Diode passing voltage in both directions.

[David lopez]

i connected a diode to a 4 volt battery. in 1 direction the diode passes 4 volts. in the other direction the diode passes 1.9 volts. is this normal? shouldn't the diode pass 0 volts in the other direction?

 

[BvU]

Hi,

in 1 direction the diode passes 4 volts

Huh ? Do you mean: I measure 4V across the diode ?

shouldn't the diode pass 0 volts in the other direction?

And an infinite current, but only if it is an ideal diode.

In reality https://www.allaboutcircuits.com/technical-articles/understanding-i-v-curves-of-non-linear-devices/ and so do batteries.

If 1.9 V is measured over the diode, the current must be considerable. You are lucky nothing blew up ...

 

[David lopez]

current flows from the 4 volt battery through the diode then into the multimeter.

 

[BvU]

through the diode then into the multimeter

You would do that if you wanted to measure the current through the diode -- is that why you used the term 'passes 1.9 V'.

Is the multimeter digital (auto-ranging, set to V ?) or old-fashioned analog ?

Did you read up on the diode I-V curve ?
Do you know how to http://www.technologyuk.net/science/physics/electrical-principles/electrical-measurement.shtml ?

 

[David lopez]

current flows out the battery, then current passes through the diode, then passes into the first prong then passes out the second prong, then into the battery.

i am using a digital multimeter. the multimeter is manually set to read voltage.
i am not 100% sure i know how to read voltage across a diode. i am making an educated guess.

 

[gleem]

You have the battery, diode and multimeter connected in series?

 

[David lopez]

the battery, diode and multimeter are connected in series.

 

[gleem]

Then you are measuring the voltage across the battery and the diode. If you wish to measure the voltage across any particular component you must place the multimeter cables at opposite ends of the component. In voltage mode the multimeter has a high resistance so in series it is significantly limiting the current. But cautions is needed. if you take out the multimeter from the present configuration the full voltage of the battery will be applied to the diode. When forward biased the 4 volts may produce too much current and burn out the diode. One more comment, your use of passes for the voltage is inappropriate. Passes means to let through and the only thing passing through a diode is the current. The proper term related to voltage is across i.e., the voltage across a component. Thus your meter should straddle the component.

 

[David lopez]

can you post a schematic of how to measure the voltage across the diode, just the diode.

 

[tech99]

i connected a diode to a 4 volt battery. in 1 direction the diode passes 4 volts. in the other direction the diode passes 1.9 volts. is this normal? shouldn't the diode pass 0 volts in the other direction?

What is a happening is that the voltmeter has a high resistance - they are meant to. In the reverse direction, the diode also has a high resistance but not high enough to drop the reading very much.

 

[phinds]

can you post a schematic of how to measure the voltage across the diode, just the diode.

Put the terminals of the meter on each side of the diode. You hardly need a schematic for that.

 

[gleem]

I have added a protective resistor to limit the current through the diode when forward biased. Note, voltmeters usually draw very little current which you can assume to be zero.

 

[David lopez]

i tried putting the multimeter leads on each side of the diode. i connected the diode to a 3.77 volt battery. in 1
direction i got 3.77 volts. in the other direction i got 0.24 volts. is this normal? should the reading be 0 in the other direction or not?

 

[gleem]

Basically. When you got 3.77 volts the same of the battery the diode was reversed biased and did not conduct current acting as an open circuit. So the meter read the battery voltage. When you reverse the leads the diode became forward biased becoming conductive. The battery voltage was so large as to force the diode to become very conductive so much so that you probably exceeded the diodes safe current limit and basically shorted out the battery pulling down its voltage.

what is the voltage now of the battery alone? If you connect the diode to the battery as you originally started do you still get the voltage of the battery? If not you blew out the diode. One usually puts a resistor in series with a diode to limit the current through it.

 

[David lopez]

i did put a 1000 ohm resistor in series with the diode. i forgot to mention that part.

 

[gleem]

OK. That was good. that limited the current to less than 3.7 mA. = 3.77/(1000 +diode forward resistance) Because the 1000 ohms is much greater than the diode resistance it had a much smaller voltage drop across it. If you measure the voltage drop across the resistor it should be 3.53 V = 3.77 V-0.24 v. If you check the specs on the diode it should tell you the max current it can handle.

 

[David lopez]

i tried replacing the 1,000 ohm resistor with a 22,000 ohm resistor. the current flows from the battery through
the diode, then through the 22,000 ohm resistor, then back to the battery, completing the circiut. i measured the
voltage across the resistor. i put the multimeter leads on each side of the resistor. if diode is in 1 direction, i get
3.6 volts. if the diode is in the other direction i got an increasing value. it starts at 0.51 volts. then the voltage slowly increases, i stopped measing the voltage across the resistor at 1 volt. why is the voltage increasing in the other direction?

 

[gleem]

The voltage is increasing because the resistance is increasing. The resistance is increasing probably because the temperature is increasing. The 22,000 ohm resistor is limiting the current to about 0.17 mA. this is pretty small for the diode and semiconductor can have a high temperature dependence of resistance on temperature and thus current. For the record what is the diode ID.

 

[David lopez]

the name of the model of diode is sb3100.

 

[gleem]

This is a Shottky diode. When the temperature increase for a forwarder biased diode the resistance increases see
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.919.6468&rep=rep1&type=pdf
It is late will continue this tomorrow.

 

[sophiecentaur]

can you post a schematic of how to measure the voltage across the diode, just the diode.

Just try thinking about what you have just asked. Have you ever used a Voltmeter before?

You keep posting this type of elementary question. Every time, some good willed PF member tries to help but your series of questions never seem to be going anywhere.

Have you actually learned anything about electronics so far? From what you write, it appears that you never look anywhere else than on PF for your answers.