Question

i am not sure why the current is really low. i have tried different values and it still is low. the forward current currents from the leds are 20milliamps and 50milliamps for the leds that i am working with. my design has a max of five volts supply. please respond with an explicit answer

PMOS # MZ JJNMOS Hi PULSE(O 5 000 5 10) tran 30 redout AN

i need the leds to be turned on and off at different times. when one is on the other one is off

Answer 1

Why the LED's current is low in the order of 20mA and 50mA is being asked. LEDs have a upper permissible limit for forward current. If the current in an LED is above that limit, the LED will get damaged and it will stop functioning. Generally, speaking normal small LEDs like 5mm LEDs have a upper permissible limit for forward current at around 30mA. But, it depends on the LED that is being used. Its permissible upper limit for forward current should be mentioned by its manufacturer.

That being said, let us analyse the circuit. There is no major error in the circuit provided the forward currents in LEDs are well below the permissible upper limit. If the maximum possible forward current is above the permissible upper limit of the diode, then we need to change the increase resistor values accordingly to reduce the forward current. But, as we can see the gate and source of NMOS are shorted in the circuit. However, if such a connection is made then the NMOS will always be in Cut off as Vgs will be zero. Then, we will near zero amps current flowing through the LED. So, one shouldn't short gate and source in the NMOS.

Now, Let us calculate the maximum possible forward currents. Assume, the supply is at 5V. One must note that when input pulse is 5 volts NMOS goes into saturation and PMOS is in cut off region of operation. Similarly, when input pulse becomes zero NMOS is in cutoff and PMOS is in saturation.

Say, the input pulse is zero. Then PMOS is in saturation. Therefore, the drain current of PMOS equals to its source current. It can be calculated writing KVL loop equation as,

-5+ (Vds)saturation + (V)cutin off diode + 100*(Id)sat = 0.

Or Idsat can also be found using formula for saturation drain current of MOSFET. ie;

(Id)sat = $\mu n \times Cox \times (w/l) \times (Vsg - Vt)^{2} \times (1 + \lambda Vds)$

Similarly, for NMOS can be found also current can be found. And, necessary changes can be made to the circuit if the current value is above permissible limit of the LED.