Adjustable Constant Current Load
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Adjustable Constant Current Load
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An adjustable power load is a piece of test equipment that often comes handy
in the development of a certain electronics projects. For example, when you
are building a power supply, it will come a time when you need to "simulate" a
load to see how well your design performs as the load varies. Adding power
resistors to the output can sometimes do in a pinch, but often you will not
have the right resistor value handy with the right power rating for the test.
This is where an adjustable electronic load comes handy. In this article, I'll
show how you can build one using common components available to the
electronics hobbyist.
The circuit
Constant current "dummy load" designs built around the venerable LM324
quad OPAMP have become quite popular in the hobbyist community. The
basic concept has been around for a long time though, and it's essentially an
OPAMP based current source design. The current source portion of the
circuit below is very similar to Dave's design. It always bothered me though
that the original design didn't use the four OPAMPs in the LM324, and that
there was limited protection in the circuit. Therefore, I decided to use one of
the two "spare" OPAMPs for thermal overload protection and another to
activate a cooling fan. This approach should result in a more reliable design
which is an important consideration for test equipment.
Figure 1 shows the circuit schematic. The "Current Source" portion should be
familiar to anyone that has seen similar designs on the web. The U2D
OPAMP is a voltage follower that ensures the voltage at the 0.1 Ohm resistor
"follows" the voltage applied to its non-inverting input. Therefore the current
at the output through the 0.1 Oh resistor is I = Vi / (0.1) = 10 x Vi. I decided to
use a 0.1 Ohm resistor (instead of the common 1 Ohm design) to allow larger
currents. The smaller resistor value reduces the power dissipated in the
resistor for a given output current. Furthermore, the source-to-ground voltage
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is also reduced thus ensuring that VGS is kept well above the MOSFET's
threshold voltage for all practical operation conditions. I used two 50N06 Nchannel MOSFETs in parallel to reduce the total Ron and improve reliability.
In this design, the maximum current should be about 7A and it is limited by
the 5W resistor I used; not by the MOSFETs. Larger currents can be
achieved with a resistor capable of 10 or 20 W dissipation (which I didn't have
handy). The input voltage should not exceed 60V (maximum VDS for these
MOSFETs). As an added protection measure, I added a power MOV from the
input to ground to protect the MOSFETs against high-voltage transients.
The U2A OPAMP is a voltage follower buffering the "set voltage" from the
resistive divider formed by the 1 M resistor and the multi-turn potentiometer
RV1. If a multi-turn pot is not available, you can use two single turn pots in
series instead (100K and 10K for example). The U2A output drives another
resistive divider before driving the panel meter. This was needed in my case
to "calibrate" the panel meter independently for voltage and current readings.
If only one reading is needed, this portion can be skipped. The switch SW2
switches between voltage and current readings. I found this to be very useful
in normal operation so you can quickly calculate the power dissipation.
Switch SW2 allows you to see the current "programmed" by the resistive
divider even if no load is connected. This is a "convenience" feature I saw
implemented in another design posted in the eevblog forum that turns-out to
be quite important. Without this switch, one cannot know exactly the
"programmed" load current until the input terminals are connected. In the
best-case scenario this is just an annoyance. In the worst-case scenario it
can actually be quite dangerous for the external circuit, as you can
inadvertently apply an excessive load to your supply under test !
As mentioned in the introduction, the two "spare" OPAMPs are used for
protection and cooling fan control. U2C forms a simple comparator between
the voltage set by the thermistor and R8 voltage divider and the R5, R6
voltage divider. The hysteresis is controlled by the positive feedback provided
by R4. The thermistor is placed in contact with the MOSFET's heatsink and
its resistance decreases as the temperature increases. When the
temperature exceeds the set threshold, the U2C output goes High. This
turns-on the BS170 MOSFET and immediately forces the control voltage at
the U2D input to ground thus shutting down the current source and protecting
the circuit. I calculated the threshold set by R5 and R6 for my particular
application based on temperature measurements at the MOSFET case. For a
more generic approach, you can replace R5 and R6 with an adjustable trimpot and set the right threshold for your design. Ensure that the protection trips
when the MOSFET's junction temperature is slightly below the absolute
maximum rating specified in the data-sheet. In normal operation, the BS170
is OFF and very little current flows through the Drain (leakage current mostly)
so it doesn't affect the circuit. The LED D2 signals the user when the overload
protection is activated and was mounted in the front panel.
The U2B OPAMP is also a voltage comparator with hysteresis and is used to
control a 12V fan (re-used from an old PC power supply). A 1N4001 diode
protects the BS170 MOSFET against inductive kick-back voltages. The
temperature threshold for the fan activation, controlled by RV2, should
obviously be lower than the overload protection threshold. One could argue
that the whole fan control circuit is not needed and that the fan should always
be on. While this is certainly a valid approach, I prefer not to deal with the fan
noise when the power dissipated in the load is low and found this circuit to
accomplish this goal quite nicely.
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Lessons Learned
I decided to build this circuit in a simple Perfboard rather than designing a
dedicated pcb. In retrospect, this was a mistake. As I later learned, the
parasitic capacitance in this layout made the circuit quite susceptible to
oscillation. To understand how to "tame" the oscillation, I ended-up reading
quite a bit on stability as a "refresher course", and actually learned a bit
(there's always a silver lining). For those interested in the topic, I recommend
Ron Mancini's excellent Apnote "Op Amp Stability and input capacitance"
from Texas Instruments. R7 and C5 was my first approach at increasing the
phase margin but proved insufficient. I ended-up using a "heavy-handed",
dominant-pole compensation technique by adding capacitors C6 and C7 to
the 50N06 Gates. This may be overkill in your particular design and layout,
but it's an effective solution when everything else fails.
The second lesson I learned is related to ground currents (which could have
been avoided with a good PCB as well). A "mysterious" increase in the
measured current in the panel meter was seen at first whenever the fan
activated. Since the fan is not powered from the load input, this was quite
unexpected. After some debugging, I found that the ground return current for
the fan was flowing through the same path as the ground return for POT1.
Since the voltage at this potentiometer is very small (a side effect of the
divider network and the small 0.1 Ohm sense resistor I chose) this was
adding a voltage drop through the ground plane that was visible in the panel
meter. Adding an additional ground path directly from the ground of Q2 to the
power ground input solved the issue. This is part of the fun in building these
circuits; you learn a lot by making mistakes!
Assembly
I'm not particularly "mechanically inclined", but I'm quite proud of the way the
assembly turned out this time. I re-used an old parallel-port switch aluminum
box as an enclosure for the project. This is a very good quality box with plenty
of space for components. I used some old AC/DC adapters to supply the 12V
for the main circuit and the 9V for the panel meter (yes; it's one of those
meters that requires a separate ground so I couldn't use a LM317 to generate
the 9V from the 12V). The front panel assembly is depicted in figure 5. The
panel meter is on the left whereas the multi-turn pot is on the right. Notice the
big MOV soldered right at the input terminals to prevent voltage surges from
propagating further down the circuit.
Putting it all together
Here's a typical test setup (see Figure 8). The DUT is the power supply on
the right set for 5V. The load in this example is drawing only 0.49A. Using the
"measurement" terminals, I connected a multimeter right at the load input, so
that load current and voltage are monitored simultaneously. This is a
relatively simple and fun project to build. It should prove useful in my lab for
years to come.
4/12
5/12
6/12
7/12
Accurate LC Meter
PIC Volt Ampere Meter
Build your own Accurate LC Meter
(Capacitance Inductance Meter) and
start making your own coils and
inductors. This LC Meter allows to
measure incredibly small inductances
making it perfect tool for making all
types of RF coils and inductors. LC
Meter can measure inductances
starting from 10nH - 1000nH, 1uH 1000uH, 1mH - 100mH and
capacitances from 0.1pF up to 900nF.
The circuit includes an auto ranging
as well as reset switch and produces
very accurate and stable readings.
Volt Ampere Meter
measures voltage of 0-70V
or 0-500V with 100mV
resolution and current
consumption 0-10A or
more with 10mA
resolution. The meter is a
perfect addition to any
power supply, battery
chargers and other
electronic projects where
voltage and current must
be monitored. The meter
uses PIC16F876A
microcontroller with 16x2
backlighted LCD.
8/12
60MHz Frequency Meter /
Counter
1Hz - 2MHz XR2206 Function
Generator
Frequency Meter / Counter
measures frequency from 10Hz
to 60MHz with 10Hz resolution.
It is a very useful bench test
equipment for testing and
finding out the frequency of
various devices with unknown
frequency such as oscillators,
radio receivers, transmitters,
function generators, crystals,
etc.
1Hz - 2MHz XR2206 Function
Generator produces high quality
sine, square and triangle waveforms
of high-stability and accuracy. The
output waveforms can be both
amplitude and frequency modulated.
Output of 1Hz - 2MHz XR2206
Function Generator can be
connected directly to 60MHz
Counter for setting precise
frequency output.
9/12
BA1404 HI-FI Stereo FM
Transmitter
Be "On Air" with your own
radio station! BA1404 HI-FI
Stereo FM Transmitter
broadcasts high quality stereo
signal in 88MHz - 108MHz
FM band. It can be connected
to any type of stereo audio
source such as iPod,
Computer, Laptop, CD Player,
Walkman, Television, Satellite
Receiver, Tape Deck or other
stereo system to transmit
stereo sound with excellent
clarity throughout your home,
office, yard or camp ground.
USB IO Board
USB IO Board is a tiny spectacular
little development board / parallel port
replacement featuring
PIC18F2455/PIC18F2550
microcontroller. USB IO Board is
compatible with Windows / Mac OSX /
Linux computers. When attached to
Windows IO board will show up as
RS232 COM port. You can control 16
individual microcontroller I/O pins by
sending simple serial commands.
USB IO Board is self-powered by
USB port and can provide up to
500mA for electronic projects. USB IO
Board is breadboard compatible.
10/12
ESR Meter / Capacitance /
Inductance / Transistor Tester
Kit
ESR Meter kit is an amazing
multimeter that measures ESR
values, capacitance (100pF 20,000uF), inductance,
resistance (0.1 Ohm - 20
MOhm), tests many different
types of transistors such as
NPN, PNP, FETs, MOSFETs,
Thyristors, SCRs, Triacs and
many types of diodes. It also
analyzes transistor's
characteristics such as voltage
and gain. It is an irreplaceable
tool for troubleshooting and
repairing electronic equipment by
determining performance and
health of electrolytic capacitors.
Unlike other ESR Meters that
only measure ESR value this
one measures capacitor's ESR
value as well as its capacitance
all at the same time.
Audiophile Headphone
Amplifier Kit
Audiophile headphone amplifier
kit includes high quality audio
grade components such as Burr
Brown OPA2134 opamp, ALPS
volume control potentiometer, Ti
TLE2426 rail splitter, Ultra-Low
ESR 220uF/25V Panasonic FM
filtering capacitors, High quality
WIMA input and decoupling
capacitors and Vishay Dale
resistors. 8-DIP machined IC
socket allows to swap OPA2134
with many other dual opamp
chips such as OPA2132,
OPA2227, OPA2228, dual
OPA132, OPA627, etc.
Headphone amplifier is small
enough to fit in Altoids tin box,
and thanks to low power
consumption may be supplied
from a single 9V battery.
11/12
Arduino Prototype Kit
Arduino Prototype is a spectacular
development board fully compatible with
Arduino Pro. It's breadboard compatible so
it can be plugged into a breadboard for
quick prototyping, and it has VCC & GND
power pins available on both sides of PCB.
It's small, power efficient, yet customizable
through onboard 2 x 7 perfboard that can
be used for connecting various sensors and
connectors. Arduino Prototype uses all
standard through-hole components for easy
construction, two of which are hidden
underneath IC socket. Board features 28PIN DIP IC socket, user replaceable
ATmega328 microcontroller flashed with
Arduino bootloader, 16MHz crystal
resonator and a reset switch. It has 14
digital input/output pins (0-13) of which 6
can be used as PWM outputs and 6 analog
inputs (A0-A5). Arduino sketches are
uploaded through any USB-Serial adapter
connected to 6-PIN ICSP female header.
Board is supplied by 2-5V voltage and may
be powered by a battery such as Lithium
Ion cell, two AA cells, external power
supply or USB power adapter.
200m 4-Channel
433MHz Wireless
RF Remote Control
Having the ability to
control various
appliances inside or
outside of your house
wirelessly is a huge
convenience, and
can make your life
much easier and fun.
RF remote control
provides long range
of up to 200m / 650ft
and can find many
uses for controlling
different devices, and
it works even through
the walls. You can
control lights, fans,
AC system,
computer, printer,
amplifier, robots,
garage door, security
systems, motordriven curtains,
motorized window
blinds, door locks,
sprinklers, motorized
projection screens
and anything else
you can think of.
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