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Thread: Alternate polarity led strings

  1. #11
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    Default Re: Alternate polarity led strings

    Hi,

    I don't think the problem is to do with the 3v supply.

    I have a couple of other strings which have the same alternate polarity but I had not tried to get them to appear as if all LEDs were lit, I was quite happy to drive them as two separate strings.

    I have tried to use the same fast on/off sequence with these and they behave in the same way. These strings are 21v so the H bridge has a separate feed at 6v with the appropriate link removed.

    Sorry I am very new to all this and don't understand duty cycles and rep rate.

    Thanks for your continuing interest, it is much appreciated and I am learning a lot.

    Mike

  2. #12
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    Default Re: Alternate polarity led strings

    Mike_Parks:
    I suppose that my understanding of your setup is still a bit muddy. I don't use Vixen so I'm not that good at its usage. However, Vixen can be setup to do updates every X-millisec, right? If you use 100ms updates in a Vixen sequence and your H-bridge is switching at say every 50ms when a Vixen update has not been done, could this variable also cause your "random" state? Perhaps this is what David_AVD is alluding to, even though you are not using DMX, but a similar multiplexing is happening betweena Vixen update frequency and the H-bridge switching frequency.

    On another note:
    When you cut your larger strings, do you add any series resistance to limit the maximum current? How many LEDs, for each polarity, would be in your cut strings? What H-bridge do you use? A schematic would be nice to get a better picture in my mind, but not essential. I also am unclear about your power source to drive your LEDs. You say a 6V power supply is used? The 21V string is before its cut?

  3. #13
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    Default Re: Alternate polarity led strings

    Interesting LED strings, I haven't noticed that style of LED string wiring here in the US (but who knows). Maybe they don't worry about 110VAC safety as much as with higher voltages.

    What you haven'd described is how you do your timing on the Arduino. You presumably turn on the H bridge in one direction for some portion of a timing loop, in the other direction for some other proportion, and "off" for the rest of that cycle, right? So the question is: how long is the overall cycle? Are you timing precisely, or just running a loop as fast as it can?

    Most likely, that H bridge updating cycle is "beating" with the update frequency from Vixen.

    For an AC dimmer on 50 Hz, the dimming cycle is 10 mSec long (an AC half cycle), with power gated on for Value/255 of 10ms (ie: "on" for 0 .. 255 steps of about 39 microseconds, out of each 10 mSec dimming cycle).

    So Vixen running at 50 mSec increments will set a dimming level, which gets used for approximately 5 dimming cycles of 10mSec each. (For 60 Hz mains, there would be about 6 dimming cycles per Vixen update). Those don't need to be syncronized - sometimes there would be more or fewer dimming cycles per Vixen update cycle.

    I'm guessing that your Arduino dimming cycle (which doesn't need to syncronize with any mains) is closer to 50 ms and the beats would make it flicker visually. The answer is likely for you to speed up your dimming cycles sufficiently that the beats don't matter visually.

    You could post your Arduino code (the relevant parts) for more ideas.

  4. #14
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    Default Re: Alternate polarity led strings

    I think I have found the reason for very fast on/off sequence cycles not working. It seems to be a restriction in Vixen itself. I have configured a very simple preview and this shows exactly the same - random flashing at around 0.1second cycles and ok at around 0.2.
    Guess I will have to try and understand how to control these strings using an ac voltage feed, zero cross detectors, etc. At least I have a whole year to understand this method !!

    Thanks for all the responses.

    Mike

  5. #15
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    Default Re: Alternate polarity led strings

    Vixen has a default update interval of 50ms. That default value was chosen as a good balance for performance. This can be changed, but it's important to not that it's a best attempt value, it's not a hard and fast interval. on a typical machine that's not overloaded, you'll get +/- 3ms.

    What you're trying to do should be done in hardware. The actual control of the circuit switching should be done by an MCU running at a reliable constant rate governed by the zero cross timing. The lighting commands coming in from the software will be asynchronous to the actual lighting circuit control.

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