A simple saturation meter to estimate the saturation current of inductors.

This project is a quick and dirty implementation, and is not intended for production use. I need to measure the saturation current of some inductors for a project, so I built this tool to help me do that.

I used only parts I had lying around, and I want to use isolation milling, so the design is not optimal.

To protect the circuit, I designed a simple enclosure that can be 3D printed. and laser cut.

All files are in the cad folder.

• bottom.stl

• sides.stl

• top.svg (for laser cutting red 0.2mm stroke)

It is based on a discussion on the EEVblog forum: https://www.eevblog.com/forum/beginners/help-with-testing-inductor-saturation-current/ The idea is to discharge a capacitor through the inductor and measure the voltage and current across the inductor. When the inductor saturates, the dI/dt increases, (ie L decreases).

To measure the voltage and current, I use a STM32F103 with its built-in ADC. I choose to not use any op-amps to keep the design simple.

I use two N-channel MOSFETs (BUK556), they are driven by the IR2112, but not in the usual half-bridge configuration.

The low-side MOSFET (Q2) is ON before the high-side MOSFET (Q1) in order to charge the bootstrap capacitor (C12). Then Q1 is turned ON to let current flow through the inductor under test.

Almost any N-channel MOSFET with a Vds rating of at least 30V and a low Rds(on) will work. For the bootstrap capacitor (C12), I recommend at least 1uF. Because the ON time of Q1 is relatively long compared to a typical half-bridge application, a larger capacitor will help maintain the gate voltage.

I don't have a proper shunt resistor, so I made a shunt using PCB traces. The shunt resistor (R9) is designed to have a resistance of 60mΩ with a 1oz PCB. Keep in mind that the copper thermal coefficient of resistance is poor, so the resistance will vary with temperature. But because the current is measured over a short period of time, the temperature rise should be minimal.

You will need to calibrate the shunt resistor by measuring the voltage drop across it at a known current, because the actual resistance may vary due to manufacturing tolerances (especially with isolation milling).

Two LEDs give visual feedback about the operation of the circuit:

• D5 (green): indicates that the circuit is powered.
• D6 (yellow): inidicates that the capacitor bank is being charged.

The layout is designed for isolation milling. It complies with the following rules:

• Minimum trace width: 0.2mm (recommended: 0.3mm)
• Minimum clearance: 0.3mm
• No plated through holes

I choose a single layer design to keep it simple, therefore traces routed on the top layer are straps. Nevertheless, I added front copper plane because the LPKG S63 I use is able to do double layer PCB. Because I cant do vias, I had to add some jumpers to connect the top and bottom layers.

Use the interactive BOM viewer

The firmware is available on its dedicated repository: https://github.com/AntoninPvr/saturation_meter_firmware

• Add test results