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Author Topic: WS2803 IREF resistor mandatory?  (Read 1051 times)
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Hilversum, Netherlands
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Hi,

I'm looking to control RGB LEDs with several WS2803s using a Teensy++ 2.0, however the LEDs already have SMD resistors attached to them. Do I still need to connect a resistor to IREF, or can I just connect it to ground? I've read somewhere (probably this forum) that the WS2803 doesn't do anything until you connect something to IREF. The datasheet says "The output current can be set by connect a resistor at IREF pin", which implies it's optional.

Thanks in advance.
« Last Edit: April 09, 2013, 03:13:28 pm by ErikMinekus » Logged

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Hello

If you just think logically: the Iref resistor is for setting the output current. That implies (to me) that, when the Iref is not connected, the output current is not set.

The datasheet states: Iout = 27.5 / Rext. If you connect it to the ground, Rext is effectively zero, meaning Iout is (in theory) infinite. In practice, this will mean a current of 30mA (the max) through your leds.
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Hilversum, Netherlands
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Wouldn't the SMD resistors then limit that to 20mA though? I unfortunately don't know their values (no magnifying glass smiley-razz), but I'm using the ARC EYE v2 (photos are half way down). Either way I don't mind using IREF even if it was just to be on the safe side, the LEDs are plenty bright either way.

On to the next problem however. Below is the breadboard I played with yesterday. I didn't have a ~1375Ω resistor, so I used 6x 220Ω (=1320Ω) in series. It worked great for a couple of minutes and then stopped working. Now I can't get that chip working at all anymore, so I guess I broke it since I tried another chip shortly which still worked. Luckily it was one of two spares, but I'm unsure what I did wrong. Should I have used a higher resistor value (my LEDs still work, so I doubt it)? Am I supposed to add a decoupling capacitor? They also mention using a 50Ω resistor on the outputs for long distances, but that doesn't apply here.

As you can see I'm very new to electronics, I'm trying to read as much as possible and look at other WS2803 schematics, but on the few clear schematics I find the only difference I see is some have a decoupling capacitor and others don't.


* Teensy++2.0_WS2803.png (80.13 KB, 956x424 - viewed 66 times.)
« Last Edit: April 10, 2013, 11:40:06 am by ErikMinekus » Logged

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It worked great for a couple of minutes and then stopped working.
Was the chip hot?
Do you have a real schematic, those Fritzing things are rubbish.

Yes a decoupling capacitor will be needed but lack of one would not cause the chip to blow only to work erratically.
http://www.thebox.myzen.co.uk/Tutorial/De-coupling.html
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The WS2803 (which is a TI TLC5940) is a constant current PWM device.  It is intended to use LEDs without current limiting resistors because that's what the chip does, it controls the current (NOT the voltage) of its outputs.

You must set IREF otherwise the outputs will become unstable.  You can try setting the max output current to higher than what your LEDs are drawing.

Wouldn't the SMD resistors then limit that to 20mA though?
Minor point.  Not all LEDs are designed to run at or are run at 20mA.  While a common number, it isn't always the case.
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Hilversum, Netherlands
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Was the chip hot?

I'm quite sure it wasn't, when I took it out several minutes after it stopped working it certainly didn't feel hot.

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Do you have a real schematic, those Fritzing things are rubbish.

You're right, I don't use Fritzing anymore either, I just figured a breadboard view would be easier. I've attached an Eagle schematic and did my best to lay out the ARC EYE. It looks like the values for green and blue are 91Ω, I'll try to get my hands on a magnifying glass to check the red value.

Thanks for the decoupling tutorial.

Minor point.  Not all LEDs are designed to run at or are run at 20mA.  While a common number, it isn't always the case.

True, however the page I linked to describes them as 20mA per LED per color:

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The board is designed to consume up to 20ma per LEDs per color, with two LEDs on each circuit board. So each button typically consumes 40ma if only one primary color is used.


* WS2803_schematic.png (12.63 KB, 901x499 - viewed 71 times.)
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It seems to have been a false alarm, I think my breadboard is just low quality because pins are regularly loose in their sockets, so when something's not working I never know whether it's just not making contact or something else is wrong. I had to push down the chip a little more for it to make proper contact. So I will just use a resistor for IREF and a capacitor and I should be good to go. Thanks again for the help.
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so when something's not working I never know whether it's just not making contact or something else is wrong
That is why I never use it or recommend anyone to use it.
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