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Voltage drop on MCU output pin

Former Member

Hi.

 

I am experiencing a voltage drop on my MCU when I connect it to a solid state relay. With the relay removed I measure 5V on the op pin as expected but with the relay connected I am only getting 1.6V. It is a  PVN021A which has about 5mA current draw. Anyone familier with these devices know where th voltage drop is coming from?

 

Thanks,

 

Mark.

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  • 0

    Input is an LED. You need a series resistor to limit the current.

     

    Currently it's only the FET in the MCU that's limiting the current (to something like 150mA, so don't leave it on for too long). You might have damaged either or both already.

     

    Try a 620 ohm resistor.

  • 0 in reply to jc2048

    Ah Silly me. I put in some 500 ohm resistors and the op pins are now back to 5V. However I had a problem which I thought that may solve. In my code below which turn on or off a particular relay whether the characters 'a','b','c','d','e' or 'f' are sent via the serial port. It works for the most part except for when I send 'd' it should turn off  'a','b' and 'c' but it doesn't. The value on the pins is the same as if nothing has been sent. Is there something silly I am overlooking here?

     

    Mark.

     

    #define F_CPU 16000000UL   // AVR clock frequency in Hz, used by util/delay.h

    #include <avr/io.h>

    #include <util/delay.h>

     

     

    #define BAUDRATE 9600

    #define BAUD_PRESCALLER (((F_CPU / (BAUDRATE * 16UL))) - 1)

     

     

    int therm_25 = 12; // 25 degrees

    int therm_37 = 10; // 37 degrees

    int therm_45 = 13; // 45 degrees

    int power = 11;

    int incomingByte = 0;   // for incoming serial data

     

     

    #define HighTemp  PORTD6  //R1 - 1K

    #define MedTemp  PORTD7  //R2 - 1k4

    #define LowTemp  PORTB0 //R3 - 1k78

    #define Power  PORTD5

     

     

    int main(void)

    {

      UBRR0H = (uint8_t)(BAUD_PRESCALLER>>8); //Set the Uart baud rate

      UBRR0L = (uint8_t)(BAUD_PRESCALLER);

      UCSR0B = (1<<RXEN0);//|(1<<TXEN0); //Enable the receiver and transmitter

      // 1<<TXEN0 interefres with serial port for MAC address read. needs to be disabled

     

      UCSR0C = (3<<UCSZ00); //Set 2 data bit length to 8-bit. (1 << USBS0) for 2 stop bits

     

     

      DDRD = 0b11100000;

      DDRB = 0b00000001;

     

     

        while(1)

        {

            while (! (UCSR0A & (1 << RXC0)) );

      incomingByte = UDR0;

           

      if(incomingByte == 'a') {

      PORTB |= (1<<LowTemp);

      PORTD &= ~(1<<MedTemp);  

      PORTD &= ~(1<<HighTemp);

     

     

      }

           

      else if(incomingByte == 'b'){

      PORTD |= (1<<MedTemp);

      PORTB &= ~(1<<LowTemp);

      PORTD &= ~(1<<HighTemp);

      }

     

     

      else if(incomingByte == 'c'){

      PORTD |= (1<<HighTemp);

      PORTB &= ~(1<<LowTemp);

      PORTD &= ~(1<<MedTemp);

      }

     

     

      else if(incomingByte == 'd'){

      PORTD &= ~(1<<LowTemp);

      PORTD &= ~(1<<MedTemp);

      PORTB &= ~(1<<HighTemp);

      }

     

      else if(incomingByte == 'e'){

      PORTD |= (1<<Power);

      }

     

      else if(incomingByte == 'f'){

      PORTD &= ~(1<<Power);

      }

           

        }

    }

  • 0 in reply to Former Member

    You messed up port letters, LowTemp is on PORTB and HighTemp is on PORTD, MedTemp you have correctly on PORTD. So it should look like this:

     

    else if(incomingByte == 'd'){

      PORTB &= ~(1<<LowTemp);

      PORTD &= ~(1<<MedTemp);

      PORTD &= ~(1<<HighTemp);

      }

  • 0 in reply to kulky64

    Indeed it is. I'v been staring at this screen for too long.

     

    Thanks guys.

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