Guru
Joined: 02/02/2017
Location: AustraliaPosts: 1480
| Posted: 03:33am 31 May 2026 |
my choke tester:
A simple thing to do, that will be a good exercise for the skills
you need to build upon if you want to make your own inverter.
basic operating idea is to have a variable length pulse (5V)
come out of the microcontroller and use it to switch a MOSFET completely ON
and so put a voltage across the inductor under test.
My circuit uses a TLP250 opto coupler to obtain the required 12V drive to fully switch on
the FET. 5V is not enough.
I need 2 signals, which are displayed on the DSO.
Yellow trace is the 5V pulse from the Nano uC
Light Blue is the voltage across a 0.02 Ohm resistor that is the last thing
in the circuit that charges the inductor before Ground.
We could test the chokes (or inductors..) at far higher voltages
but things are simple and easy when I do it with a 12V supply.
eg. 2x the voltage is the same as 1/2 the pulse time.
The basic inductance equation used in testing the devices is
L = V x t/I
L is inductance, in Henrys
I is current, Amps
t is time, seconds
V is voltage, Volts
In practice, we will look for the change in current that occurs during a
period of time.
I see the pulse length is 150 uS
I also the the change in current is about 1.5V across the 0.02 Ohm current sense
resistor. This is about 1.5 / 0.02 or 75 Amps (V = IR and of course I = V/R etc..)
Volts is 12.1 V from the bench supply.
L = 12.1 * 0.000150/75 = 24.2 uH
The BK Precision LCR meter has it at 24.6 uH
The current increases linearly and in a nice straight line too.
I like to see measured results illustrate theory so nicely as in the above graphic.
Now when I increase the pulse width a fair bit, we can see the current increase a lot faster over time after about 180 uS. This is when the choke is becoming saturated.
It still IS a choke, with some inductance. But it's value will be a lot less than when it was not saturated.
260uS, 5V over 0.02 Ohms, 12.1V supply
L = 12.1 * 0.000260 / 250 = 12.6 uH
this is the schematic I used for the PCB
Schematic_inductor-tester_2026-05-31.zip
void setup()
{
pinMode(8,OUTPUT);
Serial.begin(9600);
}
void loop()
{
float a;
int v;
v = analogRead(1);
digitalWrite(8,1);
delayMicroseconds(1023 - v);
digitalWrite(8,0);
delay(50);
}
Edited 2026-05-31 13:37 by poida