Just started MPP project

[multieagle]

Images to Step 5.

[Chet]

I have finished to Step 5, and must now wait for missing parts. I had some problems with terminology and processes. For one thing, did not realize a coil must be attached at Step 4. I am using two coils made for the Surf PI, both are copper tape shielded and work well on the Surf. 3 coils are around 300uH, 1 coil 278uH and 1.2 - 1.6Ω. However, I'm getting a reading of 1.5v at TP3 on my multimeter and that does not change with popcan or other metal. On my osiloscope, I'm getting an average 1.10v and that also does not change in the presence of metal. I may have to construct another unshielded coil for test purposes?? See attached image.

Hi multieagle

Attached are a couple of sample oscilloscope displays for TP3.

Scope_9 is with no target.
Scope_13 is with a large metal object near the coil.

Chet

[multieagle]

At step 3, I am not receiving an audible signal between TP1 and 0K. I replaced U4 NE555 and powered up. IRF740 began to overheat. I removed the coil, which returned the Mosfet to normal temperature. I have the coil wires hooked up centre cable to middle connector PL2, and shielded cable (coil has copper shield connected to negative lead) to outer PL2. Centre connectors (PL2 & PL4) are connected beneath the board. Any suggestions as to what may be the problem? Could the Mosfet be defective? What voltages should I expect from the three mosfet legs?

[Qiaozhi]

At step 3, I am not receiving an audible signal between TP1 and 0K. I replaced U4 NE555 and powered up. IRF740 began to overheat. I removed the coil, which returned the Mosfet to normal temperature. I have the coil wires hooked up centre cable to middle connector PL2, and shielded cable (coil has copper shield connected to negative lead) to outer PL2. Centre connectors (PL2 & PL4) are connected beneath the board. Any suggestions as to what may be the problem? Could the Mosfet be defective? What voltages should I expect from the three mosfet legs?

If you don't have a signal at TP1, then the TX oscillator is not running. If U4 pin3 is stuck high, the MOSFET will be hard on, and connecting the coil will definitely make it get hot. You need to fix the TX oscillator problem before proceeding any further.

[Chet]

At step 3, I am not receiving an audible signal between TP1 and 0K. I replaced U4 NE555 and powered up. IRF740 began to overheat. I removed the coil, which returned the Mosfet to normal temperature. I have the coil wires hooked up centre cable to middle connector PL2, and shielded cable (coil has copper shield connected to negative lead) to outer PL2. Centre connectors (PL2 & PL4) are connected beneath the board. Any suggestions as to what may be the problem? Could the Mosfet be defective? What voltages should I expect from the three mosfet legs?

Hi multieagle

"TP1 and 0K" Is "0K" the same as TP10? The MOSFET will over heat if the pulse is not correct at TP1. TP1 should look like your previous oscilloscope display in Post #14 or Figure 5 of the build manual. If not leave the coil disconnected and look for a problem around U4. Do not reconnect the coil until you have the pulse correct on TP1.

Regards,
Chet

[multieagle]

Ha, ha . . . that damn TP1-10. Yes connected correctly.

Thanks for your continued help, online and via email. Hightly appreciated.

Gord

[multieagle]

If you don't have a signal at TP1, then the TX oscillator is not running. If U4 pin3 is stuck high, the MOSFET will be hard on, and connecting the coil will definitely make it get hot. You need to fix the TX oscillator problem before proceeding any further.

Problem is, it was working (see post #14) and I continued on to Step 4 & 5. I was missing a part at step 4, and instead of waiting until I received and installed it, I went ahead to step 5. I picked up the needed replacement and installed it. Something happened at step 4 & 5 that caused step 3 to fail. Does that make sense?

I had to remove IRF740 and replace it. I scewed up the middle leg recepticle, the insert broke free and also the top soldering point. The bottom soldering point (ring) remained in place. I installed the replacement and am getting 12v both outer legs, and 0v centre leg. I'm thinking it may relate to that?????? I should maybe replace R3, as there was a lot of heat applied to the Mosfet at that point.

Pin values
U4
1: +12 # 5: 0
2: +9.7 #6: +9.7
3: +10 #7: +9.7
4: 0 #8: +11.8

R3 +4V

TP! +11.8v

[6666]

as there was a lot of heat applied to the Mosfet

When I am makeing such projects as a metal detector I mount mosfets, damping resistors or others components that may need replacing on PCB pins,
does not look pretty but only takes a few seconds and the job is done, does not require a lot of heat and does not damage the PCB.

[multieagle]

Excellent idea!

[multieagle]

Chet, to save you some trouble, I replaced the 555 and almost immediately the Mosfet began to overheat. Replaced that with another, same result. I have rechecked all solder points and everything looks okay. Went over it with a magnifier and there were no bridges. Therefore, it is probably associated, IRF740, R2 or R3. Will first replace the 740 and go from there. Thanks for your help.

[Chet]

Chet, to save you some trouble, I replaced the 555 and almost immediately the Mosfet began to overheat. Replaced that with another, same result. I have rechecked all solder points and everything looks okay. Went over it with a magnifier and there were no bridges. Therefore, it is probably associated, IRF740, R2 or R3. Will first replace the 740 and go from there. Thanks for your help.

Hi multieagle

Remove the 740 then check for a pulse from U4. If you don't have a pulse do not install another 740 until you have a pulse.

Regards,
Chet

[multieagle]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

[6666]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

You can series link more than one value to get your 470 ohm :IE 200 +200 + 70
and it takes the stress off just one R.

[Chet]

I need to replace R1 (470 ohm 2W). Locally, can only find 470 ohm 5W. Any problem using that? Otherwise there is a 470 ohm 1/2W.

Hi multieagle

The 5W is no problem unless it is a wire wound resistor. A wire wound resistor may add inductance or capacitance to the circuit. Other wise the leads on the 470 may be too large to go through the holes. You might splice some smaller leads onto the fatter 5w resistor leads. 470 ohms will probably be a temporary value. You will probably change the value after final assembly to get the best performance with a particular coil.

Regards,
Chet

[multieagle]

Removed the MOSFET and R1. Am again getting a pulse. See image. Will pick up the 470 tomorrow and do a temporary install, along with a new 740. Fingers crossed.