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In the past to create voltage dividers or protect against back emf I have used resistors, but reading about signal diodes those situations are given as examples. I can't really comment on the inductor example because I have never used one but I am familiar with MOSFETs.

So I was wondering what are the advantages of using signal diodes instead of resistors.

These are the examples I have seen:

first example

second example

Images from signal diode tutorial here!

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    \$\begingroup\$ It is difficult to tell what is being asked here. Diodes and resistors are quite different components, so without specific circuits to compare their usage in there is nothing to answer. \$\endgroup\$
    – Olin Lathrop
    May 4, 2012 at 20:00
  • \$\begingroup\$ @OlinLathrop I have put images of 2 circuits in, but they keep disappearing, they show up in the preview and then disappear \$\endgroup\$
    – DNN
    May 4, 2012 at 20:10
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    \$\begingroup\$ @DNN - There was a problem with embedding images from the other site. I've moved them to our default image host, imgur, which works fine. \$\endgroup\$
    – Kevin Vermeer
    May 4, 2012 at 20:20
  • \$\begingroup\$ @KevinVermeer Thanks, I was going a bit crazy trying to fix the markdown! \$\endgroup\$
    – DNN
    May 4, 2012 at 20:21
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    \$\begingroup\$ @DNN A 'signal' diode generally implies a diode rated for low voltage and current, and generally wouldn't be used in a clamping configuration. \$\endgroup\$
    – Adam Lawrence
    May 4, 2012 at 20:56

1 Answer 1

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Note that the diode is reverse-biased in that inductor example. What happens when the power is cut is that the inductor's collapsing magnetic field creates "counter-EMF"; a voltage. This voltage is opposite to the voltage which had been applied, and forward-biases the diode. The diode at that point conducts, allowing the energy to be dissipated. Without the diode, the energy will still be dissipated, but elsewhere. It can create an electric arc that can damage other components or start a fire.

The MOSFET semiconductor doesn't generate counter EMF, but is a sensitive device. A diode can protect it from a discharge coming into the semiconductor from the outside by acting as a short around the MOSFET. In there is only one diode. The assumption is that the external discharge will be in opposite polarity to the power supply rails; i.e. the threat that is being recognized by this defense is that of the flyback voltage from a power supply that contains inductive components. A single diode like this won't adequately guard the MOSFET against static discharge.

The "advantage" of a diode compared to a resistor is that its resistance depends on the magnitude and polarity of the voltage which is applied, whereas an (ideal) resistor has a constant resistance. This is also an advantage of the resistor with respect to a diode.

We can't replace the diodes with resistors in these uses, because we need the resistance to be very small, and such a small resistance would unconditionally behave as a short circuit.

A diode (to a first approximation) acts as a short only when there is a forward-biasing voltage on it (or it has been driven into reverse breakdown). Between these extremes, it is an open circuit. So it's like a voltage-dependent switch.

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