Custom High-Power 24V Pulse Induction PI Detector Circuit Review

[Aashif]

Hello everyone, I am building a custom high-power 24V Pulse Induction metal detector and I need your expert advice to review this circuit architecture and find any potential flaws, mistakes, or necessary corrections.
Key Architecture in attached PDF:
- Power Stage: 24V 6S BMS input, 50V 4700µF bulk capacitor, driven by FGH80N60 IGBT (80A, 600V) and IRF3205.
- Regulation: LM317T (+15V for TLP250), L7812CV (+12V logic), and ICL7660S (-12V analog rail for NE5532 dual supply).
- Protection & Damping: MUR1100E and 1N4148 input clamping diodes, 1N4746A gate zener, and a 330 Ohm 50W parallel damping resistor with CBB81 0.1uF 630V film capacitor.
- Timing & Sampling: NE555 timer clock, CD4011B delay window, 2N3904 analog switch, and NE5532 + LM386 audio output.

Please analyze the attached PDF schematic, point out any mistakes, and suggest what needs to be changed or corrected for stable deep-seeking performance.

[Alexismex]

No schematic to analyse?

[Aashif]

currently drawing the schematic manually and will upload it shortly."

[Detectorist#1]

Hi Aashif,
Will be good if you mention the information for desired deepness and size of the targets as aim of this project.
Before our analyze of the design of the schematic, one precaution - the IGBT transistor FGH80N60 is in "end of life" state. Last delivery date from the manufacturer is 08 Apr. 2022. Also, for the "brutal" TX power, maybe 600V transistor will be not suitable. Maybe 1200V transistor will be need. It is very probable to need of use 1700V SiC type transistor in this project and Teflon wire with 2500V isolation voltage for the coil.

[Detectorist#1]

The idea to use 6S LiPo accus for powering of power PI MD is very good. This allows to use cheap and popular BMS charging modules for 6S Lipo. Using of schematic of popular "Polones" DIY project as a base have to be tested. For big distances the size of the TX coil will be 1x1 or 2x2 meters. This coil is impossible to be moved quickly and the lack of EFE samples maybe will be not so essential. EFE signals are proportional to the speed of the moving (the law of electromagnetic voltage generation).

[Aashif]

Hi Detectorist#1,
Thank you for the expert analysis and the solid advice! I completely agree with your precautions regarding the brutal TX power and the 600V limitation.

To answer your question: My ultimate aim is to achieve a detection depth of about 30 to 35 feet. At this extreme depth, I am strictly targeting massive metallic anomalies (like deep-buried chests or large hoards), not small surface targets.

Based on your suggestions and my own project goals, I am implementing a highly modular and upgraded architecture:

1. Transistor & Wire Upgrade: I am replacing the FGH80N60 with a much heavier 1200V IGBT (such as FGA60N120 or IHW40N120) to safely handle the extreme inductive spikes. I will also strictly use Teflon wire with 2500V isolation for the coils.
2. Modular Tx/Rx Coil System: Instead of a standard mono-loop, I am implementing a Dual Tx/Rx coil geometry. Furthermore, the system is designed to be modular with interchangeable coil arrays. Depending on the field requirements, I can plug in 1x1 ft, 2x2 ft, 3x3 ft, or 4x4 ft dual-coil setups.
3. Variable Depth Control: I am integrating a dedicated potentiometer circuit to dynamically scale the TX power/pulse-width. This will act as a "Depth Control" knob, allowing me to manually adjust the penetration depth anywhere from 1 foot for shallow scanning, all the way up to the maximum 30-35 feet range.

I am currently drawing this updated and modular schematic on my PC using EasyEDA. I will upload the visual diagram here very soon for you and others to analyze.

Thanks again for pointing me in the right direction!

[Detectorist#1]

Hi,
Your aim is really very ambitions but Elon Mask shows to all us - the dreams have to be big! I will try to be useful in your efforts.

[Detectorist#1]

Hi,
Do not be afraid from 1700V SiC transistors - they are controlled not more complex in comparison of IGBT transistors. Before appr. 10 years ago, I used this type in my first bipolar PI MD with 24-36V TX voltage MD. Also the price of 1700V SiC transistors is near to the price of 1200V IGBT.

[Aashif]

Hi Detectorist#1,
That is incredibly reassuring information, thank you!

If the control circuitry (gate driving) remains similar and the cost is comparable, it makes absolute sense to go for maximum safety. I will take your advice and upgrade the design to use a 1700V SiC MOSFET instead of the 1200V IGBT. This will give the circuit an indestructible safety margin against the massive back-EMF from the 24V TX and large coils.

It is amazing to know you successfully ran a 24-36V PI system with this setup 10 years ago! Your practical experience is a huge help to my project.

I am finalizing the schematic in EasyEDA with this 1700V SiC update and will share it shortly.

[Detectorist#1]

Some datasheets for 1700V SiC transistors:

[GeoMax]

Hi Aashif,

You wrote in your document the following:
".... Furthermore, it incorporates strict hardware fail-safes, "zero-ohm" potentiometer limits, and extreme transient protection mechanisms to ensure total circuit stability without the reliance on an oscilloscope for final calibration...."

How would you like to achieve the later part of this goal (...to ensure total circuit stability without the reliance on an oscilloscope for final calibration...)?

[Aashif]

Hi GooMee,
Thanks for pointing that out. Achieving total circuit stability without an oscilloscope comes down to building a bulletproof hardware-level protection system rather than relying on manual scope tuning.

Here is how it is handled in this design:
* Precise Current Limiting: Using strict hardware-level thresholds and zero-ohm safety limits to automatically prevent overcurrent conditions before they can damage the power stage.
* Transient Voltage Suppression: Integrating heavy-duty snubbers and robust clamping mechanisms to safely absorb inductive kickbacks from the large coil.
* Self-Regulating Feedback: Designing the gate driver and timing stages with fixed, hardwired safety margins so the circuit self-stabilizes under varying load conditions without needing visual waveform tweaking on a scope.

In short, the hardware itself is engineered to enforce its own safety limits dynamically.

[GeoMax]

Hi Aashif,

okay... but an oscilloscope you will still need to fine tune your electronics for optimal settings. It would be interesting to see a draft of your circuit diagram to determine whether you are on the right track.

Others:
I wouldn't use an ICL7660s in the power supply. Instead, I would use an LT1054 chip. It can handle a maximum input voltage of 15 V.

[Detectorist#1]

Hi,
We look forward to the first version of the schematic. Rest assured, there will be plenty of comments—after all, that is precisely the point of this popular and useful forum! In my practice, I alwis try to design the unbeatable schematic for all possible pot settings - but this is really hard task.