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Advertising Index
ADM Instrument Engineering....... 39
Altronics.................................. 68-71
Australasian Exhibitions & Events.. 27
Elcomtel..................................... 103
Element14...................................... 3
Embedded Logic Solutions.......... 80
Emona Instruments........................ 5
Futurlec........................................ 10
Hare & Forbes.......................... OBC
Ask SILICON CHIP
. . . continued from page 103
that this circuit would meet my needs.
Accordingly, I built the circuit on a
Veroboard layout.
I have doubled-checked, even-triple
checked, my layout and can find no
fault, however even with a modest
load of half an amp the 7812 becomes
so hot it is impossible to touch it
within 15-20 seconds. Surely this is
not normal? I have checked the pinout
of the TIP2955 and the 7812 and as
I mentioned earlier, cannot fault my
layout connections.
I know this is presumptuous of me
but is the circuit and component listing correct to provide the resultant
12V at up to 3A? Could you perhaps
suggest an alternative power supply,
published by SILICON CHIP, that I could
use? (K. J., via email).
• That Jaycar current boost circuit is
fairly standard (see www.eleccircuit.
com/boosting-regulator-current-foric-78xx-by-mj2955/).
The 3.3Ω resistor between the base
and emitter of the TIP2955 means that
there will be 0.6V across the resistor
when the transistor is conducting, allowing the transistor to bypass some of
the current that would otherwise pass
through the regulator to the output.
When the transistor is conducting, the
current supplied by the regulator will
be 0.6V/3.3Ω or 182mA.
Whether the regulator runs hot or
not when delivering current depends
on the voltage difference between the
input and output. If you are feeding
in, say, 15V to the input for a 12V
output, there is about 3V minus the
0.6V drop across the 3.3Ω resistor.
The regulator then dissipates 2.4V x
182mA or 437mW.
104 Silicon Chip
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Harbuch Electronics..................... 95
Instant PCBs.............................. 103
Jaycar .............................. IFC,49-56
Keith Rippon .............................. 103
KitStop.......................................... 10
LED Sales.................................. 103
Notes & Errata
Low Energy Developments........ 103
CLASSiC DAC, February-May
2013: the element14 part number
listed for the AP5002S IC was
wrong (page 41, April 2013). It
should be 1825351.
Mikroelektronika......................... IBC
Demonstration Circuits for Human Colour Vision, July 2013: The
RGB LED specified for the circuit
of Fig.3 is a common cathode type
and its Jaycar catalog number is
ZD-0012. Do not use the ZD-0010
which is a common anode type.
Radio, TV & Hobbies DVD.............. 8
Microchip Technology................... 23
Oatley Electronics........................ 79
Ocean Controls............................ 37
Quest Electronics....................... 103
RF Modules................................ 104
Rockby Electronics....................... 89
Sesame Electronics................... 103
Silicon Chip Binders..................... 95
Silicon Chip Online Shop........ 96-97
The thermal resistance of a TO-3
package is typically 4°C/W junction
to case and 35°C/W case to ambient.
The thermal resistance of a TO-220
package is typically 4°C/W junction
to case and 50°C/W case to ambient.
So for a TO-3 package, the regulator will run some 39°C/W x 437mW.
That’s 17°C above the ambient temperature. For a TO-220 style package,
the regulator will run at 54°C/W x
437mW or 23.6°C above ambient.
The 23.6°C above ambient (for a
TO-220 device) would mean that the
regulator tab will be 48.6°C and that is
fairly hot. If you have a higher input
voltage, then the regulator will run
hotter again.
One way to reduce temperature is to
reduce power dissipation within the
Silicon Chip Subscriptions......... 102
Syndetic Pty Ltd........................... 11
Tekmark Australia...................... 7,59
Tenrod.......................................... 81
Verbatim Australia.......................... 9
Wiltronics...................................... 33
Worldwide Elect. Components... 104
xLogic............................................. 8
regulator. To do that, the input voltage
needs to be reduced. Note that you do
require at least 2V between the input
and output of the regulator in order to
maintain output regulation. Otherwise,
use a heatsink to keep the regulator
SC
running cooler.
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