Hello!
I'm trying to learn the behaviour of a diode. In particular, I would like to know if it is able to set the current on its branch.
As far as I understood, once the diode is on, its voltage drop \[ V_D \] should be constant, regardless of the rest of the circuit: so, the diode should "decide" to "absorb" a specific amount of current, such that the voltage drop on itself keeps being a specific value. This value should also be equal to the threshold voltage of the diode: so, \[ V_D = V_{\mathrm{th}} \]
I tried to simulate the simple attached circuit, with a DC voltage generator, a resistor and the 1N4148 diode with SPICE:
When R1 = 1k, I obtained a voltage drop across the diode
\[ V_D = 677.2 \ \mathrm{mV} \]
with a 4.323 mA current. From the datasheet of the diode, I can not clearly understand what should be the threshold voltage of this diode. However, if I try to raise the resistor R1 value, I obtain each time different voltage drops across the diode: they become smaller.
With R1 = 10k,
\[ V_D = 565.8 \ \mathrm{mV} \]
With R1 = 100k,
\[ V_D = 456.7 \ \mathrm{mV} \]
and of course the current is smaller each time.
Why does this happen? Shouldn't the diode try to absorb a current such that its V_D keeps being constant (with minor variations due to the non-ideal components)?
Who sets the current in this circuit?
Instead, if I keep R1 constant and make a DC sweep analysis for V1 from (for example) 0 to 20 V, once the diode is on, the current and the voltage drop across the diode only slightly grow, as V1 raises, showing the behaviour I was expecting. Why varying V1 it's different than varying R1?
Thanks for having read,
Jimmy
I'm trying to learn the behaviour of a diode. In particular, I would like to know if it is able to set the current on its branch.
As far as I understood, once the diode is on, its voltage drop \[ V_D \] should be constant, regardless of the rest of the circuit: so, the diode should "decide" to "absorb" a specific amount of current, such that the voltage drop on itself keeps being a specific value. This value should also be equal to the threshold voltage of the diode: so, \[ V_D = V_{\mathrm{th}} \]
I tried to simulate the simple attached circuit, with a DC voltage generator, a resistor and the 1N4148 diode with SPICE:
When R1 = 1k, I obtained a voltage drop across the diode
\[ V_D = 677.2 \ \mathrm{mV} \]
with a 4.323 mA current. From the datasheet of the diode, I can not clearly understand what should be the threshold voltage of this diode. However, if I try to raise the resistor R1 value, I obtain each time different voltage drops across the diode: they become smaller.
With R1 = 10k,
\[ V_D = 565.8 \ \mathrm{mV} \]
With R1 = 100k,
\[ V_D = 456.7 \ \mathrm{mV} \]
and of course the current is smaller each time.
Why does this happen? Shouldn't the diode try to absorb a current such that its V_D keeps being constant (with minor variations due to the non-ideal components)?
Who sets the current in this circuit?
Instead, if I keep R1 constant and make a DC sweep analysis for V1 from (for example) 0 to 20 V, once the diode is on, the current and the voltage drop across the diode only slightly grow, as V1 raises, showing the behaviour I was expecting. Why varying V1 it's different than varying R1?
Thanks for having read,
Jimmy