HELP Choosing a design for first digital PI build

[Qiaozhi]

OK - that's good. Problem solved.

[detectormods]

For God’s sake stop putting series resistor on opamp 1st stage amps, at least use a couple of depletion mode mosfets

[algan]

For God’s sake stop putting series resistor on opamp 1st stage amps, at least use a couple of depletion mode mosfets

Post the circuit you say will work better so I can test it.

[detectormods]

[Detectorist#1]

Hi detectormods,
For God's sake, please explain to us what is the advantage to use two depletion mode transistors instead of simple resistor for safety of the first stage input. Thank you in advance!

[Orbit]

Hi Detectormod! I would say that the circuit with two 797's is paired like that for noise suppression I tried the LT 1028 it does suppress but not as much as I wanted the resistor in series also creates noise. Detektormods can only two depleted mosfets be used?Is a depilation mosfet used because of the high rds?

[eduardo1979]

For God’s sake stop putting series resistor on opamp 1st stage amps, at least use a couple of depletion mode mosfets

Hello Detectormods.
I've used your concept in my PI detector, and smoke is coming from the BSS139s. I've attached the schematic where I've implemented it. Could someone tell me what I've done wrong?

Thanks in advance.

[detectormods]

Try increasing the value of the 100 ohm resistor.

[Detectorist#1]

Hi eduardo1979,
In the scheme from detectormods these transistors are connected to RX1 and RX2 signals (post #34) - maybe not directly to the TX coil. Also Uds max of BSS139 is 250V. It is possible to destroy the BSS139 from voltage on the TX coil higher than 250V after switch-off of power MOSFET. Also the use of 3W 1oHm resistor in the source of the power MOSFET is non sense. The real power on this resistor is not more than 0.25W even if the average current is 0.5A (0.5V x 0.5A = 0.25W).

[eduardo1979]

Try increasing the value of the 100 ohm resistor.

I increased the resistance value to 10K and smoke is still coming out. Could this be because I'm using a MONO coil, as described by Detectorista#1?

[detectormods]

That is really strange, I still have mine on a detector and it’s working fine.

The answer is that the Minelab has clamped fly back at 185 volts, so my circuit never sees full flyback, you can use a string of zeners to drop flyback or use higher voltage rated depletion mode mosfet.

[Detectorist#1]

Yes, this indicates that my assumption that overvoltage is the cause of the white smoke on the BSS139 is correct.

[Altra]

I got it to work using DN3535 mosfets. My circuit uses a 180v snubber circuit, so the assumption of overvoltage is probably correct. The transistors were running warm. This blocker circuit works the best out of the ones that I have used. By the way ML didn't invent this current limiter. See fig4 in attached pdf. Photos of the add on blocker pcb and scope shot. Yellow trace Tx snubbed at 190v, Green input opa1611 limited to 1.8v with fets and diodes. Blue output opa1611

[eduardo1979]

Hi Altra. Could you please attach your diagram again, as the PDF file you're referring to doesn't exist?

[eduardo1979]

That is really strange, I still have mine on a detector and it’s working fine.

The answer is that the Minelab has clamped fly back at 185 volts, so my circuit never sees full flyback, you can use a string of zeners to drop flyback or use higher voltage rated depletion mode mosfet.

Thanks for your help!