Project: Active load with microcontroller – Part 6: Validation and conclusion

This is a follow-up post to my previous project page Part 5 (PC software).

This is the last post concerning this project. I have done some tests with the final hardware and software. Furthermore I have written a small conclusion to this project.

Transient response

An interesting test is the transient response on a sudden change of the setpoint current. In this case I stepped the setpoint current from 2 A to 0 A. The following picture shows the voltage across the shunt. You can see a voltage drop of 200 mV (corresponding to 2 A) within a time span of around 20 ms. The time could be reduced by decreasing C18 and C21 in the schematic. The latter one should be adjusted carefully because a too small capacitor may destabilise the control loop.

Step response from 2 A down to 0, measured across 0.1 Ohm shunt.
Step response from 2 A down to 0, measured across 0.1 Ohm shunt.

 

 

 

 

 

 

Heatsink and temperature

One requirement of this project is a permanent power dissipation of 12 W. The heatsink has a temperature coefficient of 5.3 K/W. That leads to a temperature rise of 63.6 °C at 12 W. Considering a room temperature of 20-21 °C, the steady-state temperature should be around 85 °C at the heatsink.

I have attached a screenshot of my Active Load Tool while doing this little experiment. You can see that the steady-state temperature of 85 °C is reached after about 13-14 minutes. It is worth mentioning that the junction-to-case coefficient of the power transistor is 2.5 K/W. That is another temperature rise of 30 °C at the junction compared to the measured heatsink temperature which results in 115 °C in the FET. Considering the maximum operating temperature of 150 °C, it is a good idea to keep some headroom. So 12 W permanent power dissipation seems to be a good choice – also because the overtemperature protection is not yet triggered.

Active Load: Average dissipated power of 12W over a time period of around 13 minutes. Steady temperature is 85°C.
Active Load: Average dissipated power of 12 W over a time period of around 13 minutes. Steady-state temperature is 85 °C.

During this experiment I noticed a small design flaw. The actual current flow (as measured by an external ampere meter) decreased by an amount of around 20 mA while performing this temperature test. That’s because of the ambient temperature rise around the heatsink which heats up the components of the control loop sitting directly beneath it. Due to the temperature coefficients of the components (opamp, resistors, MCU) the measurements deviate from the “exact” (calibrated) ones. It would have been better if I had placed the heatsink far away from other devices. But anyway… the results suffice my use cases.

Current draw

There is not very much to say here. The minimum current draw (setpoint current set to zero) is around 5-10 mA (depending on tolerances in the components) as written before.

The maximum allowed current of 3 A can also be sinked by the device. Keep in mind that the temperature may rise very fast in this case when the voltage is greater than 5 V. 😉

Conclusion

This has been a fun (although small) project. Nearly everything worked from the beginning and the results show that the requirements could have been fulfilled.

Of course nothing is perfect. If I were to redesign the hardware I would move the heatsink to a more remote location so that the components do not heat up so much.

Another thing which could be added is a relais or another FET which disconnects the source from the load. In this case the minimum current draw from the source would go from around 10 mA to zero.

All in all I think this device (remote controlled from a PC) allows for some useful applications in the electronics laboratory.

Don’t forget: Everything is in my GitHub repository.

So thanks for reading. If you have some questions regarding the design, don’t hesitate to ask. Also feel free to drop me a line if you decide to build or improve this design. 🙂

André

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