# DIODE RESISTANCE-Urgent

Discussion in 'The Projects Forum' started by yellowstone, Dec 16, 2008.

1. ### yellowstone Thread Starter New Member

Dec 16, 2008
2
0
Is there a calculator available for determining the series resistance of the diode.(ie RS which is given in the data sheets)

Apr 5, 2008
15,796
2,384
Hello,

A diode can not be expressed in resistance.
It conducts current one way (so a low resistance) if the voltage is over the Forward voltage.
It isolates in the counter direction (very high resistance).
Here is a page of the EDUCYPEDIA with a lot of links to information on DIODES.
http://www.educypedia.be/electronics/composemiconductorspassif.htm

Greetings,
Bertus

3. ### mik3 Senior Member

Feb 4, 2008
4,846
63
If you are talking about the bulk resistance, you can calculate it if you measure the current through the diode and voltage across it. The voltage across the diode will be the sum of the junction voltage and the voltage across the bulk resistance. If you subtract the junction voltage (from the datasheet for a given current) from the voltage across the diode you will get the voltage across the bulk resistance. Then divide the voltage across the bulk resistance by the current through the diode to get the value if the bulk resistance.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Can you link to a datasheet we can look at? Not all diodes spec Rs.

5. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
The parameter would be Rs - R sub s, or resistance depending on whatever s stands for - if such an animal can be found.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
S stands for series, as the OP said in his first post.

7. ### steveb Senior Member

Jul 3, 2008
2,433
469
This specification seems to be more relevant for switching diodes. I attached a PDF for a switching diode that specifies the forward resistance.

Also, I've attached a paper that discusses a more complex diode model which includes series resistance and capacitive effects.

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8. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
The BA483 spec is for Rd. Rs appears to be another animal.

9. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Rs appears to be a bulk semiconductor resistivity.

10. ### steveb Senior Member

Jul 3, 2008
2,433
469
The nomenclature can be confusing. It's difficult to be sure if everyone conforms to one standard that Rs is the bulk resistance and Rd the diode resistance. So far we see the following.

1. The BA846 data sheet says Rd is the forward resistance, which would seem to imply that bulk resistance is included. Granted it may be negligible, but still confusing.

2. The modeling paper I referenced uses Rs for the bulk resistance and Ri for the diode resistance.

3. Ron H. shows a reference with Rs the bulk resistance and Rd the diode resistance.

4. The OP is looking for the series resistance RS

It's difficult to be sure which spec the OP is looking to calculate, but hopefully he can figure it out from what is available. The paper I referenced gives a formula for the bulk resistance (Rs in that paper), but this depends on design parameters that may be unknown. The diode resistance is well known and the formula is commonly available and depends on current and temperature.

11. ### yellowstone Thread Starter New Member

Dec 16, 2008
2
0
i am trying to find the total resistance of the diode ( ie resistance of a p type diode+ contact resistance.)