September 2017 61
load! Let this circuit “run”
for a while and test the
7805 case temperature
with your fingertip (carefully).
2. If you have a thermometer or temperature measuring probe, take a
temperature reading of the case. Calculate the junction temperature of the
regulator by using the datasheet’s value for θja: Tj = 0.7W x 5°C/W = Tambient +
3. 5°C. The case should be just noticeably warmer than the surrounding air.
3. Attach the 5Ω load and turn on the power. The case of the regulator
will get QUITE hot, so don’t touch it. The regulator has to get rid of 7V x 1A =
7W! What is Tj now (Tambient + 35°C) The regulator will quickly enter thermal
shutdown where it cuts off the output current at high temperatures. You will see
the output voltage drop as the IC shuts down. This is a feature — not a bug — as
it keeps the IC from burning itself up and often whatever it’s connected to as
well.
4. Attach a heatsink as illustrated in the figure. If you don’t have insulating
hardware, be sure not to let the heatsink touch any other part of the circuit since
it’s connected to the regulator’s case directly. Turn on the power once again
and check the temperature one more time. Cooler, isn’t it? If you can measure
temperature, calculate Tj.
5. Experiment with different input voltages, types of heatsinks, scraps of
printed circuit board material or metal, a metal enclosure, whatever’s handy.
Measure the temperature with a fan blowing on the heatsink. Put the circuit
under a cover to simulate an enclosure with no air flow.
You’ll quickly get a feel for what works as a heatsink and what the effects
are. NV
Beyond the Breadboard
Using heatsinks is just one aspect of building your own electronic stuff. As you
gain experience, bit by bit, you’ll find yourself tackling more advanced projects.
To help you get on the way to some serious bench time, check out the ARRL’s
Tech Portal at www.arrl.org/tech-portal and the Circuit Construction page at
www.arrl.org/circuit-construction. There are tons of good tips for electronic
aficionados just getting started and moving beyond the breadboard stage of
building.
- USB
- Ethernet
- Web server
- Modbus
- CNC (Mach3/4)
- IO
- up to 256
microsteps
- 50 V / 6 A
- USB configuration
- Isolated
- up to 50MS/s
- resolution up to 12bit
- Lowest power consumption
- Smallest and lightest
- 7 in 1: Oscilloscope, FFT, X/Y, Recorder,
Logic Analyzer, Protocol decoder, Signal
generator
- up to 32
microsteps
- 30 V / 2. 5 A
- PWM
- Encoders
- LCD
- Analog inputs
- Compact PLC
www.poscope.com
PoScope Mega1+
PoScope Mega50
s
- LCD
- up to 32
; FIGURE 3. The three-terminal voltage regulator
circuit for experimenting with power dissipation.
Insets show how to attach the regulator’s TO-220
package to a heatsink. The circuit uses a power
resistor as the load, and power is supplied by the
clip leads.
