5V pin showing 3.3v?

Created Mon, 24 Feb 2014 23:53:31 +0000 by mitchjp

mitchjp

Mon, 24 Feb 2014 23:53:31 +0000

Had to add an RS422 transceiver onto a custom shield and I'm debugging away when I realise the 5v pin on the my max32 is showing at 3.3v.

Bit weird. Unplugged everything I have in it, tried again.

Normally I run from a 12v battery.. when I tried with USB it showed 5v.

I thought the 5v reg powered the 3.3v reg?

Am i missing something?

EDIT: This is ALL of the 5v pins. J7 and J10 EDIT2: The jumper for the regulator is in the REG position.

majenko

Tue, 25 Feb 2014 00:06:42 +0000

It sounds like you might have one of the ones with the wrong regulator.

[url]http://chipkit.net/forum/viewtopic.php?f=17&t=1462[/url]

mitchjp

Tue, 25 Feb 2014 00:08:26 +0000

Thanks for the reply.

What a pain! Oh well, I guess I'll contact them and get it sorted. I only bought this recently. Must of been in stock for a while.

majenko

Tue, 25 Feb 2014 00:11:50 +0000

How's your soldering? A replacement regulator costs about 20p. It's also a regulator I keep in stock in my parts trays, as it's a very common one and one I use in many of my own products.

mitchjp

Tue, 25 Feb 2014 00:18:27 +0000

Hmm well I can solder it on. Just need to buy the part. Or maybe my work will have one in spare somewhere.. I could just put any old 5v reg before and BYP the regulator but I have already created my custom shield and theres not a lot of room on there.

I guess it is probably more of a pain for Chipkit to have to send me a new one, rather than me not be a lazy sod and just do it myself haha.

majenko

Tue, 25 Feb 2014 00:23:10 +0000

Heh. I'd be surprised if you didn't have one at work, or one in some other scrap device you could scavenge one from. The 1117's are probably the second most fixed voltage regulators around, after the LM7805. Maybe even the most common these days actually. They're made by everyone from Diodes INC, through ON Semiconductors, right up to ST Micro and Texas Instruments. Just doing a search for "1117" on Farnell UK, and filtering for 5V output and SOT-223 yields 32 products (not all in stock though)...

mitchjp

Tue, 25 Feb 2014 01:25:00 +0000

Yep, just ordered a couple. (Of course we have plenty of them in stock here... but they are the 3.3v ones.... ahh)

Should be here by tomorrow and ill do it then and mention here that it's ok.

Thanks again!

mitchjp

Wed, 26 Feb 2014 22:58:20 +0000

Adding the new one on. Seems to work OK. Thanks again.

Also,

How hot should the 3v3 reg get? When I am putting 12v into the barrel it gets really hot. Too hot to touch. Is this normal?

majenko

Wed, 26 Feb 2014 23:12:26 +0000

Is this the 3.3V regulator you're removing, or the existing 3.3V regulator that is meant to be 3.3V?

If the former, yes, you can expect that to get hot when run off 12V. Linear regulators waste their excess voltage as heat, and 12->3.3 is quite a large drop for a linear regulator. Even 12->5 is quite large, depending on current draw (the higher the current the more heat you'll get).

mitchjp

Wed, 26 Feb 2014 23:31:21 +0000

Sorry, I meant the 3.3 that is supposed to be on there.

It would drop 12 -> 5 -> 3.3, right?

Total current cons is about 160 mA.

majenko

Wed, 26 Feb 2014 23:35:17 +0000

It should be dropping 5V -> 3.3V, yes - assuming the 5V is actually 5V now ;)

5 - 3.3 = 1.7

1.7 * 0.16 = 272mW

That shouldn't be getting TOO hot, but you would expect some heat there...

mitchjp

Wed, 26 Feb 2014 23:58:11 +0000

Ok so the 5v is also getting very hot. I am guessing it's ok. It's rated up to 120 degrees C so shouldn't be a problem I would guess...

mitchjp

Wed, 26 Feb 2014 23:59:08 +0000

Sorry just read that now. Yep the 3.3 is getting warm. But not too crazy. It's the 5v which i must of touched which was hot to the touch.

Thank you a lot majenko you always have the answers for my questions!!!

majenko

Thu, 27 Feb 2014 00:09:41 +0000

Yeah, off 12V you can expect the 5V to get rather warm. 12-5 = 7, 7 * 0.16 = 1.12W.