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Topic: Elimination of PWM noise from TLC5947 LED (Read 1 time) previous topic - next topic

mitchjs


hey all, i need some help

I got a project, that i use a TLC5947 to drive some mostfets that drive 12v LED Strips
the thing works perfectly, well...execpt... that its bing used in a limo
(it controls cabin lighting)

but its putting noise on the AUDIO system.... when i look at the 12v supply to the LEDs, i can see a square wave on the power feed from the battery
that has some relation to the PWM...putting a 470uf cap across the input, does cut it down, but not enough

im think its a car, and there is other noise on the power, ie alternator... so... its the audio system not doing its job
but if i can kill audio at the source here... id be way ahead

this is my output stage
3 channels worth
thanks!
mitch

Boffin1

What current does each chan supply max?
With my mobile phone I can call people and talk to them -  how smart can you get ?

fungus


putting a 470uf cap across the input, does cut it down, but not enough


"470uf" makes me think you used an electrolytic capacitor.

If so, add a ceramic capacitor in parallel to the electrolytic (two capacitors across the same spot). Electrolytics don't have a very fast response time so they're not very good at decoupling.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

mitchjs

each chain can supply up to 3 amps...

yes, 470uf is electrolytic cap

yes i would add a ceramic if it turned out that this was the solution...
prob adding a ceramic now wouldnt change the noise
ill try tonight

im putting the cap across the 12v input to the rgb strips, and the common anode
(so pins 1 and 2) on the output connector JLED0

Grumpy_Mike

Quote
yes i would add a ceramic if it turned out that this was the solution.

Even if it is not the solution you should fit it and keep it fitted. You might even have to use an inductor ahead of those capacitors.

Probably the noise is coming from the PWM in the LEDs and radiating off the connecting wires. Start by disconnecting all the LEDs and adding them one at a time. Until you can just here the noise. Then what ever you change will be more noticeable. If something improves things but does not affect a complete cure, keep it in and look for the next small improvement. You can also try dropping the LED current, adding screening, getting a proper ground scheme and so on.

It could be that you can't fix this at all, it would not surprise me if that were the case.

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