Hi. This circuit works perfectly when I only plug in the USB. When I put a 3A at 12v supply in as well as the USB I smoked two arduino and one lcd.
It's running a k type thermocouple an lcd icII and a potentiometer.
I thought at first it was a simple short that's why I tried again. I configured pins 2@3 to Output high and low for 5v and ground.
Can anyone help? I have also just added the circuit sketch and the code is under my reply to Mark T.
![circuit[2].jpg](https://europe1.discourse-cdn.com/arduino/original/3X/6/b/6b9e587a83b328c3f825441a24950f3b78996678.jpg)
Where did you apply the 12V? Why do you thing Arduino pins can be used to provide 5V
and ground to power other devices (they can't)?
Have you checked the polarity of your supply? There are two ways to connect and both are used.
Weedpharma
Hi,
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png or pdf?
Tom.... 
Mark T - I got the idea that arduino 5V pins can power other devices by the fact all the tutorials told me to plug then in. Since the power supply is 5V I can't power off that as I need 5.. So I think I am missing the point of your advice. Are you saying I should be grounding to the power supply instead of direct from the board?
Here is the code configuring the pins:
#include <OneWire.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
#include "max6675.h"
/-----( Declare Constants )-----/
static float gTempLast;
LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE); // Set the LCD I2C address
byte newChar1[8] = {
B00100,
B01110,
B10101,
B00100,
B00100,
B00100,
B00100,
B00100
};
byte newChar2[8] = {
B00100,
B00100,
B00100,
B00100,
B00100,
B10101,
B01110,
B00100
};
int thermoDO = 6;
int thermoCS = 5;
int thermoCLK = 4;
MAX6675 thermocouple(thermoCLK, thermoCS, thermoDO);
// use Arduino pins - but at the moment only use VCC and Ground Pins
int vccPin = 3;
int gndPin = 2;
//Potentiometer Stuff
const int analogInPin = A0; // Analog input pin that the potentiometer is attached to
const int analogOutPin = 9; // Analog output pin that the LED is attached to
int potValue = 0; // value read from the pot
int outputValue = 0; // value output to the PWM (analog out)
void setup() {
Serial.begin(9600);
lcd.begin(20,4); // initialize the lcd for 20 chars 4 lines and turn on backlight
// use Arduino pins
pinMode(vccPin, OUTPUT); digitalWrite(vccPin, HIGH);
pinMode(gndPin, OUTPUT); digitalWrite(gndPin, LOW);
//LCD Create the Chars
lcd.createChar(1, newChar1);
lcd.createChar(2, newChar2);
// ------- Quick 3 blinks of backlight -------------
for(int i = 0; i< 3; i++)
{
lcd.backlight();
delay(250);
lcd.noBacklight();
delay(250);
}
lcd.backlight(); // finish with backlight on */
gTempLast = 999; //impossible start value
//-------- Write characters on the display ----------------
// NOTE: Cursor Position: CHAR, LINE) start at 0
lcd.setCursor(1,0); //Start at character 1 on line 0
delay(2000);
lcd.clear();
lcd.setCursor(0,0);
// wait for MAX chip to stabilize
delay(500);
}
void loop() {
lcd.setCursor(0,0); //home
float currentTemp0 = thermocouple.readCelsius();//DEBUG
float currentTemp1 = thermocouple.readFahrenheit();//DEBUG
float fTempDiff = currentTemp0-gTempLast; //between this read and the last
if (fTempDiff != 0){ // gone up or gone down since last time
//Serial.println (currentTemp0);
if (currentTemp0 > gTempLast)
lcd.write(1); //up arrow
else
lcd.write(2); //down arrow
lcd.print(" "); //clear previous char
lcd.print(fTempDiff);
//Serial.print(fTempDiff);DEBUG
}
else {
lcd.setCursor(0,0);
lcd.print("<> "); //overwrite the first line
// Serial.println ("<>");DEBUG
}
// read the analog in value:
potValue = analogRead(analogInPin);
// map it to the range of the analog out:
outputValue = map(potValue, 0, 1023, 3600, 1);
// change the analog out value:
//analogWrite(analogOutPin, outputValue); //Fade an LED if required OR control motor
lcd.setCursor(0,1); //goto second line
lcd.print((char)223);
lcd.print("C: ");
lcd.print(currentTemp0);
lcd.print(" ");
lcd.print((char)223);
lcd.print("F: ");
lcd.print(currentTemp1);
lcd.setCursor(0,3);
lcd.print(potValue);
lcd.print(" ");//clear previous char
gTempLast = currentTemp0;
delay(10000);
}
Hi tomGeorge.. attached to the first post I made a crude sketch... I have a nasty feeling that I should be grounding to the main supply and using a breadboard instead of just these scewshields. The problem is, I know software but when it comes to electronics with any load on them I tend to make a lot of smoke.. All at sea so to speak. I hope I can learn.
Your pot is shorting out the supply.
The wiper , middle pin should be connected to A0
Boardburner2:
Your pot is shorting out the supply.The wiper , middle pin should be connected to A0
many apologies, I drew it wrong. The middle pin is connected to A0 so that's not the problem. I will correct the drawing in an hour or so.
:-X Pin 3 is 5V software configured.
I hope that is not the supply for the ic.
Post a link for the supply if you can.
How did you connect it ? Centre pin polaity ?
Have you measured the PSU output ?
Boardburner2:
:-X Pin 3 is 5V software configured.I hope that is not the supply for the ic.
I don't know. All I know is the code is a downloaded example. Pin 3 is on the analog pins. IT is a power output to the thermocouple. sorry - I am not that conversant with Arduino YET.
Boardburner2:
Post a link for the supply if you can.http://www.maplin.co.uk/p/maplin-36w-switched-mode-acdc-fixed-voltage-12v-power-supply-n22ju
Boardburner2:
How did you connect it ? Centre pin polaity ?
The centre pin is positive and the outer pin is negative. It ran for a while OK, its only when I plug BOTH USB and PSU in the smoke escaped.
Boardburner2:
Have you measured the PSU output ?
Put a multimeter across it. Gives 12 Volts. Seems like a reputable make.
weedpharma:
Have you checked the polarity of your supply? There are two ways to connect and both are used.Weedpharma
centre positive
MikeDutton:
Put a multimeter across it. Gives 12 Volts. Seems like a reputable make.
I have used that without bother.
MikeDutton:
The centre pin is positive and the outer pin is negative. It ran for a while OK, its only when I plug BOTH USB and PSU in the smoke escaped.
Which order usb first or psu first
also is your pc a laptop or mains unit.
Hi, what are you using as your 12V supply.
Tom.....
He is using this which I have used myself and can vouch for.
http://www.maplin.co.uk/p/maplin-36w-switched-mode-acdc-fixed-voltage-12v-power-supply-n22ju
One problem I sometimes encounter is pc rebooting when plugged into powered board.
Not had smoke from this but when dual powering I tend to use batteries to avoid the problem.
Boardburner2:
I have used that without bother.Which order usb first or psu first
also is your pc a laptop or mains unit.
Hi - Power supply is plugged in and it works. Then plugged in the USB and it smoked. Two boards in a row. The PC is a mains unit, on a multihub.
I take it that the circuit looks basically ok?
The only other thing I can see is maybe if a pin shorted but it was on a polystyrene foam pad. BUT here is the thing. The ICII blew too OK? Why I ask. Then I looked. The VCC and Ground lead supplying the ICII LCD screen are on spade clips that are not insulated (but not touching as I can see). If they touched and I did not notice... could that take out the board AND the LCD screen?
MikeDutton:
Mark T - I got the idea that arduino 5V pins can power other devices by the fact all the tutorials told me to plug then in. Since the power supply is 5V I can't power off that as I need 5.. So I think I am missing the point of your advice. Are you saying I should be grounding to the power supply instead of direct from the board?
An Arduino pin (you mentioned 2 & 3) cannot source/sink enough current for many devices, and is not
stiff enough anyway to be a stable supply (about 30 ohms output resistance), and
is not rated to handle the current surges needed to charge up the decoupling
capacitors on the load. You switch loads with FETs or transistors which can handle
a lot more current and are much lower impedance.
MikeDutton:
could that take out the board AND the LCD screen?
Its possible.
Any short could potentially burn your house down.
In general they are a bad thing.
MikeDutton:
The PC is a mains unit, on a multihub.
Is that self powered or does it have its own supply , link ?
Where did the smoke escape from , on the board.
Typically its near the power socket but not always.
Edit
And what value potentiometer are you using.
MikeDutton:
grounding to the main supply and using a breadboard instead of just these scewshields. The problem is, I
There is nothing wrong with using screwshields.
Provided you connect to the right pins.
Reread marks posts.
Your instructable or tutorial is wrong.
