pin currents

[aamirsherkhan]

hi i m new in microcontroller i wnt to know that what will the current at the pin of AT89C51 microcotroller when it is cofigured as output and is high.i m using 5V supply.if i have to connect a relay 12v with the pin through transitor,how can i calculate the resistance used in series with the NPN transistor

 

[dknguyen]

The current at the pin will be whatever the load tries to draw for the voltage the pin outputs...up to a limit, usually somewhere between 5-25mA depending on the MCU. It will say in the datasheet what the maximum is. Beyond this current, the output voltage will rapidly drop.

The relay will have a voltage rating based on the current it needs and the coil resistance. THe coil resistance will make it so you do not need a resistor at all if you run the coil at its rated voltage (since the coil already has its own resistance). So if you put 12V through a 12V relay coil, no resistor is needed. If you put 12V through a 5V relay coil, then you need a resistor to limit the current and voltage across the the relay coil.

 

[aamirsherkhan]

i m using At89C51 wtih 5V supply i m talking about the resistance that lies between the MCU pin and the base of transistor.The 12v relay needs 50mA of current.i m Using C1815 NPN transistor.how can i calculate the base resistance

 

[ericgibbs]

hi i m new in microcontroller i wnt to know that what will the current at the pin of AT89C51 microcotroller when it is cofigured as output and is high.i m using 5V supply.if i have to connect a relay 12v with the pin through transitor,how can i calculate the resistance used in series with the NPN transistor

Hi,
This circuit will cover most low power relay drivers.

What current/resistance is the relay.?


If the MCU pin is at +5V and the transistor has a Vbe of +0.7V, this gives 4.3V across R1.

If the relay draws say, 100mA and the gain of the transistor is 50,then ideally the base current would be 100/50= 2mA

but ensure saturation of the transistor I would provide a base current of about 4 to 8mA.

So, 4.3V/0.004 =1075R npv = 1K0

Does this help.?

 

[dknguyen]

I_desired = NPN_Beta * (Vpin/Rbase) [where Vpin/Rbase is the current sunk by the uC pin].

It's not terribly critical though. Just make sure the resistor isn't so small that the uC pin doesn't get burned out (using I = V/R and the output voltage and max current of the pin). As long as it's just a little bit more current than the relay needs, the relay resistance will regulate the rest of the current for you.

 

[aamirsherkhan]

this is thedata sheet of AT89C51 the output current from the pin when it is high?

 

[aamirsherkhan]

will the base current requred by the transistor be provided by the pin .upto howmuch it can provide .see the data sheet

 

[dknguyen]

It seem to say 15mA max per pin under "electrical characteristics" so, that means that if your relay current > 15mA x NPN_Beta, then you are fine.

 

[aamirsherkhan]

It seem to say 15mA max per pin under "electrical characteristics" so, that means that if your relay current > 15mA x NPN_Beta, then you are fine.

thanks 4 helping

 

[aamirsherkhan]

Hi,
This circuit will cover most low power relay drivers.

What current/resistance is the relay.?


If the MCU pin is at +5V and the transistor has a Vbe of +0.7V, this gives 4.3V across R1.

If the relay draws say, 100mA and the gain of the transistor is 50,then ideally the base current would be 100/50= 2mA

but ensure saturation of the transistor I would provide a base current of about 4 to 8mA.

So, 4.3V/0.004 =1075R npv = 1K0

Does this help.?

thanks 4 helping