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Constant current measurement load.



IL02 is a constant current load for testing small power supplies.
The current range is 1 mA to 64 mA in 1 mA steps plus additional 10 mA and 20 mA settings.
The input voltage range is around 1 V to 100 V within the power limitation of the circuit.
The circuit requires a 20 V power supply to work.


This circuit can work with higher voltages than is generally considered safe.
Unless you really know what you are doing, please do not build it ( or keep the voltages within safe levels ).


IL02A schematic.
Fig.1: IL02 schematic.

I+, I- is the current input, GND, +20V is the power supply. Supply current is around 10 mA.
D1 and D5 prevent repairs if ( when ) I connect the board with the wrong polarity.
I1 is a 1 A fast fuse that give some protection in case of a fault. It will not save the FET, but hopefully prevent it from exploding.
The FET shown is an obsolete 600 V, 2.6 A type. Almost any modern TO-220 FET with sufficient voltage rating will work. Its drain-source leakage current should be less than 100 µA and its channel-gate leakage current less than 10 µA.
The maximum current in the FET is 92 mA with all switches on. If you want to use the circuit at high voltages with this current for more than a minute or so, the circuit will require considerable more heatsink than shown on the photo below.
Do not use the circuit as shown with too high voltages as the strip-board will flash over. I will suggest an absolute maximum value of 100 V.
R5 and R6 is a voltage divider that set the voltage at U1B non-inverting input at 210 mV.
With all switches open, the voltage across R1 is the same as the voltage at the lower ( GND ) end of R6 as R5 = R9 and R3 = R6.
Shunting some of the current from R9 to GND will reduce the current in R3 and the voltage across R1 will increase.
As the voltage on U1Bs inverting input is constant, the currents in RS0x..RS8x will add and the output current will be the sum of the values for each setting.
R8 is an offset adjustment for U1B and should be adjusted with a current setting of 1 mA ( all switches off ).
It works with the component values shown, but should be easier to adjust with R7 = 270 kΩ, R8 = 10 kΩ and R14 = 330 kΩ.
J1..J8 can be a pin-row with jumpers or a DIP-switch.
D3, D4 prevents U1B from going into saturation if the current in the FET is below the set current.
R4 allows the circuit to be used with FETs with a very low threshold voltage as it can pull U1Bs output to 0 V while maintaining regulation.
U1C detects if the current in the FET is below the set current and turns on D6.
R13 should be adjusted so D6 just turns off at the highest current setting while U1B is in regulation.
The second LED on the photo below indicates that VCC is present.
C1 is decoupling for U1 and C2, C3 decoupling for U2.
The resistor values shown in the schematic is what I had at hand.
There is a spreadsheet for calculating the resistor values in the Downloads section

Table 1: Resistor values.
 Prototype valuesBest E24 values
R3 = R614.3 kΩ15 kΩ
RS0287 kΩ11.3 kΩ300 kΩ27 kΩ
RS1287 kΩ11.3 kΩ300 kΩ27 kΩ
RS2150 kΩ0160 kΩ5.1 kΩ
RS369.8 kΩ5.60 kΩ82 kΩ910 Ω
RS434.8 kΩ2.94 kΩ36 kΩ5.6 kΩ
RS518.7 kΩ200 Ω20 kΩ750 Ω
RS69.31 kΩ147 Ω10 kΩ390 Ω
RS730.1 kΩ3.48 kΩ36 kΩ910 Ω
RS815.0 kΩ910 Ω16 kΩ1.5 Ω

Table 2: Current at 5 V and 40 V. The circuit is adjusted at 40 V, 1 mA.
Nominal currentCurrent at 5 VCurrent at 40 V
1 mA1.00 mA1.00 mA
2 mA2.02 mA2.01 mA
3 mA3.02 mA3.01 mA
5 mA5.00 mA5.00 mA
9 mA8.99 mA8.97 mA
17 mA17.0 mA17.0 mA
33 mA33.1 mA33.1 mA
10 mA10.0 mA10.0 mA
20 mA20.0 mA20.0 mA

IL02 photo.
Fig.2: Photo of the board.


Download a spreadsheet for calculating the resistor values here.

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Copyright © Poul Petersen 2018. First published: 20181007, Last update: 20200918. Valid HTML!