I got a 20 Watt solar panel to play with from eBay. I wasn’t sure if it could deliver the rated 20W. I had long suspected the transmission loss in the wire was very significant. If I had a constant current load I could test the power delivered by my solar panel, characterize my other power supplies, test batteries and do a whole range of stuff with it. Lab constant current loads are expensive and there were no alternatives available. I really liked the concept of a constant current load and what I could do with it, so I decided to build my own
I followed Dave’s guide (eevblog) on YouTube on how to design a constant current load. I liked the basic design but I decided to add a micro-controller and an LCD to the design to add other functionality
Very simply put, this circuit uses an opamp and a mosfet as a voltage follower circuit. I can dial in any current I want and the circuit will adjust itself to draw that much current from the power supply regardless of the supply voltage. It can be 3 Volts, 5 Volts, 15 Volts whatever it might be and the load will adjust itself to draw the selected current
I designed the circuit to have the following features
- To draw up to 2A of current
- Dissipate up to 25 watts of power
- Connect a supply up to 30 volts
- To have an LCD panel to show
- Current drawn
- Voltage across the load
- Power dissipated in the load
- Recommend to turn on the fan if a power dissipation increases
- Give warnings if power/current/voltage range is exceeded
- Give the user the choice to run the circuit from the load supply thereby eliminating the need for an external power supply
I designed and built the prototype on breadboards, extensively tested the design. Then I designed a PCB of the circuit in eagle. After extensive design verification, I finally gave the order to have the board professionally manufactured. In 2 weeks the PCB was delivered. I populated the PCB, wrote the code, tested the board and finally, I was done. Phew. This whole process took me over 9 months to complete
I actually learned a great deal in this process which cannot be completely described, yet here are the key ones
- End to end product design (from conceptualization to final product)
- PCB design for manufacture
- How to properly select the specifications of a circuit and then design the circuit according to those specifications
I must say I am very pleased with the final results. It is indeed a work of art!
Constant Current Load demonstration
I of course used it to test the power output of my solar panel. I discovered that I could only draw a maximum of 7w from my 20w panel. Most of the losses were due to the resistance of the wire as well as the dust accumulated on the solar panel which was mounted on the terrace.
More recently I extensively used the CCL to charge Li-Ion batteries which I used for Implementing the room occupancy system. I also used it to discharge the Li-Ion batteries and plot the discharge characteristics and calculate the exact power rating of the battery. It worked flawlessly for the entire time I used it without any hiccups.