2010, page 25. I got a lot of feedback
on this one! Many people sent
charts and calculations but the best
(in my opinion) is this link from Rick
L. and Charles Ryberg: http://circuit
calculator.com/wordpress/2006/01/31
/pcb-trace-width-calculator/.
Dear Russell:
Re: February issue, Countdown
timer, page 18. If I read the schematic
correctly (which I may not have), the
minutes display would actually be
hundreds of seconds. I tried to think
of a simple way to preload 60 on the
tens of seconds when borrowing
from the minutes, but it got very
complicated very quickly.
— John Orndorff
Response: You are so right; I
completely spaced it. When the 10s
counter hits zero, it goes to nine
instead of six. My solution to that
problem is Figure A. When the
borrow output goes low, the analog
switch is turned on, which loads six
into the counter. The analog switch
is able to overcome any number that
may be loaded in the program
switch. At startup, the borrow output
is probably low causing six to be
loaded, but then the borrow output
goes high and the program number
is loaded (providing the start switch
is closed). The hex inverter is used
to delay the load command until
after the data is stable. Otherwise,
the loaded number might not be
correct.
Thanks for bringing this to my
attention.
amps. The emitter-resistor dissipates
10 watts so that is 330 watts not in
the heat-sink. Now I need to
calculate the transistor junction
temperature to see how much safety
margin there is. I don’t want to
operate near 200 deg C because the
reliability will be poor and the failure
mode is a short circuit (which could
result in transistors popping all over).
The junction temperature is found
from:
I found a chart (Figure 7) of the
thermal resistance of sheet aluminum.
I was not able to find the thermal
resistance of a TO-3 case to heatsink,
so I did an experiment. I had a 9.5x12
inch sheet of aluminum diamond plate
(hardware store item); I mounted an
unknown PNP transistor with heatsink
compound and 4-40 screws, and
tightened it as much as possible (see
Figure 8). At 45 watts, the case
temperature was 84 deg C and the
heatsink was 81 deg C. I figure this is
3/45 = .07 deg C/watt. The area of my
heatsink is 736 sq cm which I find on
the chart in Figure 7 to give Rha =
1.5. The ordinate is not labeled but I
assume it is deg C/watt. Ambient
temperature is nominally 25 deg C, so:
Mouser part number 280-CR10-0.5-
RC; cost is $0.47 per 10.
INEXPENSIVE DRAFTING
SOFTWARE
Tj = P*(Rjc+Rch+Rha) + Ta
where:
P = Power dissipation in watts
Rjc = Junction to case thermal
resistance in deg C/watt
Rch = Transistor case to heatsink
thermal resistance in deg C/watt
Rha = Heatsink to ambient thermal
resistance in deg C/watt
Ta = Ambient temperature
Rjc is given on the datasheet as
0.87 deg C/watt
QBeing a newbie, I am
trying to identify an
electronic/mechanical
drafting program to buy and
use. For the projects I will doing, I want
to be able to draft out a block diagram
for the components, a schematic for
the electrical diagram, a flow chart for
programming the microcontroller, and
capture all the notes I would like to
post. I just have not seen any software
that jumps out at me. Can you please
give me a recommendation(s)?
Tj = 52*(.87 + .07 + 1.5) + 25 =
152 deg C
— Mr. Lynn Wyatt
This is for the plate mounted
vertically in still air. If you blow air on
it, the temperature can be reduced
considerably or you can use a
smaller heatsink, depending
on your need for reliability.
The 0.5 ohm resistor is
■ FIGURE 7
AI have very limited
experience with that
kind of software; I have
been using AutoCAD’s
Autosketch v.2.1 for 15 years and
have built up a library of electronic
parts plus other parts. I have used
it for schematics and layouts,
mechanical layout,
and architectural
design. The newest
version is 10 and
costs $235 but
version 9 is available
on eBay for under
$50. Perhaps
readers will have
recommendations for
good, inexpensive
software (something
that you have used
■ FIGURE 8
and like, please). NV
26 April 2010
