Would this 78SR-5/2-C work as a drop in replacement?

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Sunnyback
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Would this 78SR-5/2-C work as a drop in replacement?

Post by Sunnyback » Fri May 11, 2018 2:06 am

Hello all, :cool:

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 this work as a drop in replacement? http://www.kynix.com/Parts/3326975/78SR-5%2F2-C.html
or do i need to redesign? :cry:

dyarker
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Re: Would this 78SR-5/2-C work as a drop in replacement?

Post by dyarker » Fri May 11, 2018 8:55 am

Replacing one linear regulator with another linear regulator having higher max current rating solves part of the problem. But, heat (power) generated is the voltage drop across the regulator (not the output voltage) times the current. Increasing the current (even if still under max) requires a bigger heatsink. By lowering the output voltage (12 to 5) while (probably) keeping the supply the same, you've increased the voltage across the regulator. Doubly (V and I) increasing the heat from the regulator.

From what I see about 78SR-5/2-C at Digikey, it is a switch mode regulator, so it will run a lot cooler than a similar rated linear. And spec sheet says it is a linear replacement. So, yes (AFAICT).

Cheers,
Dale Y

Sunnyback
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Re: Would this 78SR-5/2-C work as a drop in replacement?

Post by Sunnyback » Sun May 13, 2018 7:40 pm

Yeah, even the datasheet says so, I see it now, and @dyarker what do you think of adding a a 5.6V or similar zener diode at the beginning of my neopixel strip, now that those neopixels are really sensitive to input surges and their long distributed capacitance makes it really easy to over-shoot and destroy the first pixel...hmmm, i guess a zener might do me good. Thanks for you reponse mate.

dyarker
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Location: (somewhere), Afghanistan; from Rochester, NY
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Re: Would this 78SR-5/2-C work as a drop in replacement?

Post by dyarker » Mon May 14, 2018 9:20 am

You mean across the regulated 5V (to ground) before the display? Yes, but zener knee is not very sharpe. 6.xV or 7.xV would probably be better. Check the curves on the data sheets. You want 5.2V to be definately in the cutoff zone of the zener. (higher would defeat the purpose of using it though!)

Cheers,
Dale Y

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