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To get some more depth out of the SD2000 I replaced the IRF710 with a T0-247 package STW20NK50Z. This is the same FET that is used in the GP 3000 series of detectors. To make it fit you will need to cut the legs off but leave 4mm and solder thin wires to the remainder of the FET legs. This will make sure it fits. The other mod I did was install an 8 volt regulator so I can run the detector from a 12 volt Gell Cell. Make sure that you make a good connection to the heatsink block as this FET gets rather hot.
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Woody watch out what voltages you produce with your fets,the cap next to it is 200V.
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That could go off with a bang, might change them to 250 V rating.Comment
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Any idea why the FET is getting hot?Comment
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The FET should not be getting hot, when testing my 2000 the FET can run warm but not hot unless I have shorted the case to the positive ground!!
When I was designing the PI back it '99 I had a similar problem, at first the engineer said that maybe the switching wasn't real square-waves, I found out it was the damping resistor. Changed that and no more hot FET. I would say that the FET is just not compatible enough. Stick with the 840 can't really see what you would gain with the one you are using now. A FET should not get real hot if everything is designed to suit it. You could do like I did and change the dampening resistors (remember they use 12+ in the 2000!)
Regards Ismael
www.minelabmods.comComment
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Hi guys,
What does that 200V capacitor do? I have heard that it stores the flyback voltage and then
the PI reuses it to charge the coil. Is it true?
I have seen such high voltage capacitors in Lorenz PIs. I think it is right since the Lorenz PIs
work at 1000+ frequency and high pulsewidth, although their consumes are low.
Best regards,
1843Comment
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1843, is that we know as the Energy Recuperation?Comment
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The MOSFET is probably getting hot because the gate capacitance of the replacement MOSFET is quite high compared to the original 810. What this may mean is that the gate drive is not sufficient to give a fast transition during switching on or off due to the increased capacitance of the gate.. The effect of this is that you may find high frequency oscillation of the MOSFET. This will cause the MOSFET to overheat. I would check the the gate drive.
Regards,
StefanComment
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You fellows are right, the gate drive circuitry is only good for around 1200pf as when I loaded the Fet input the drive pulse started showing delayed rise time causing the fet to get hot. I found a Fet that has only 900pf input C and its rated at 9 amps for .45 ohm R. It is a STP11NK40Z.Comment
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rated 9 amps and it's getting hot? have you considered water cooling?
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MOSFET drive
Hi Woody,
Consider a TC426 (Rockby Electronics Clayton VIC have some).
They can switch 1000pF in 30nS.
regards
bugwhiskersComment
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I had a GoldScan 5 that I had modified to work on 24 volts and even though the series resistance was 6 times higher than Mr Minelab I still had to install a computer fan into the box to cool the output fet. A lot of the heat generated in the M/L machine is from uping the voltage to 8 Volts extending the on time and the .5 ohm coil resistance.
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Using a Fet driver would be the best bet but then it would need making a sub board and fitting it in. Someone else should do the Fet driver as a project and put it up on the forum.
http://ww1.microchip.com/downloads/e...Doc/21415C.pdfComment
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That is a good idea with the driver IC, the only thing that has me puzzled is what if anything are we going to gain by changing the Transmit FET anyway? I understand that we may well get a certain increase in output characteristics but we still haven't changed any of the output transmit frequencies which will still be governed by the detector circuitry. So all we can hope to gain is maybe an increase in compatibility with coils of different design. I don't think that changing the FET is going to improve the noise or stability of the detector and that is really the challenge. Stabilising the detector would make a 2000 a formidable detector. With all the mods we perform to it as it is it still gives all the GP range a run for the money and in the last 8 years I have played with these I have yet to see a GP beat a modified 2000 (or standard) on a big deep target. In fact I have professional prospectors that have all the newest offerings from ML and still they say the 2000 leaves them for dead on big deep targets. They have proved this time after time. Just recently here in WA at a test patch my mates were invited to check out with the 2000GPM they found all 4 targets easily where as the 3500 couldn't find the last one (the deepest) and the owner of the patch says that all detectors up to now had never found all 4 targets. The targets were buried at 50cm, 60cm, 80cm and 105cm. The 105cm being a squashed coke can!
But back to the thread, upping the voltage definitely does increase depth but also will increase noise so to offset this we have to make the receive circuitry more stable and sensitive but at the same time quieter, similar to the GP methods. We have successfully given better sensitivity but so far stability is still an issue especially threshold. The standard FET or even a IRF740/840 still handle voltages up to 9.5V comfortably but after 8-8.5V the gain is negligible so I think stick with the 2 FETs I mentioned. Rather than expand this thread here on stability I am going to start a new thread to get some feed back...
Regards Ismael
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