# How do you the find impedance of a Schottky diode?

Discussion in 'General Electronics Discussion' started by Shivraj, Mar 10, 2010.

1. ### Shivraj

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0
Mar 10, 2010
I just wanted to know how one can find the impedance of a shotkey diode? I am using it in a full wave rectifier circuit.

2. ### Resqueline

2,848
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Jul 31, 2009
It doesn't matter if it's a Schottky or an avalance diode or any other non-linear device.
Impedance = Delta Vf / Delta I.
You need to know/establish/measure two points on its Vf/I curve, one high-current & its corresponding Vf, and one low-current with its Vf. Subtract the voltages and the currents and divide the results by each other.

3. ### davennModerator

13,653
1,886
Sep 5, 2009
will have to remember that one

not that I have ever needed it so far but ya never know

would I likely to find that info as part of a standard data sheet for a diode ?

thanks Resqueline

Dave
VK2TDN

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
You will likely find a graph indicating the voltage drop across the diode at various currents. The impedance at a particular current is given by the slope of the line tangential to the curve at that point.

That is a fancy way of saying exactly what Resqueline said, and would be familiar to you if you recall the limits stuff you may have done prior to doing differential calculus.

An example for a diode at random. There is a graph showing typical forward voltage vs forward current.

You will note that at low currents, a relatively small increase in current produces a comparatively large increase in forward voltage. As the current increases, the forward voltage is far less sensitive to changes in current. This shows that thte diode exhibits a decreasing impedance with increasing current.

Note also that the graph is log/linear so you can't simply measure the slope. You need to read values off the graph and calculate as per Resqueline's instructions.

It's also worth pointing out that these curves are for a typical device at some constant temperature. In the real world the actual values will change between devices and with temperature.

5. ### neon

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Oct 21, 2006
TRY BOLTZMAN CONSTANT 26MV

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
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Jan 21, 2010
I can see where this relates to semiconductor junctions, but not how it applies to the question being asked (and here I show my lack of schooling). If you could explain, or point me to an appropriate source of information I would appreciate it.

Thanks.