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What does "Vf" mean? I have searched every facet of the internet and I cannot find an answer.

I just want to know what Vf means.

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closed as unclear what you're asking by Leon Heller, Andy aka, PeterJ, Daniel Grillo, placeholder Sep 29 '14 at 12:45

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Possibly related: electronics.stackexchange.com/questions/10962/… \$\endgroup\$ – David Sep 29 '14 at 7:38
  • \$\begingroup\$ So you do not care about standards, and expect others to care about answering this? \$\endgroup\$ – venny Sep 29 '14 at 7:40
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    \$\begingroup\$ It would help if you clarify where you saw this - context might be very important to getting the right answer. \$\endgroup\$ – David Sep 29 '14 at 7:40
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    \$\begingroup\$ en.wikipedia.org/wiki/Ventricular_fibrillation ? \$\endgroup\$ – Andy aka Sep 29 '14 at 8:17
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    \$\begingroup\$ If you want a serious answer you need to ask a serious question. A look in Wikipedia shows 15 meanings. None are what you probably want. You MUST give some context - if W' can suggest 15 wrong meanings why should people here be able to guess what you want. Please provide more information. \$\endgroup\$ – Russell McMahon Sep 29 '14 at 15:23
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Vf is the filament voltage of a vacuum tube.

Some tubes, especially those intended for battery operation, had a single wire that was both the cathode and the heater. This was often called the filament. The differential voltage controlled the heater, and the common mode voltage the cathode. Some types had a center tap to more easily set the common mode voltage in the middle of the cathode.

Look up a 1H4 as a example of about as simple a triode as there is. The filament has no center tap and requires 2 V at 60 mA thru it for it to work as the cathode. In contrast, the 3V4 is a power pentode meant for battery operation and has a center-tapped filament. This is the type of tube you would have found in the audio output stage of a small portable radio.

Other types of tubes had a separate heater that was not electrically connected to the cathode. For those, there was a separeate heater and cathode, so Vf was not used.

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    \$\begingroup\$ THIS ... is the answer the question deserves! \$\endgroup\$ – Brian Drummond Sep 29 '14 at 13:24
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    \$\begingroup\$ A good try - but Vf is never (correctly) used to indicate fillament voltage. Nor liniment voltage, filagree voltage, flippancy voltage, or filllament voltage. But there may be a filament of truth in the descriptive material.| I wonder, might Vf or a filament also be correctly said to be its forward voltage? \$\endgroup\$ – Russell McMahon Sep 29 '14 at 15:18
  • \$\begingroup\$ @Russell: It took me a little to get your point, but the spelling error has now been fixed. \$\endgroup\$ – Olin Lathrop Sep 29 '14 at 15:29
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Vf is quite possibly the forward voltage of a diode. This is used in relation to normal diodes, light emitting diodes or the diodes inside a transistor. Essentially it is the "on voltage", or the forward potential below which a diode will not conduct.

A typical value for "normal" diodes is 0.7 volts, germanium and Schottky diodes will be lower. Light emitting diodes have typical values from 2V for red to 3.6V for blue (found from this great chart).

There is a good explanation of the physics on Wikipedia and there are some excellent answers to a related question on this site.

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  • \$\begingroup\$ Vf is called the "forward voltage" of the LED. What that means is that is that you need to apply a certain voltage across the LED to get a certain amount of current flow. \$\endgroup\$ – achingwrist Oct 3 '14 at 8:12
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    \$\begingroup\$ @achingwrist then why do you ask? \$\endgroup\$ – clabacchio Oct 3 '14 at 9:48

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