off to a low idle current.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Without a circuit diagram it would be difficult to know what to change.
    What does the inverter do when the strobe pack tries to charge, shut down?
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    If there's a charge inhibit input, startup could be sequenced so only one flash unit draws peak current at a time.
     
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