power supply help for Neopixel led strips

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lapoza
 
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power supply help for Neopixel led strips

Post by lapoza »

Hello all,
For the past several years I have been making little glowing signs using an arduino and 12v Dumb led strips, to control the led strips I used the Allegro A6281 chip. I had custom PCB's made with the allegro on them using an arduino pro mini. To power these boards I had a 24VAC supply as the power source. I ran these through a 4 diode rectifier circuit, then into a 12v 7812 regulator with a 10uF 35v cap on the input, and a 4700uf 35v cap on the output. The output then was used to power the raw input of the arduino, as well as the Allegro and the led strips. I was able to run 27 leds successfully like this with no problem. This is actually more than i need, I only need 18 leds for my little signs. Also on the pcb's i had a pad exposed that i soldered the heat sink of the regulator to.

now on to my current problem. So I saw the neopixel strips and these are awesome, i can do more than one color which open the door to a lot of cool stuff. So what I thought I would do is just change my 12v regulator for a 7805 5v regulator. I ordered new boards, but forgot the heat pad for the regulator. So I made two of these for some friends and then on the third one a guy asked for 27 leds so it would be brighter. well when i went to full white with the colors it actually locked up and turned a pinkish color. if i shut it off and wait a minute it comes back no problem but will do the same thing if left on white. The circuit was exactly the same except no Allegro chip and a 7805 instead of a 7812. So I was waving my hand and felt a lot of heat over the regulator. Got out the heat gun (temperature reading) and measured the 7805 at 275 F at full white right before it locks out. Then I did my mathification and realized the 27 leds were pulling well over 1A. So I went back down to 18 leds, measured the current using a fluke meter at .78A at full white on 18 leds. But, I was still having some issues of overheating on full white even with just 18 neopixles. Switched to a 1.5A 5v regulator and still had this problem even though i am at less than 1A total current.
I did buy one of adafruits 5v 2A switching regulators and had no issues with it, also i can power this and the leds using the arduino FTDI usb cable and it won't lock up either. So I am wondering can i buy something like this and drop in where the regulator goes and be ok? or is it a component issue on my simple power supply itself? I am thinking at 12v the dumb leds wouldn't be drawing as much current, plus the 12v regulator was soldered to the pcb ground plane of the entire pcb for heat dissipation. I have tried the bolt on heat sinks for the 5v regulators but they didn't help, I'm worried that if i soldered the 7805 to the ground plane the heat could go into the arduino through the ground pin and effect it somehow. Just looking for advice on how to be able to use these neopixles without having to use a 5v wall wart or some other huge type of switching power supply.

Would these work as a drop in replacement? http://www.mouser.com/ProductDetail/Mur ... jcObOA8%3d

or do i need to redesign?

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adafruit_support_bill
 
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Joined: Sat Feb 07, 2009 10:11 am

Re: power supply help for Neopixel led strips

Post by adafruit_support_bill »

We have these that are drop-in replacements for the 7805: https://www.adafruit.com/product/1065
They are compact, cool-running and work quite nicely with the Neopixels. The product in the Mouser link looks like it should work also.

lapoza
 
Posts: 2
Joined: Sun Apr 13, 2014 8:14 am

Re: power supply help for Neopixel led strips

Post by lapoza »

Thanks for the quick reply. So according to the web page I should keep the 10uF on the input but I can get rid of the 4700? Thanks for the quick reply

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adafruit_support_bill
 
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Re: power supply help for Neopixel led strips

Post by adafruit_support_bill »

The 4700uF cap is not required for regulation. But it will help smooth out instantaneous current demands from rapidly switching LED patterns.

You could probably get away with a smaller one. And you don't need the 35v rating. Something in the 6-10v range should be fine.

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