 # does this diode arrangement contradict ohms law?

theres something i dont understand about electrcity maybe someone kind with knowledge can help me

voltage and current go according v=i*r

so if i put two resistances in line in parallel with a single resistance with 1 ohm and 1 volt throught the two in line will go half ampere and through the single one one ampere

but if i do the same with diodes i verified all current goes through the single diode and nothing will go through the two diodes in line unless i cut the single diode line, this arrangement behaves like a transistor

i dont understand how this agrees with what i studied maybe someone can shed some light?

But basically diodes are not resistors so do not obey Ohm's Law in the same way. A diode has a forward voltage Vf BEFORE it starts to conduct. So if your 2 diodes have total Vf more than 1V and you're only applying 1V then they won't conduct. But I have no idea what "i cut the single diode line" means.

Steve

Does this diode arrangement contradict ohms law?

Nothing contradicts Ohms Law, or if it does then there is probably a Nobel prize for Physics awaiting the person who proves it.

The thing you are missing is that diodes are non-linear in the way they conduct. Up to a certain forward voltage (about 0V6 for a simple PN silicon diode) they don’t conduct at all, then ever so slightly above that voltage they go from not conducting to conducting very well. The If you arrange 3 diodes in series / parallel as you describe (if I understood the description, schematic please) then there will be 0V6 across the single conducting diode, meaning there is also 0V6 across the 2 diodes in series, so 0V3 for each one. At 0V3 thy don’t conduct, which is what you are seeing.

1 volt through

Voltage does not go through anything, current goes through. Voltage is the difference in potential between 2 points, so is ‘across’.

++Karma; // For experimenting and thinking about what you find.

here the disposition i mean, by cutting the line i mean disconecting the cable

If you apply 1V to a diode like that with no current limit then the current will be whatever the power supply can provide, until the diode disappears in a puff of smoke.

The right hand diodes won't conduct with 1V if they are normal PN silicon diodes, they would need at least 1V2 to 1V4 to conduct. Either way, they need something else to limit the current, such as a resistor.

Doesn't Ohm's law say the relationship between voltage and current is linear? I thought the law simply did not apply to semiconductors, which weren't invented/discovered when Mr Ohm was around (~1827).

PerryBebbington:
Nothing contradicts Ohms Law, or if it does then there is probably a Nobel prize for Physics awaiting the person who proves it.

Pretty much everything contradicts Ohm's Law, because it's not a fundamental law, it's an empirical relation that models the resistance of some types of materials. In reality, resistance is nonlinear, and highly dependent on temperature and frequency. For an accurate model, you need Maxwell's equations, solid-state physics and a fair bit of quantum mechanics.

For simple electronics, Ohm's law provides a decent approximation, but it's not an unbreakable law.

PaulRB:
Doesn't Ohm's law say the relationship between voltage and current is linear? I thought the law simply did not apply to semiconductors, which weren't invented/discovered when Mr Ohm was around (~1827).

Exactly.

Pieter

PaulRB:
Doesn’t Ohm’s law say the relationship between voltage and current is linear? I thought the law simply did not apply to semiconductors, which weren’t invented/discovered when Mr Ohm was around (~1827).

Ohm’s law describes the relationship between voltage, current, and resistance of a circuit. The complication here is that the resistance of a diode is not a fixed value. Rather it is (primarily) a function of the voltage across the diode terminals.

For an ideal diode the resistance is infinite with a negative anode to cathode voltage and zero with a positive anode to cathode voltage. Real semiconductor diodes have forward resistance that is nominally an exponential function of forward voltage.

an intensity as a curve would expect some intensity going through the two in line diodes

but it goes all or nothing

its more like a square than a slope

what im finding by myself is that this 3 diodes configuration behaves like a transistor npn or pnp depending on how you arrange the diodes

im trying now the same configuration but instead of using single diodes using two diodes in opposite sense

really im liking it much more now im getting my hands dirty that when i did my engineering PieterP:
Pretty much everything contradicts Ohm's Law.

MrMark:
Ohm's law describes the relationship between voltage, current, and resistance of a circuit. The complication here is that the resistance of a diode is not a fixed value. Rather it is (primarily) a function of the voltage across the diode terminals.

Exactly, Ohms Law applies to non linear devices, it's just that the result varies according to the applied voltage, I don't see a problem with this.

farolero777:
It goes all or nothing

It's more like a square than a slope

farolero777:
really im liking it much more now im getting my hands dirty that when i did my engineering Playing around with real circuits is usually more interesting than staring at equations.

That said, the Wikipedia page on diode modeling is a very well done overview of the math and commonly used approximations: Diode modelling - Wikipedia

PerryBebbington:
Exactly, Ohms Law applies to non linear devices, it's just that the result varies according to the applied voltage, I don't see a problem with this.

Ohm's law explicitly talks about a linear relationship between current and voltage:

Wikipedia:
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship: I=V/R
where I is the current through the conductor in units of amperes, V is the voltage measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.

Of course, you're right that you can still define the I-V characteristic, but talking about "Ohm's law" in the nonlinear case is incorrect and confusing.

You can linearize a non-Ohmic component about a fixed operating point, so Ohm's law applies locally, for very small variations of current and voltage, which is the derivation posted by MrMark.

Thank you Pieter, I can always learn something new from you. ++Karma

I think with the OP's circuits and similar, 2 laws are better than 1. (they have a law for everything)

Interesting discussion.

For the OP's benefit. A diode isn't defined with ohms. One might say they don't have any ohms***.
So it is not that they defy Ohm's law but Ohm's law does not apply to diodes.

*** for the purest, a small part of the diode characteristic can be modeled as a resistor so in some small way it has "some" ohms but for the case in hand it can be ignored.