READER-TO-READER TECHFORUM
>>>YOUR ELECTRONICS QUESTIONS
ANSWERED HERE BY N&V READERS<<<
reset button sets the display to 00.
Pushing either counter button once
will cause the display to count up or
down once. Holding down either
counter button will cause the display
to continuously count up or down.
Pushing the flash button will toggle
the display to either flash or stay on.
Make your own display by mounting LEDs onto a piece of 12" square,
1/8" thick plastic such as McMaster
8747K112. To make the display easier
to see, mount a clear, gray-tinted
plastic of the same size over the LEDs,
such as McMaster 85805K26.
Each segment is made up of five
LEDs spaced 0.8" to 0.9" apart, center-to-center yielding 8" to 9" high digits.
LCDs consume far less current, but
finding the sizes you've requested can
be difficult and pricey. Using LEDs
lowers the cost considerably and gives
you more options such as size, colors,
etc. For an indoor environment, use
only diffused LEDs rated about 200
mcd or less.
[#6117 - June 2011]
AC Resistance Needed
How can I find the AC resistance
of the emitter diode of a power
transistor?
AC resistance of the emitter diode
of a transistor (re) can be calculated
by:
re = .026v / Ie
where Ie is the DC emitter current.
This is the solution for the resistance at
room temperature of 25 degrees
Celsius. You can find the emitter
current by measuring it directly (open
the emitter lead and insert the
milliammeter), indirectly, (if there is
a single external emitter resistor,
measure the voltage drop on the
emitter resistor and use Ohms Law to
find the current), or by simulation.
To simulate, download LTspice
(free) and input the transistor circuit
you want to use. Place your cursor
over and around the emitter until you
see the clamp-on ammeter icon, then
left-click. The emitter current will be
plotted.
Bill Bradrick
Pittsburg, KS
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