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Adjustable Electronic Load

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Introduction

After I did my Powerbank debunking project , I felt the need for an adjustable electronic load. Next time I need to test the capacity of a powerbank, I can simply hook it up to the electronic load and adjust the amount of current to be drawn from it.


Schematics

The circuit is based on Dave Jone's design, and the load will be limited to a maximum output of 2 amps (Max. Power: 20W). Here's the first draft of the circuit.

R103 is a 10 turn pot acting as a voltage divider, and is used for adjusting the input voltage to the OPamp. The MCP6002 has two OPamps in one package, so I wired the first OPamp as a voltage follower (Unity gain). R102 is a trimpot used for fine adjusting the voltage on input B on the OPamp (and the gate voltage on Q101-Q102). R104 is a pull-down resistor used for grounding the gate of the mosfets when there is no output voltage from the OPamp.

Q101 and Q102 are two N-channel mosfets which are connected in parallel and controlled by the OPamp. R105-R109 acts as a 1 Ohm dummy load, and the voltage drop across the dummy load is used by the OPamp to adjust the gate voltage of the mosfets.

Principal of operation

Have a look at the following picture.

The path highlighted by the red and blue wires show that the second OPamp in the MCP6002 is also connected as a voltage follower. Since it's wired as a voltage follower, the voltage at point A will equal the voltage at point B. For example, if we adjust the pot R103 so that we have 1V at point A, the voltage at point B will also be 1V, and we'll have a 1V voltage drop over the group of resistors (R105-R109). 1V over a 1 ohm resistor equals 1 amp. The OPamp will adjust the gate voltage of the mosfets so that the voltage levels att point A and B are the same.


The power supply to be tested is connected to J102, and the current drawn from it is adjusted by R103. The current will flow through the ammeter (J103), which will display the amount of current drawn with its built-in display. As a safety measure, a polyswitch is connected in series with the J102 connector. If too much current is drawn from the power supply, the polyswitch will break the path by increasing its resistance.

Hardware

In order to test if the circuit works, I created a prototype on a veroboard.

The prototype performed very well, keeping a constant current even if the PSU voltage changes. Since I have verified that the prototype is working, I can move on to creating the PCB layout.



I created the PCB layout in Diptrace. The circuit board has 2 layers, where the bottom layer is a ground plane.


I soldered all the components onto the PCB, and placed the PCB in its enclosure.


The power switch, current adjustment pot, and input jacks will all be connected to the lid of the enclosure, so I had to drill a couple of holes. I also added a couple of holes for reducing the temperature in the enclosure during heavy loads.


I tried a new method when adding text to the front panel. After adding text to the panel with a labelwriter, I used an X-acto knife to cut out all excessive paper, leaving only the letters on the panel.


A thin adhesive film on the front panel will protect the letters from getting scratched. I used an X-acto knife to cut the adhesive film where I have drilled holes.


Finally I added the power switch, current adjustment pot, ammeter, and input jacks to the front panel.


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last edited: 2017.04.01

Components used

MCP6002 OPamp

IRLZ44 N-Ch. Mosfet

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