Inductor DCR Insanity For SDA's

[gimpod]

WARNING: If simple math & basic electronic theory gives you a headache you may want to move on.

Let's get on with it. So you've been reading DarqueKnight's threads on replacing the stock board level inductors in his SDA SRS 1.2TL's and SDA CRS+'s and the good results he's gotten from doing so using Jantzen perfect lay inductors from Parts Express (Special order) Link to thread and you've gotten the inductor upgrade bug. Well your in luck if you've got ether the SDA SRS 1.2TL's or the SDA 2B/CRS+'s as DarqueKnight has done all the heavy lifting on doing this mod and was kind enough to supply part numbers and a contact person at Parts Express to order the inductors from.

Now what do the rest of us do who don't have the 1.2TL's or 2B/CRS+'s and want to upgrade there inductors?

First thing we need to do is find out what inductors are being used in our crossovers, here is a list of the different inductors used in the 4th & 5th generation SDA's



All other measurements were made by using a common DMM, a fixed current power supply and Ohm's Law for figuring an unknown resistance, R = V / I. Where R = resistance, V = Volts and I = Current.

So how do you find out what the DCR is, Simply put you measure it. But hold on a minute that may not be as simple as it sounds because you need a multimeter that is accurate to at least 0.01 ohms DCR such as a Fluke 8050A multimeter or something in the range of the Fluke (Big $$$) and no that Radio Shack or run of the mill DMM will not get the job done it just doesn't have the resolution and accuracy needed.

Now some of you may be thinking "No problem I'll use my LCR meter" that most likely wont work ether because most LCR meters don't measure DC Resistance they measure AC Resistance or what's call ESR or "Equivalent Series Resistance" and is typically larger than the actual DC Resistance. Also there is an inherent problem with using any meter to measure very small amounts of resistance and that is call "The parasitic resistance of the test leads" and is usually less than 1 ohm and needs to be subtracted from the reading you get from your meter.

Now if you don't have one of those high end, high dollar meters that can measure DCR down to a hundredth or thousandths of an ohm you can buy one ($$$+) if this is not an option for you, you could take them to a shop or a friend that has such a meter to get them measured.

Now there is a way to measure very small amounts of resistance using any run of the mill DMM, a couple bucks worth of parts, Ohms Law and what's call "4-wire (Kelvin) resistance measurement method" that will give you a very accurate reading with a resolution of better than a millionth of an ohm.

Ohms Law for calculating an unknown resistance states that if you push a known fixed amount of current through the device under test (in this case an inductor) and measure the voltage drop across the device under test you can calculate the resistance using the formula R = V / I, were R = Resistance, V = Volts and I = Current.

Let's use a real world example to illustrate this. If we take a 22 Gauge 0.4mH inductor and push 0.100 amps of current through it and measure a voltage drop of 0.0687 volts across it we can calculate the resistance using Ohms Law like this:

R = V / I
R = 0.0687 / 0.100
R = 0.687 Ohms

If you read DarqueKnight's thread I linked to earlier you will notice that he measured the same inductor with his Fluke 8050A multimeter and got a reading of 0.68 Ohms, How's that for the magic of math.

Here is a link to a tutorial on how to build the power supply and a video explaining the 4-Wire (Kelvin) method for measuring resistance.

Simple Low Resistance Measurement

Now if some of you guys measure the DCR of the stock inductors and post your results here like this,

0.300mH, 22 Gauge, DCR=X.XXX, Make & Part Number you replaced it with only if you had good results with the replacement.

I will update the above table, Hopefully we'll get the table filled in so we'll all have something to go by when replacing our inductors.

Hopefully I didn't confuse or bore the crap out anyone with this.

Happy Modding.

[FTGV]

Nice thorough work gimpod.

[drumminman]

Tony this is a great idea.

[PolkieMan]

Simple yet accurate.
So as long as you know the exact voltage and the exact current the resistance can be calculated simply by the drop in the voltage.
The kit was just to make it easier by providing a known amount of current divisible by 10.

[headrott]

Tony, this is an outstanding thing you did in your post! Thank you sir!

[VR3]

Do you guys want me to add this to the page?

Solid work Tony!

[headrott]

Yeah Trey, I think adding this to your mods page is a good idea.

[VR3]

Posted under "Replace Inductors" and "Schematics"

http://vr3mods.com/LCSDAUpgrade.html

:)

[vmaxer]

I want to do this sometime. Good info!

[DarqueKnight]

Thanks Tony. Great info. I thought this was going to be a thread calling us insane for changing SDA inductors.

[PolkieMan]

Wow this would be cool leaving the old CO board intact and using "all" new parts on the Gimpod boards