Following some inspiration from both Dave Jones' and Martin
Lorton's sites, I decided I will build a constant current dummy
load. This is a nice simple circuit, which is easy and cheap to
build, and very useful, especially if you want to design power
supply circuits. Basically, you dial in a current level, and it
will draw that amount of current, regardless of the voltage.
Much better than just sticking some power resistors in as your
How It Works
Basically, a voltage is set by a multiturn potentiometer (a
single turn pot will work if you prefer). The op-amp buffers
this and sets a voltage on the gate of the MOSFET. This causes
the MOSFET to allow some current through the drain to the
source. The 1 Ohm power resistor helps share the power with the
MOSFET and assists in providing feedback to the op-amp. This
feedback enables the current to stay constant if the voltage on
the 'Load Input' changes.
Update (December 2013)
I have revised the circuit to use a logic level MOSFET. This
means you can use a power supply much smaller than the 24V shown
below in the circuit diagram. Any logic level MOSFET with an
appropriate voltage/current rating should do, but I used an
F12N10L, which is available from Farnell. The power supply is
simply a 9V PP3 battery. Simply swap out the IRF540 for the
F12N10L, and swap the 24V supply for a 9V supply, and it should
An LM324 op-amp was used here. An LM358 will also work. Any
other bog standard op-amp will also probably work provided you
get the power supplies correct.
The voltage input in this circuit goes up to 24V. This is due
to the use of an IRF540 MOSFET. Ideally, a logic level MOSFET
should be used instead, as this will allow a much smaller
voltage to be used (around 5V).
The 1R resistor must be able to dissipate quite a bit of power.
In my brief testing, I found that it could dissipate around 10W,
but even the 50W resistor got hot, so you may want to attach it
to a heatsink, especially if you use a smaller one.
The IRF540 MOSFET was also attached to a heatsink. In my brief
testing, this can get very hot, and I did in fact kill one
transistor when using a heatsink which was too small, so I
swapped it for a bigger one. I used a 2.7°C/Watt heatsink which
is available from Farnell as product code 171-0614 for about
I built it on a breadboard. I had to attach the MOSFET to some
wires as there is no room on the breadboard for the heatsink.
In this photo, the multimeter shows the dummy load drawing
1.02A from a 5V power supply. The oscilloscope shows that there
are no oscillations, hence the circuit is stable. I have used it
up to about 3.4A.
It is quite easy to get this circuit to oscillate. This is
unwanted and can make it difficult to set the current
accurately. If the capacitor in the above circuit diagram is
removed, the circuit will oscillate. I got some fun looking
waveforms on my oscilloscope, which vary as the current is
I tried the capacitor in other locations and still got
oscillations, so settled on having it in its current location.
Click here to see the oscillation
A slightly more permanent build
I built the circuit up on some strip board and added some
banana connections. This was only intended to be temporary till
I make a PCB, but it actually works OK.