adequate. The choice rests mostly on the maximum
current you need to draw for the supply voltages you plan
to test. The higher the light bulb power rating, the higher
the current you will be able to draw in your load.
For example, a single 20W 12V bulb would nominally
draw about 20/12 = 1.67A at 12V. This circuit allows you
to draw less current than this by adjusting the
potentiometer, but not more current.
This is why I paralleled three bulbs as I
wanted to draw at least 4A maximum
at 12V (and I had quite a few unused
bulbs available). However, if you don’t
routinely need to test supplies with this
much current, you may only need one
or two bulbs.
You should also be careful not to
burn the light bulbs by applying
voltages above their rating (above 12V
in this case) for extended periods of
time while drawing maximum current.
You can always place more bulbs in
series if this is a requirement for you, or
use 24V bulbs instead.
Safety Warning: Incandescent light
bulbs get very hot and can burn you!
Make sure you don’t touch the bulbs
during operation, and include a way to enclose them so
the risk of accidental contact is minimized.
As this is a relatively simple circuit using only through-hole components, I decided to build it using a prototype
44 July 2016
ITEM DESCRIPTION SOURCE
U1 78L05 5V Linear Regulator Digi-Key LM78L05ACZFS-ND
U2 PIC12F683 Microcontroller Digi-Key PIC12F683-I/P-ND
Q2 N-Channel Power MOSFET Digi-Key IRF540NPBF-ND
Q1 2N2222 NPN Transistor Digi-Key PN2222AFS-ND
D2 1N4001 Diode Digi-Key 641-1310-1-ND
D1 Red LED
RESISTORS — 1/4W 5% axial carbon film
C1 0.1 μF / 100V Ceramic
C2 10 μF / 16V Electrolytic
C3 0.1 μF / 100V Ceramic
C4 4. 7 nF / 100V Film
C5 470 μF / 50V Electrolytic
C6 680 μF / 50V Electrolytic
C7 0.1 μF / 100V Ceramic
L1 Toroid Core Inductor 220 μH, eBay http://tinyurl.com/zmoutld
4A or higher
RELAY SPDT 10A Relay Digi-Key 255-2065-ND
RV1 10K Multi-turns Potentiometer eBay http://tinyurl.com/zoqs4ru
Light Bulbs (see text) Automotive parts store
■ FIGURE 6. Assembled perf board circuit.