Help with giant light show project

Discussion in 'The Projects Forum' started by Robert1, May 22, 2013.

  1. Robert1

    Thread Starter New Member

    Apr 22, 2009
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    I am building a GIANT spectrum analyzer for our church’s VBS program. I am using an array of solid state relays driving 120 VAC lights and am “tapping into” a small commercial spectrum analyzer to control the relays. Problem is that the unit I am using multiplexes its array of LEDs (rows and columns) so that each is on for only a small time period. Unfortunately, the solid state relays pick up on this and the 120 VAC are dim and flashing noticeably. If I add a 22 uf cap directly to the the output of the driving logic (74VHC595) to “hold the voltage through the off cycle, the 120 VAC light works great. Does this sound OK or could this modification damage the 74VHC595 over time? (don’t know if the current required to charge or discharge the cap might be exceeding the IC’s capability). Thanks for any help.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I never thought a church would need a spectrum analyzer. Nice work!

    As for your project, please post a schematic of your original and modified circuit so we can make some real comments. Picture is worth a 1000 words or more in electronics talk.
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    I'm a little confused by that, since I think the output can source or sink the same current. So when it goes low it should discharge the cap just as fast as it charged it on the high pulse. But maybe I'm missing something.

    I was thinking adding a diode to prevent back discharge would help reduce AC current (what frequency ?) thru the 74VHC595, and probably allow using a smaller capacitor as your peak detector.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I think he is concerned that an unrestricted flow of current into a 22uF cap until it is charged might damage the chip. I am hoping he posts the schematic so his question and our solutions can be clarified.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I get the question about placing a capacitive load on an AC output, but not why the capacitor has worked so far. To my mind it would only help if there is a difference between the charge and discharge rates.
     
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  6. Robert1

    Thread Starter New Member

    Apr 22, 2009
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  7. Robert1

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    Apr 22, 2009
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    Also - I do think there is a difference between the charge and discharge rates. There are 8 rows of LEDs each is controlled by a different FD337N. So I would think the charge rate would be 1/8 of the period and the discharge rate would be 7/8 of the period. Frequency is probably pretty low (whatever would be necessary for visual flicker free operation > 30 Hz?)
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Usually 100Hz or more is better, what is the model number of the solid state relay,

    Also have you put the relay in parallel with the exisiting led, or removed the old led and put the opto in its place?
     
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  9. Robert1

    Thread Starter New Member

    Apr 22, 2009
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    The relay is a CLARE CPC1976Y (the load is a 100 W bulb). I removed the existing led so it would not draw any current.
     
  10. wayneh

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    Sep 9, 2010
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    Oh, I get it. The capacitor isn't really holding it on any more than it's holding it off, but it does act as a low pass filter, a sort of integrator.
     
  11. Robert1

    Thread Starter New Member

    Apr 22, 2009
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    I figured the capacitor was charging up (or charging down) during the short period when its corresponding transistor was on. Then when the transistor was off it was providing enough charge to keep the LED of the solid state relay on which keep the 120 VAC light from flickering. So, is it ok to hang a bunch of these capacitors directly on the outputs of 74HC595? Also, the FD337N could be supplying the ground path for up to 15 of these Caps. Am I going to fry something?
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Maybe I'm way off - very possible - but my understanding from the data sheet is that when the IC goes "off", it's not disconnected or somehow preventing reverse current, it goes to ground. That would sink current and discharge the cap as fast as the internal resistances allow.
    I don't think that's been established.
    Do you have the data sheet? Check both max instantaneous and max continuous current. Both specs should well exceed the sum of your 15 circuits. I'd be nervous about anything less than 2X.
     
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  13. Robert1

    Thread Starter New Member

    Apr 22, 2009
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    Thanks. I think the way the spectrum analyzer works is that the IC (the 74HC595) puts the LED pattern for one column on its outputs. Then the uC displays that column for a short period by turning on the corresponding FD337N to display that column. Then the uC turns off that FD337N, loads up the 74HC595 with the data for the next column, then turns on the next column's FD337N to display it. So during this period where the other columns are being "drawn". the cap has no path to ground for reverse current through the IC. All it can to is supply current to the solid state relay LED.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    Ah, so the logic IC is still "on" when the MOSFET turns off. That's what I was missing. Instead of turning off immediately when the MOSFET turns off, the relay stays on briefly as the cap discharges through it.

    I suppose this means that, in addition to seeing a discharged cap when it goes high, the logic IC might also see a partially charged cap when it goes low. So, worst case, the IC is seeing a full capacitive load on it's output. The impact of that depends on frequency, and the fact that you've gotten away with it so far says something.

    It'd be nice if someone could chime in if they have experience with capacitive loads on 74HC series logic ICs.
     
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