home made coils

[Mark-VA]

If I wanted to attempt making a coil for my TDI where could I find the needed information?

[Koala]

I believe the TDI coils are 22 awg Teflon coated stranded wire and the inductance is about 350uh

[LukeJ]

I've made several coils for my SPP, which is a simplified version of the TDI. They use interchangeable coils.

I've wound coils using 22 AWG on down to 26AWG, stranded and solid. With Teflon insulation or just PVC.

Larger coils need 18-20 turns for 300-330uH, smaller coils 22-26 turns. The 3DSS coils work good too...

My coils are 1 to 3 ohms, depending on the wire size and number of turns. I've got one that's 1.5 ohm and it works great, and I've got another that's 3.3 ohm and it works good too.

If you stay in the ranges I've suggested, you should be fine. One thing I will say, and it may pertain to all PI detectors.... Don't disconnect the coil while the machine is on. You may let the smoke out. I did... One of the mosfets popped and I had to do some surface mount surgery. Since then, no issues.

Making a Fast Pulse Induction Mono coil is a great article and helped me quite a bit when I was getting started. One of you guys please help me out on the title if I didn't get it right. Thx.

Good luck !!

Luke

[LukeJ]

One more thing: When you wire the connector... Pin 1 is Receive Ground, Pin 2 is Receive signal, Pin 4 is Transmit signal, and Pin 5 is Transmit ground.

For a mono coil, you'll jumper pin 2 and 4 inside the connector and solder your co-ax center wire to the 2-4 connection. The coax shield will be connected to pin 5.

Have fun !!

Luke

[Carl-NC]

Luke pretty much summed it up. The TDI is very tolerant to coil inductance and resistance. You can also use separate TX & RX coils. I've never had a problem disconnecting a live coil and have done it hundreds of times.

[6666]

Hi Carl
does the TDI use conventional resistor damping for the coil ?
any idea what value the resistor is ?
thanks .

[LukeJ]

Luke pretty much summed it up. The TDI is very tolerant to coil inductance and resistance. You can also use separate TX & RX coils. I've never had a problem disconnecting a live coil and have done it hundreds of times.

Thanks Carl. I was talking to Reg Sniff quite a bit when I was first learning about the TDI and making coils for it. He was a bit surprised when I told him what happened and he suggested that perhaps it was flawed (the mosfet) to begin with. I replaced the two, they work in tandem, and the detector works again. I just didn't want to take the chance... or send anyone else down that road. Thanks for the info !!

Luke

[Mark-VA]

That being said are there any VLF coils that will work on a TDI?
I know that sounds kind of stupid to ask, but you know if you don't ask.......


All this comes to the need I have for a smaller than stock coil, maybe a coil very similar to the White's DFX coil that I think is a 5x10 or so?
I might can make do with a 6 or 7" round for the heavy, heavy trash and iron.

Well anyway is there any particular winding patterns.
I guess I just don't understand enough, being a VLF user since 1980 and only using the TDI for 1 1/2 years now I do realize the TDI has a far greater potential than all the other machines in the past.

[6666]

Generally a tdi coil is in the 300 uH range and a single wind,
most vlf coils are many times higher than this value and are not suitable for tdi.
There are a number of brands in the 6-8 inch range for tdi.

[Mark-VA]

I guess I'd like someone to draw it out for me.

[FatBob]

Generally PI coils have low inductance say 150 - 500 uh (unless your looking for big objects).
The TDI being a PI type MD should have around 300uh to see the size objects it was designed for.
You can make a DD coil for it but must use a low inductance for it to work. A PI is nonresonant
and needs low inductance to see the quickly decaying signal when the TX is removed.

VLF's generally have much larger inductances (say 1mh to 30mh) which are resonated to
frequencies in the 1 - 100 KHZ range.

If you attach a VLF coil to a PI detector you won't see much of anything. If you make a
VLF like coil with the proper inductance then you could possible use it on your TDI...

[Mark-VA]

I take it that there are NO internal components in the coil, no resistors etc? If I meet the inductance range I should be fine, correct?

Luke pretty much summed it up. The TDI is very tolerant to coil inductance and resistance. You can also use separate TX & RX coils. I've never had a problem disconnecting a live coil and have done it hundreds of times.