Coil for the GS4

[Paul Harris]

Hi All,

I've built the front end of the GS4 on a vero board for testing, although i have it working the depth is not very good. Now I understand it could be down to the fact I need to build the rest of the circuit.

But I am leaning to the fact I may have built a coil that is not the stated 436uH. The gate pulse on-time to the mosfet is 196uS. Anyone know the correct on time pulse duration?

Also, when the mosfet turns on the coil voltage only reaches -8 volts peak, at which time the mosfet turns off, I was expecting it to be closer to supply. Is the 8 volts correct?

On the circuit diagram there are two 1k Ohm 1 watt resistors, is that correct, couldn't I just use one 500 Ohm 2 watt resistor.

Finally, does anyone have the correct number of turns and wire gauge for an 11 inch diameter coil for this machine? The coil I built was using the inductor calculator.

Many thanks

[Qiaozhi]

Hi All,

I've built the front end of the GS4 on a vero board for testing, although i have it working the depth is not very good. Now I understand it could be down to the fact I need to build the rest of the circuit.

But I am leaning to the fact I may have built a coil that is not the stated 436uH. The gate pulse on-time to the mosfet is 196uS. Anyone know the correct on time pulse duration?

From the circuit diagram (if we're both looking at the same one) it says that the TX-on time should be 100us, but the component values indicate that the duration is actually set for 219us, which is closer to what you have.
The adjustable frequency range is also shown as 450 to 550 pps, but the component values will give 489 to 577 pps.

Also, when the mosfet turns on the coil voltage only reaches -8 volts peak, at which time the mosfet turns off, I was expecting it to be closer to supply. Is the 8 volts correct?

If your coil's d.c. resistance is very low, then the coil voltage will not flat-top before the end of the TX-on time. Since the voltage is rising to 8V, I calculate that your coil resistance is only 0.24 ohms.
The tau (L/R) of the coil during TX-on is then 436u/2.44 (R = 0.24 + 2.2 = 2.44 ohms) which gives 178.7us. For the coil voltage to reach 12V, this would take approximately 5 * tau, which is 893.5us.

On the circuit diagram there are two 1k Ohm 1 watt resistors, is that correct, couldn't I just use one 500 Ohm 2 watt resistor.

The diagram shows a 1k damping resistor on the PCB with a second 1k damping resistor inside the coil shell. I suspect that this is to allow for tweaking of the damping resistor value depending on the coil / probe used. Also, it helps to remove some of the dissipated heat from the coil. Of course, you could replace the two resistors with a 500 ohm if you desire.

Finally, does anyone have the correct number of turns and wire gauge for an 11 inch diameter coil for this machine? The coil I built was using the inductor calculator.

Many thanks

The coil calculator will get you very close to the correct number of turns.
Do you not have an inductance meter?

[green]

If your coil's d.c. resistance is very low, then the coil voltage will not flat-top before the end of the TX-on time. Since the voltage is rising to 8V, I calculate that your coil resistance is only 0.24 ohms.
The tau (L/R) of the coil during TX-on is then 436u/2.44 (R = 0.24 + 2.2 = 2.44 ohms) which gives 178.7us. For the coil voltage to reach 12V, this would take approximately 5 * tau, which is 893.5us.

Coil current rises, volts should start at near supply volts and drop off if mosfet or supply can't handle the current.

[Qiaozhi]

Coil current rises, volts should start at near supply volts and drop off if mosfet or supply can't handle the current.

You're right ... I'm was being stupid (doh!). I was thinking of the coil current.

[Paul Harris]

Thanks for the replies guys much appreciated.

Managed to take a look at the circuit and found the reason for the coil not charging to near supply. I was using a irf840 mosfet (all I had at the time) instead of the irf740. The latter has a lower Rds on, well thats what I put it down to. I also changed the feedback resistor in the preamp from 1.5M to the stated 1M, these two changes have improved depth, but not enough to get exited about - ?1 coin at 4 inches in air. I still feel the coil is a problem, although it could be the fact I only have the preamp and need to get the other stages built to see any significant gain. Should be getting my hands on a coil today, its off a GS II, but I'm told the resistance of this coil is 1 ohm and not the 3R5 of the GSIV. The coil I built has a d.c resistance of 1.1R. It'll be of a much better build quality than mine so I have high hopes. The freq range of my circuit is 436Hz - 573Hz which is pretty much inline with whats expected.

Thanks for the coil construction tips/pics that info is going to be very useful.

Been playing about with the fathers DeepStar detector, one of Erics. Has nice depth but the dad has another one built by Eric which he tells me has more depth again, but he won't let me near that machine.