Questions on making big mono for gp 3000

[ivconic]

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Thank you very much!
I will rearrange DP and try this....
But i'll have to raise freq. on 400Hz even more...cose this coil is not suitable for lower freqs...Piece of cake...just need few hours...
Regards!

[Reg]

Hi,

The coil should work at the lower pulse rate without any mods. The pulse rate on a PI has little to do with the coil design. As long as the coil is made for a PI, it should work at about any pulse rate.

Reg

[Righteous]

ivconic i also apologize for not spelling you name correctly.

Reg thanks for all your input and your pic's you have certainly help me out.

Righteous.P.S. it certainly isent as easy as one would think to make a coil.

[ivconic]

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Name....who cares about name!? Dont mention it...

But i am scared not to blow my IRF740 if connect this without modding!?
Usually my coils have resistance over 4ohms...mostly 6 ohms...so fet is cool and steady...
This coil has very low resistance....Adding serial resistor will distort signal much cose it will need higher resistance to add than coil itself has!??
Most of power will disipate on serial resistor...!
Once i had 2 ohms coil and added 4 ohms resistor, i noticed after a while that resistor is hot, coil worked as canarian...etc.etc...
After than i am keeping all my coils over 4 ohms....so no need for resistor..
Also noticed that 30-35 windings on usuall sizes makes DP works much better, calmer and depth is very good....
Always wandered how Germans done this in Pulse Star, since its coil (45cm) has only 1.9 ohms resistance? I havent noticed any serial resistor inside device...Fet is IRF740 too...
regards

[Unregistered]

ivconic
is not there two irf740 in pulse star and also big cooler aliminium plates.

okantex

[Reg]

Hi Again,

I still wouldn't worry too much about blowing the FET. As long as the pulse on time isn't too long, the current won't be as high as you might think.

The pulse current will rise at the rate of the time constant of the coil and other associated resistances. So, by lowering the resistance of the coil, you make the time constant longer. This means if the pulse on time isn't extremely long, the actual peak current should be well below the rated current.

Keep the pulse on time below 200 usec and the pulse rate at the original 400 hz rate and the FET shouldn't get that hot. I am guessing the coil current will peak at somewhere between 6 and 7 amps. However, the duty cycle is quite low so the average current should be very low.

If one is worried, they can further reduce the pulse on time.

Reg

[Unregistered]

Hello,

I am new to all this & i need some advice, i know the thread is a bit old but it is relevant to some queries i have regarding testing some things out.

In this thread there is an image of the plug wiring by Fred, i would like to check my metal detector on an O-Scope & want to verify the connections so no mistakes are made.

In the image supplied for MONO Coil configuration pins 2, 3, & 4 are positive & pins 1 & 5 are negative, is this correct.

If so do i just connect all the pins 2,3 & 4 to positive on an O-Scope & pins 1 & 5 to the Negative on the O- Scope.

Sorry for the very basic question but i have to start somewhere to learn more about things.

Also on the image for DD wiring configuration there is XMIT & REC, what do these mean??

Thanks

[Unregistered]

Sorry i meant IMAGE supplied by REG not Fred---DUH!!

[Reg]

Hi,

This is Fred, I mean Reg.

Ok, if you want to look at the signals at the coil plug of a mono coil on a ML, you connect the scope probe to pin 2 for sure or pin 4. The drawing I was given had pin 3 connected also but pins 2 and 4 are used for sure. Those are the two that have to be used that I know of. Then you connect the probe ground lead to pin 1 or pin 4. Normally, pins 1 and 4 are connected either at the plug or inside at the pc board.

Now, make sure your scope has a 10:1 probe and that probe is rated at a voltage higher than 400V. Also, make sure the probe is selected in the 10:1 position. I prefer a probe rated at 600V. If you do not have a 10:1 probe, make absolutely sure your scope input is rated higher than 400V or you might blow something. Actually, I would prefer the scope be rated higher than 600V to be safe.

I use cheap scope probes I get from MPJA. See www.mpja.com for details. A 60 meg probe rated at 600V is only $10. I have used them for years and finally had one get a little flakey, so I tossed it and pulled out another. At the mentioned price it pays to have a spare or two.

Now, XMIT is short for transmit and REC is short for receive. So, the signal from the Transmit FET goes to the transmit pin 3 via the center conductor of the coax. The receive circuitry is connected to pin 2 via the center conductor of the receive coax. Pins 1 an 4 connect to the coax shields.

In the case of a mono coil being made, there is only one coax needed since the receive and transmit connect to the same place. So, on many if not most coils, the center wire of a single coil cable may connect to pins 2, 3 and 4 or it may just connect to pins 2 and 4. With this connection, the transmit circuitry is connected to the receive circuitry. I have build mono coils with the simple connection between 2 and 4, but I have a different PI with the same connector.

I hope this makes sense.

Reg

[Unregistered]

Hahaha, Thanks Fred!

I'll have more of a read to make sure i get it right.

Thanks again, appreciated greatly!