Fixed Power Supply Issues

A project log for Drum Master

Electric drum set based on Teensy 3.1 and Audio Board

the-big-oneThe Big One 12/24/2015 at 08:064 Comments

So I was noticing that the board would brown out and reset occasionally; usually when writing to the SPI flash chip. I could bypass the problem by plugging into USB, so I didn't bother looking into it too much until now.

I was confused because the regulator I was using was rated for 250mA, but I was pulling much less than that. I seem to average 100mA when playing the drums, and 140mA when copying files from SD card to SPI Flash. Obviously the extra current used when copying was the problem, but how to best fix it?

It turns out that simple physics is the answer. My wall wart is 12v; I was stepping this down to 3.3v. This is 9v, times 150mA worst case scenario, which is about 1.3W dissipated in a SOT-23 package. Yeah, that's not going to happen for long. (Honestly, what I am surprised about is that the 100mA constant current draw allowed things to run at all; that is still just under 1 watt, which is more than I should be able to get from the SOT-23. I guess having a nice large ground plane as a heat sink without using the thermal relief pattern on the pins helps keep things cool.

The correct solution would be to get a 5v wall wart, but I am lazy and cheap, so instead I just threw another larger regulator with a heat sink onto the power lines before they go into the board. This steps the 12v down to 5v, which is further stepped down to 3.3v with the SOT-23. I can now load samples all day long without any issues.


The Big One wrote 12/27/2015 at 07:11 point

FYI I just picked up a 5v regulator from a friend and got rid of the hack. The regulators are now running completely cool to the touch with no brownouts. Yay

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Michele Perla wrote 12/27/2015 at 12:44 point

Nice stuff! Waiting for revision 2 with built-in regulators ;)

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Michele Perla wrote 12/24/2015 at 11:48 point

Nice hack indeed!

Next time you could get a beefier regulator, the price tag is not that different. Of course, never forget the ground plane as heatsink!

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The Big One wrote 12/24/2015 at 17:24 point

Heh, yes it is indeed a hack!  Long term I will get a real wall wart (anything from 5 - 7v would do just fine).  Even longer term, if I make a rev 2.1 PCB, yeah I will have to put a beefier regulator on there.  It was just laziness on my part for not doing that from the start... I had a footprint for the MCP1702 SOT-23 package that I had used in previous designs and knew to be good, so I just used it.  I knew that the total current consumption was well under 250mA (the limit of the 1702), but I didn't clue in that I was using a 12v wall wart.  Oh well, it works and I learned something (or rather was forced to remember something that I already knew).

Merry Christmas!

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