Minimum inductance for a Whites TDI coil..

[Ism]

I know in past posts, Carl said to shoot for 300uH for a TDI coil.

So in building a Litz wire 18" mono coil for my TDI, I wound the coil but when I went to test the inductance, my "good" inductance meter was DOA.
So I used a cheap LCR meter to get me in the ballpark (320-340uH) and ordered a BK Precision LCR meter.

My cheap LCR was off by 80-100uH so my coil winding which is already spiral wrapped and ready to shield measures at 241uH.

My question is, Can the TDI be operated safely and effectively with a 240uH coil or would I be better served by starting over?

Randy

[Ism]

After some thought, I realized the question only addresses my situation.
The question of minimum and maximum acceptable inductance for a TDI coil would serve the forum community better.
Also, would the performance be affected throughout that range?

I would have edited my original post, but I waited too long and that option timed out.

[Carl-NC]

I measured a TDI coil and it's actually 347uH and RL=3.1Ω. As I recall, it has 20T of 24awg wire; I may be wrong but we'll use it for an exercise. Assume that half the coil resistance in in the windings and half is in the cable. Also, there is an additional internal series resistance of 2Ω.

Let's assume a nominal supply voltage of 14V. The flat-top current is therefore 14V/(3.1+2) = 2.745A. The actual peak current is



where t is the "on" time of 100us and tau is 347uH/5.1Ω = 68.1us. Therefore the peak current at turn-off is 2.113A. The agrees with my recollection of about 2A.

The same can be done for your coil. Let's assume the same diameter*; using the same wire, 241uH suggest 17 turns instead of 20, which suggests a reduced total coil resistance of 2.85Ω instead of 3.1Ω. Now the tau is 51.1us, the flat-top current is 2.89A, and the peak current is now 2.501A.

Increasing the current from 2.1A to 2.5A does not concern me much, it's the equivalent of running a 16.7V battery which I know some people have done. It will create a little more heat and drain the batteries a little faster but should be OK.

You might think that the extra peak current (18.4%) is going to give you an edge in detection. However, the same coil is also your RX coil, and the receive signal is proportional to turns. You would now have 17 turns instead of 20, so the overall round-trip sensitivity is

(2.501/2.113)*(17/20) = 1.006

So you're still at almost the exact same sensitivity of a stock coil, but with higher power consumption.

In winding coils of different diameters we tend to try to match the inductance and give the resistance little thought. In reality, it's always a good idea to do a final math check on the as-built parameters, including measuring the SRF and checking that damping resistor will still work.

- Carl


*Yes, I know your coil is 18" and the stock coil is 12". This is an exercise so it's easier to demonstrate with 2 like coils. But you can apply the same math to your 18" coil and see where it lands.

[Ism]

Great explanation Carl!

My coil is for cache/relic hunting. Hence the reason I am doing the 18" in mono. I thought of correcting the inductance by making it dual field coil but I don't want it to be sensitive to smaller targets near the surface.

So as a follow up question.
With the lower receive sensitivity due to the windings of my current coil, will that help reduce sensitivity to smaller surface targets more than large deeper targets, or would it be linear across the scale?

Thanks much for your thoughtful explanation to my coil question.

[Carl-NC]

It should make no difference at all.

[Ism]

Thanks again for sharing your knowledge, Carl.