Precision adjustable currentsource with HC595 and Raspberry PICO RP2040

I use from the HC595 shift-register the first 5 bits as DAC and the three other for changing Uref from 2V to 800mV, changing from current to voltage-mode ( I-base / U-gate) and swifting between n-/ p-devices . The output is isolated from the controller (with cheap opto-coupler PC817) and from power with DC / DC converter. . The polarity of the output can be changed with Opto-relais like Omron G3VM61.

For testing I used the rapid developmment board ( look at: https://www.elektormagazine.de/labs/rapid-development-board-for-raspberry-pico ).

I will use the current-source for a transistor curve-tracer.
With this device it would be easy to test devices with different polarity like npn/pnp transistor or
n/p J-Fet or n/p Mos-Fet.

I have used only cheap parts, only the DC/DC 1W converter (5V to 30V isolated) is more expensive (and if you want to change the polarity with electronic switches you nead 5 mos-relais (2-3 €) each.

Description:

5 Input-pins for Gnd, ser.Data, Latch, Clock and + 5V (about 120mA),
Isolation Power with 1W DC/DC converter( different Layouts dependend from producer. and IsolationVoltage),
Isolation logic-inputs with opto-coupler (non inverting !, collector to + 5V !).
5V-Reference ( about 2k Ohm from +30V ), also used +5V for HC595 and Opamp (in currentsource).
Outputs from HC595 to Mosfets (inverters) for switching the resistors from DAC to Gnd (bit 0 to 4).
bit 5 to 7 for swiching Vref (currentsource) from 2V to 0.8V, switching fom base-current mode (bipolar) to gate-voltage mode (jFet/Mosfet) and switching polartity n/p devices.

The 4 Mos-Relais work like a dual-swich, changing outputs :
n-device: output 1 (base) more positive compared witch output 2 (emitter) .
p-device: output 1 (base) more negative compared witch output 2 (emitter) .

The currents go from 0µA, 50µA, 100µA ... to 1.55 mA (reference = 2.0V )
or from 0µA, 20µA, 40µA ... to 0.62 mA (reference = 0.8V )
or from 0µA, 10µA, 20µA ... to 0.31 mA (reference = 0.4V ) (in work!)
The voltages for J-/Mos-Fets are similar, depends from resistor at switch current to source.

Project Elements

RP2040_HC595_setPins_6.zip (ZIP)

test-program for setting pins HC595 with own ShiftOut() (for RP2040)

HC595R.zip (ZIP)

schematic for 4 bit DAC and changeable reference 2.0V for 50 µA incr, 0,8V for 20 µA incr and 0,4V for 10 µA incr.

HC595H.zip (ZIP)

5 bit DAC, changeable Reference from 2.0V (50 µA incr.) and 0.8V (20 µA incr.)

Discussion (3 comments)

Build This Project

Bring this design to life with the Elektor PCB Service, powered by Eurocircuits. Upload the project files and order professionally manufactured PCBs or assembled boards through a proven European production platform.

Supporting KiCad, Eagle, Gerber, and ODB++ formats, the service is suitable for everything from prototypes and validation builds to series production and volume manufacturing.

Made in Europe. Fast. Reliable. Professional.

Fabricate This PCB