Basic Diode theory help

[b_karv]

Hi Everyone,
I am trying to understand the theory of diodes and have following question on basic diode theory, which is really confusing me.

If you had a DC circuit with 3 x standard diodes (0.7v drop) say current rating 20mA, with a 5v source what would be the current flow and power for this circuit?

How would I then calculate the resistance required to make up the rest of the 5v source and would what would be the final power for circuit?


Appreciate the help, I am just getting very confused on diodes and calculating power as most examples show diodes with resistors.


Thanks in advance.
Cheers,
Karv

 

[Diver300]

If all you do is connect a 5 V source to 3 diodes in series, there will be a lot of current, and even smoke.

If you want to limit the current to 20 mA with a resistor, then the voltage drop of 3 * 0.7 V is 2.1 V, so (5 - 2.1) V = 2.9 V remains across the resistor. That means you need a resistor of 2.1/0.02 = 145 ohms

The total power is 5 * 0.02 = 0.1 W, with 0.014 W of heat generated in each diode and 0.058 W of heat in the resistor.

 

[b_karv]

Thanks this clears it up then, so it almost like a short circuit?

 

[Diver300]

Thanks this clears it up then, so it almost like a short circuit?

Yes.

The voltage drop of the diodes reduces the voltage available, and the resistance of the diodes further reduces the current. However, you didn't say the resistance of the source or the type of diodes, so the short circuit current could be just about anything.

 

[b_karv]

Thanks again you so much for the response.

Yes I understand now, appreciate it alot as it has been tearing me up all night yesty.

Cheers,
Karv

 

[b_karv]

Problem solved mod's can close thread if like! Cheers