## Coil Inductance

27 08 2011

Let’s take a look at the coil inductance.

A fun and easy place to start is look at some AC going through the coil (suggested in comments).

I used the oscilloscope’s test wave generator. I’m using two 10:1 probes connected thusly:

Channel 1 is shown above channel 2 on these oscilloscope screens:

Not much distortion at  500Hz:

At 50kHz we start to see some distortion:

At 500kHz we have obvious distortion:

Not sure what this tells us, but I’m sure it tell us something!

Now lets try a more abstract approach to the problem. We have a 6cm diameter air core coil with 60 turns. I found this handy inductance calculator. I looked up the thickness of 18 AWG wire and the relative permeability of air.

Theoretically the coil has 0.56mH of inductance. Sound right?
Given inductance and frequency, you can calculate impedance…which is what we’re after. But what’s the frequency of a single pulse?

UPDATE: The rectangular function may be useful for calculating frequency.

## Ground Resistance

27 08 2011

Now that I have a milliohm meter, I want to measure the resistance of everything.

Ground quality is important for safety and signal quality.  So I measured the resistance of my grounding network. I made two 5 meter long Kelvin probes to do so:

I have two main ground connections. One is the ground pin on a standard wall outlet. The second is a braided wire to a water pipe:

Normally these ground exits are star connected. As a test I disconnected them from one another and measured the resistance between them: 700mΩ.

For various test points on the machine and rack we see from 10 to 100mΩ

I realized that this potentiometer is not grounded at all!

I will ground this forthwith!