>>>READER-TO-READER QUESTIONS AND ANSWERS
frequency current from the laptops is
getting into the power supply; try
capacitors across the supply line to
suppress transients. It might be
necessary to use a 4,000 µF capacitor
in parallel with a 1 µF tantalum and
0.1 µF ceramic capacitor because
the larger capacitors lose effective
impedance at higher frequencies.
It would also be useful to place
some inductance in series with each
laptop so as to isolate glitches in one
from the other.
#2 You don't have 12 volts in a car.
When a car starts, the amp draw of
the starter lowers the voltage to as low
as 10. 5 volts on a cold winter
morning, followed by the alternator
recharging the battery at 14. 5 volts. I
hope you see why the adapter you
have is unstable. The soluton I use is
the reliable 12V DC to 110V AC
converter. This costs less than $30 and
you use your laptop AC adapter which
also gives some filtering to make sure
you are getting proper voltage for your
New Castle, IN
#3 I have been using the circuit in
Figure 1 to power my HP Pavilion
ze1210 Laptop in my car for some
time now. It works quite well and
never even gets warm. The current
drain is maximum 3. 95 amps but average current is about three amps. I
believe the circuit will power two
laptops without any problem. All the
parts were obtained from Digi-Key
except for the Schottky diode which
was obtained from www.bgmicro.com.
The price of the LT1270A is $16 from
Digi-Key and it will handle up to 10
amps (120 watts). The two 1,000 µF
94 May 2009
caps are low ESR types from Digi-Key,
part number P11223-ND. Total price
for all the parts is approximately $25.
Connect the input to a 12 volt DC
source and adjust the voltage at the
output for 19 volts. A suitable heatsink
is also available from BGmicro, part
number ACS1415 for 49 cents.
[#2096 - February 2009]
Constant Current 30 VDC
I need a schematic for a 30 volt
DC power supply running at a constant
current of 800-900 microamps,
A precise, constant, current
source is easy to build. Almost any raw
supply voltage in the range of 35 to 50
volts can be used. In Figure 2, the
supply current is about 3. 5 mA. A
TL431 precision reference stabilizes
the supply voltage to about 30V as a
shunt regulator. The reference voltage
of approximately three volts is generated through a divider and compared
against a voltage generated by the
load current in R8. A PMOS transistor
then serves as an output stage while
R7 prevents high frequency oscillation
in this stage. The gate current and the
currents flowing into the op-amp
inputs are miniscule, resulting in an
accurate output current.
[#2097 - February 2009]
Sensing AC Fan Motor
I’m looking for an easy way to
sense when my home AC/heater fan is
running (110 volts).
I have an electronic timed pump
air freshener that I want to spray into
the AC/heater plenum ONLY when the
fan is running so it will carry the
fragrance throughout the house.
An easy way to sense airflow is
with a flag attached to a microswitch,
for example, Honeywell V3-23-D8 in
the Jameco catalog. Figure 3 shows
an extension rod and aluminum flag
attached to the microswitch actuator.
The extension needs to be long
enough and the flag large enough to
snap the switch closed in the flowing
air. It must be light enough to snap
back off with no air flow.
Otherwise, a more complex
solution is to use a CT (current
transformer) to measure the current
drawn by the motor. Triad Magnetics
CSE187L-P from Digi-Key (at $2.89)
turns ratio primary to sense 1: 500,
with a suggested burden resistor of
60Ω outputs 110 mV/amp. A 1/4 HP
motor draws approximately 2A; thus,
220 mVAC out of the C T with the 60
burden resistor on the sense winding.
Figure 4 shows a peak detector and
gain of 22 stage to give about 6 VDC
out for 2A in to the CT.