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RPi + OpenELEC Wire/cable thickness

Discussion in 'Hardware Support' started by Biggles, 28 March 2017.

  1. Nikhil

    Nikhil Member

    Messages:
    30
    Hardware:
    RPi3, +Arduino, +PhilipsHue
    Oh, not both. Still reddening towards the center.

    Thicker wires maybe?
     
  2. Biggles

    Biggles Member

    Messages:
    59
    Hardware:
    RPi1/Zero
    My current plan, following on from Penfold42's advice, is 12awg speaker wire to cover the 2m from PSU to start of strip to cover initial voltage drop, then drop down to something 1.5mm or similar to attach to the start, end and corners as this will be more manageable both for soldering to the strip and routing around the TV. I'm only using those thin wires to connect the data lines and will probably worth them as much as possible too.
     
  3. BioHaZard1

    BioHaZard1 Member Developer

    Messages:
    55
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino
    I just use 22AWG for the PSU to LED Strip and for the corners. I adjusted my PSU to counter Voltage Drop.
     
  4. Biggles

    Biggles Member

    Messages:
    59
    Hardware:
    RPi1/Zero
    Interesting, can you elaborate?
     
  5. BioHaZard1

    BioHaZard1 Member Developer

    Messages:
    55
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino
    Sure, there is a little screw (trimpot) near the output terminals. You adjust this to counter Voltage Drop. Obviously, you'd need to adjust it to your wire length depending on how long/short it is. Just make sure you have a decent PSU, I got a cheap one at first but the voltage readings were erratic at times. I got a decent one from Amazon for £13, voltages are perfect, nice and steady too.
     
  6. Biggles

    Biggles Member

    Messages:
    59
    Hardware:
    RPi1/Zero
  7. BioHaZard1

    BioHaZard1 Member Developer

    Messages:
    55
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino
    Yeah that should be fine. The only way to know for sure though is to check with a volt meter. Before you adjust the voltage check with the volt meter probes on the LED strip.

    In case you are unsure, the trimpot is the little orange one near the output terminals on your PSU.
     
  8. penfold42

    penfold42 Active Member

    Messages:
    746
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino, +nodeMCU/ESP8266
    This could be bad tho - when all the lights are off :
    - the current drops to nearly zero
    - The voltage drop over the power wires also drops to near zero
    - your increased voltage could now be too high
     
  9. Biggles

    Biggles Member

    Messages:
    59
    Hardware:
    RPi1/Zero
    Good point, it'd be up and down like a yo-yo through an action film. Running that calculator with all other settings the same gave 9.5% drop at 15A, down to 0.5% at 1A.
     
  10. penfold42

    penfold42 Active Member

    Messages:
    746
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino, +nodeMCU/ESP8266
    Which means a potential unloaded voltage of about 5.5v which is right on the maximum for the apa102 data sheet I looked at.

    For a ws2812 this is bad as it's max VCC is 5.3 v
     
  11. BioHaZard1

    BioHaZard1 Member Developer

    Messages:
    55
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino
    Correct me if I'm wrong, I'm not an expert in this field.

    But isn't one of the main purposes of trimpot to adjust the output voltage to counter the number of volts dropped over the length/gauge of the wire. If the lights are on or off the wire supplying the power is still going to be the same length/gauge?

    So even in the off state, the voltage wouldn't go higher?

    At least with my lights, this is the case, both on and off I get 5.01v.
     
  12. penfold42

    penfold42 Active Member

    Messages:
    746
    Hardware:
    RPi1/Zero, RPi2, RPi3, 32/64bit, +Arduino, +nodeMCU/ESP8266
    Will do - You're wrong! *laughs*

    The voltage drop is given by ohms law.

    V = I * R
    I is the current flowing
    R is the resistance of the wire

    The resistance of the wire is mostly fixed and determined by the wire material and thickness.

    The current will vary based on how bright the LEDs are. 1 led at full white pulls about 60 mA but close to 0 mA when off.

    On a good quality well regulated voltage regulator, the trimpot sets the voltage at the regulator - it has no idea what the voltage is at the other end of the wires.

    If the load pulled a constant current, adjusting it would work for some things ( but not all)