Q&A
the popular EPIC from
microEngineering Labs (shop around
for the best price). Kits and DIY plans
for PIC burners are also plentiful.
Be aware, though, that not all
PICs are created equal and the HEX
code for the “Hand-held Messenger”
won’t work with just any PIC microprocessor. Make sure you get a burner that supports the PIC16C84 called
for in the project. Here’s a short list of
suppliers:
Dontronics
www.dontronics.com/
diyk96.html
up the cabin’s drafty interior. (Car
heaters in those days left a lot to
be desired.)
If you’d rather not waste the
power, you can build a simple, step-down converter using an LM2678
switching regulator (Figure 7). The
circuit was created using National
Semiconductor’s Webench online
design software ( www.national.
com/appinfo/power/webench/).
The only critical component is the
inductor, which must be rated at 12
µH at 5. 5 amps, like the PM2120-
120K from JW Miller (Digi-Key
#M6184-ND).
the lamp using a rather intricate
formula.
To spare you the calculations,
I’ve assembled a table for a few of
the most popular lamps. If you don’t
find your lamp listed, cross it to the
voltage and wattage columns to find
the inrush thermistor that’s right for
you. The thermistor should be
inserted between the fan and the
bulb so that it doesn’t limit the fan’s
cooling air.
Thanks ... For
Nothing!
EPIC Plus
www.jameco.com
Protecting
Projection Lamps
NOPPP
www.covingtoninnovations.com/
noppp
PICALL, available from
Amazon Electronics
www.picallw.com
Q. Can you provide a circuit to
slowly power-up projection
lamps in video and movie
projectors? These high wattage
lamps are expensive and short lived;
I would bet a slow turn on would
extend the life of the lamps by limiting inrush currents.
Classic Car
Conversion
Mike
via Internet
Q. A friend of mind has an old
Ford that he restored to excellent
condition. Against my advice, he converted the car to 12 volts. The original
radio works fine on the 6 volts it was
designed for. I can change the six
parallel filaments to three sets of two
series filaments (7A7s, 7B6, 7B8,
7C5, and 7Y4). I was wondering, if I
hit the power transformer with short,
12 volt pulses, would it work as if it
was seeing 6 volt vibrator pulses? I
want to take this route before
designing and building a 60 watt
switching power supply.
Merlin Baker
via Internet
A. I assume you are talking about
tungsten filament lamps and not
the halogen or metal halide versions.
The answer is an NTC thermistor.
This thermistor starts out with a high
resistance that decreases as it heats
up from current flow. You can buy
them from Digi-Key for under $3.00.
The trick is to match the thermistor to
Q. Some time ago, I requested a
scramble/descramble diagram
for my 2.4 GHz wireless video camera, but you’ve entirely ignored my
request.
Then, I really wanted an USB
trans-receiver with an USB module
from www.linxtechnologies.com
They are actually a Nuts & Volts
advertiser. So much for helping them
increase their sales by featuring a diagram with their USB module! Are
they aware of your help in not helping
them sell such modules — especially
when they indirectly sign your pay
check?! Once again, you ignored my
request. Maybe other readers like me
would have been interested in
building such a project. Have you
thought of that?
Then, I requested a self-powered
timer for an auto ceramic heater (I
even sent you a JPEG photo of the
heater) to heat up inside a coil and
Lamp
Type
Lamp
Volts
Lamp
Watts
Lamp
Amps
GE Thermistor
Model Number
Cold
Ohms
Hot
Ohms
CLS
120
300
2.5
CL- 80
47
0.6
CZA
120
500
4.2
CL- 50
7
0.13
CZX
120
500
4.2
CL- 50
7
0.13
A. When I converted my ‘ 55 Ford
to 12 volts, I used a 1 ohm, 50
watt resistor in series with the hot
lead of the 6 volt radio. If you remember the cars of that era, the resistor
served double duty by dropping the
voltage from 12 to 6 — and heating
SEPTEMBER 2004
DDB
120
750
3. 6
CL- 30
2.5
0.09
ELH
120
300
2.5
CL- 80
47
0.6
EXR
82
300
3. 7
CL- 70
16
0.3
EHJ
24
250
10. 4
CL- 11
0.7
0.03
Inrush Thermistor Selection Guide
35