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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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    \$\begingroup\$ This question is now so far from its original form (see edit chain) that it has become a joke. \$\endgroup\$
    – tobalt
    Nov 1, 2021 at 13:13

2 Answers 2

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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\$
    – user16324
    Sep 29, 2014 at 13:24
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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\$ Oct 3, 2014 at 8:12
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    \$\begingroup\$ @achingwrist then why do you ask? \$\endgroup\$
    – clabacchio
    Oct 3, 2014 at 9:48

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