crazy PIC current - Page 1
Forum section: electronics / beginners

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 25, 2018, 04:38:46 pm
content: I'm using a 16F1827 in a simple project to display temperature.I starting testing my work but cannot get the thing going because of the massive voltage drop across the PIC.So, to cut a long story short, I took the PIC and isolated it completely. I then connected the power supply set for 5V and 100mA: +ve to Vss and -ve to Vdd. No other pins are connected to anything.The voltage was showing as 0.86V with the full 100mA flowing. As I turned the current up the voltage started to rise, of course.I thought that maybe I'd fried the PIC somehow but it worked fine in another circuit (which only drays about 10mA in total!). I'm too scared to raise the current in case it blows.Why on earth is it drawing so much current when doing nothing?Thank you.

author: dmills
author_rank: Super Contributor
timestamp: May 25, 2018, 04:47:49 pm
content: Vss is generally the negative pin and Vdd the positive, think source (ss) and drain (dd) on a N channel mosfet.You have the power hooked up backwards to the PIC!Regards, Dan.

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 25, 2018, 05:01:39 pm
content: HOLY SH.T!!!I cannot believe I did that!Thank you for your quick reply and thank you also for not laughing … though I do deserve it, I realise that.I'll not delete the thread as a permanent lesson to me!

author: Howardlong
author_rank: Super Contributor
timestamp: May 25, 2018, 05:56:16 pm
content: There are those of us who have, and those of us who will.Thankfully, current limiting PSUs and ESD diodes frequently come to the rescue.FWIW, as a general practice I pretty much always wind down the current limit to 50 or 100mA when first hooking up a project, and frequently add a reasonably meaty reverse protection Schottky to save me from myself.For the old farts among us...Back in the old days, there wasn’t the same polarity conformity there is now about Vss and Vdd, when dies were fabbed uniformly either as PMOS or NMOS. Over time NMOS became more popular and then CMOS, and the NMOS “standard” for supply polarity stuck even though the underlying Vss & Vdd nomenclature is meaningless for CMOS.The Vcc and GND bipolar standard came from RTL, DTL and TTL which used only NPN transistors and that stuck very early on. The earliest I worked with in new designs was Motorola DTL, but much more recently I came across RTL for the first time when repairing some aviation DME navigation equipment from the 70s.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2018, 09:14:22 pm
content: It's good the PIC survived the abuse, but I wouldn't recommend using it in anything critical in case it's been damaged. Quite often an IC which has been abused will seem to work perfectly fine, but can have problems with reliability later.

author: PerranOak
author_rank: Frequent Contributor
timestamp: May 29, 2018, 08:19:43 pm
content: Howardlong: I'm glad I'm not the only one. I don't know why but Vdd makes me think of ground - damn!Hero999: Ah right, thanks. I'll use that one in my "experiments"!

author: JS
author_rank: Frequent Contributor
timestamp: May 30, 2018, 03:35:59 am
content: Quote from: PerranOak on May 29, 2018, 08:19:43 pmVdd is voltage to the drain of N-channel CMOS or other FETs who drains the electrons, Vss is the voltage to sources of N-channel CMOS or other FETs where electrons are sourced from. Electrons flow the wrong way when looking at the current arrow, that is, from negative where there are plenty of those electrons (who are negativly charged) to positive where they are missing.Don't think about p-channel FETs, they are evil.JS