# Voltage - Forward (Vf) (Max) @ If

Discussion in 'General Electronics Chat' started by usmanqadir91, Oct 2, 2015.

Sep 17, 2015
15
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I am going to select some diodes on the basis of there forward voltage but i am quite confused in the term "Voltage - Forward (Vf) (Max) @ If". What does this term means? As an example please explain what does "230 mV @ 100 mA" (mentioned in the category Voltage - Forward (Vf) (Max) @ If) specifies?

2. ### ericgibbs Moderator

Jan 29, 2010
6,233
1,186
hi,
The forward voltage drop across the diode is not linear, it follows the curve shown in the attached image. For a BAT43 type diode as an example.

Typically the higher the current thru the diode the greater the voltage drop across the diode.

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Sep 17, 2015
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Forward voltage drop specifies the voltage at which the diode will start conduction. Am i right?

4. ### ericgibbs Moderator

Jan 29, 2010
6,233
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Regular diodes are usually made either from Silicon or Germanium.
A Si diode will start to pass a very small current when the forward bias voltage is around 0.65V, a Ge diode will start conducting at approx 0.4V.
The actual start of conduction depends upon the composition of the PN junction material.

The voltage drop will increase as the current thru the diode increases, so to say Forward voltage drop specifies the voltage at which the diode will start conduction is not correct

Forward voltage drop is the drop at a specific forward current, as per the image I posted.

5. ### crutschow Expert

Mar 14, 2008
20,263
5,737
A diode has a logarithmic voltage versus current relationship and starts conducting at a low forward voltage (albeit at a very low current) as shown in this graph.
There is no specific voltage at which it "starts to conduct".

A silicon junction diode is often stated to have a forward drop of 0.65V-0.7V, since that is the drop at typical circuit currents but, as can be seen, the forward voltage has a large variation if you go to very small currents from those typical values.

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6. ### MikeML AAC Fanatic!

Oct 2, 2009
5,445
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Zapper beat me to it, but I was creating this plot while he was posting...

What is the V(f) of this commonly available Si diode?

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7. ### dannyf Well-Known Member

Sep 13, 2015
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Contrary to popular believe, diodes and transistors start to conduct at very small current, and with very small forward voltage drop.

The parameter you cited is basically a max figure for such voltage drop at the specified current.

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8. ### Bordodynov Well-Known Member

May 20, 2015
1,986
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I still make you happy. With a single, short front voltage across the diode 1n4148 be more than 4 volts! This is important for the protection circuits of the input amplifier stages. The same effect, I watched for 1n4007 diodes. This was observed when applying short current pulses (20 microseconds) at a frequency of 2 kHz.
This is the effect of modulation of the base resistance.

9. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,953
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"Voltage - Forward (Vf) (Max) @ If"

It is a guarantee by the manufacturer that when applying that same If current you will measure a voltage less than that stated maximum.

This is a useful thing to know as it is also guaranteeing the maximum power the device will dissipate for that current.

10. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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2,001
No. Vf usually is specified at typical and maximum operating currents. These are voltages when the diode is well into its conduction region. The conduction region starts at a much lower voltage and it varies from one part number to the next. It usually is not specified because there are no standard values for start-of-conduction test parameters.

ak

11. ### #12 Expert

Nov 30, 2010
18,093
9,679
The graph provided by crutschow goes down to about 10 nanoamps. I did the actual measurements with a transistor and measured the "turn on" voltage (base to emitter) for collector currents from 1 nanoamp to 1 milliamp. This is relevant because a base-emitter junction resembles a diode. Here is my graph. I think one of the labels is backwards, but the data is easier to read with some accuracy on my graph. What it means is that there is no zero for any practical purpose and there is no, "turn-on" voltage that is fixed across any range of current. Volts per amps is a continuum from as low as you can measure to as high as the diode can go without melting.

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12. ### crutschow Expert

Mar 14, 2008
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How did you measure the voltage at 1 nA since a standard multimeter has too low an input impedance?

13. ### #12 Expert

Nov 30, 2010
18,093
9,679
Slick trick which has been posted on this site previously: My Fluke 27 has a 10 meg impedance on the volt scales. Place it in DC millivolts, in series with a very low current, and the current shows up as voltage across 10 million ohms. A nanoamp times 10 meg displays 10.0 millivolts on the 320.0 millivolt DC scale. Easily within the range of the meter because it has a digit left over for confidence in the quantity that I claimed to have read.

For the worry-warts, the "burden" voltage is the voltage on the display, 10 millivolts!

or did I miss the intent of your question?
The voltage measured was at the base of the transistor, which can be set at a low impedance. The paragraph above was about how I measured the current.

Like this:

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14. ### WBahn Moderator

Mar 31, 2012
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The key is to parse the statement correctly.

The manufacturer is saying that the maximum value of the forward voltage across the diode when the current flowing through the diode is the specified value of If is the stated value.

15. ### eetech00 Senior Member

Jun 8, 2013
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Per the OPs question...I believe it means that the forward current through the diode is 100ma when a DC forward bias voltage of 230mv is applied across the diode anode and cathode terminals.

16. ### WBahn Moderator

Mar 31, 2012
23,194
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No, that is NOT what it means. It means just what it says: When If is 100 mA -- "@ If" --, the maximum forward voltage -- "Voltage - Forward (Vf) (Max)" -- that will appear across the diode is 230 mV.

A manufacture simply cannot say that a particular current will flow at a particular voltage -- far too many variables for that.

17. ### dannyf Well-Known Member

Sep 13, 2015
2,196
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"forward current through the diode is 100ma when a DC forward bias voltage of 230mv is applied across the diode anode and cathode terminals. "

you are almost there: forward current through the diode is *** at least *** 100ma when a DC forward bias voltage of 230mv is applied across the diode anode and cathode terminals.

the above statement is identical to the one in the datasheet under the same conditions.

18. ### WBahn Moderator

Mar 31, 2012
23,194
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While that is true, it is not the best or safest interpretation. If you apply 230 mV across the diode is it possible that the current will be so much as to damage the diode and, if that happens, then you have no basis to complain.

The better interpretation is to read it for what is says. The forward voltage across the diode will be NO MORE than 230 mV when there is 100 mA of current flowing through it.

19. ### eetech00 Senior Member

Jun 8, 2013
1,221
248
But that's not what the curve shows....otherwise it would be useless information..