>>>READER-TO-READER QUESTIONS AND ANSWERS
batteries tend to wear out. I want to
use the controller for timing zone five
and would appreciate a circuit that
could do this.
The controller outputs 28 VAC as
measured at the zone outputs, but by
the time it reaches the valve box, it
drops by 3V. The ideal solution would
be to have an encoder/decoder circuit
that would work directly off of the
controller. I can then give the fourth
and fifth valves different addresses so
they could share one line. The draw is
about 250 mA at 24-26 VAC for 15
minutes per day.
An alternate approach might be to
use the ground as the fifth wire and
ground the control box to a copper
pipe. However, it is plastic pipe in the
ground that goes to the valve box. If I
simply drive in a ground rod at the
control box for my ground, will this
solution work as well?
It would be impossible to get AC
electric out to the valve box. If I used
the second approach, a possibility
exists that I can tie in the main house
[#6076 - June 2007]
Is the new Microsoft operating
system Vista back compatible? I have
many programs on Astronomy from
Windows 98 vintage, on floppies, that
cannot be duplicated.
Vista is back compatible. To run
the files in compatibility mode, right
click on the files and click properties.
Next click on the compatibility tab.
Check the box before “Run this
[#7071 - July 2007]
I want to run a 24 VAC camera off of a 12 VDC battery.
It is rated 6W at 24 VAC.
#1 The camera will also run off of a DC voltage, as the
camera's internal rectifier and filter convert the AC voltage
to a DC voltage.
The equivalent DC voltage is calculated as follows:
24VACrms x 1.414 = 34Vdc, round up to 36V (Vout)
to compensate for the diode drops.
The current required is 6W/34Vdc = 166 mA, round
up to 200 mA (Iout).
Vin = 12Vdc, Min.= 10V Max = 14V
Now that we have our requirements, we need to boost
the 12 volt battery to 34V.
The easiest way is to use National Semiconductor's
Power WEBENCH software, which can be found at
www.national.com. It's free to use!
Inputting the Vin, Vout, and Iout parameters will bring
up a list of components for Boost and Flyback converter
topologies. The easiest one is the LM2586 Simple Switcher
in the Boost configuration.
Note: When a boost regulator is shut down, the input
voltage will appear at the output. If this is not desirable,
add a switch in series with the battery.
WEBENCH will provide the schematic, bill of materials,
and provide circuit and thermal analysis.
You will also be able to get up to five free samples of
the LM2586-ADJ, or you can purchase an unassembled
custom kit at a reasonable price.
The resistor values can use the standard 5% tolerance,
as the voltage requirements are not critical. R1 = 43K ohm,
Rc = 2.0K ohm
Cin1: 22 μF 25V ESR:<1ohm
Cin2: 0.1 μF 50V ceramic
Cc: 0.27 μF 50V ceramic
Cout: 100 μF 50V ESR:< 340 milliohms
D: 1A 50V Ultra-fast Diodes, Inc., B180-13 or equivalent
L: 470 μH 3.3A DCR< 0.12 ohms JW Miller PM2120-471K
R1: 42.2K ohm
R2: 1.5K ohm
Rc: 2.05K ohms
Lake Zurich, IL
Float : ON
#2 The "easiest" way would be to
get a very inexpensive 12 VDC to
120 VAC inverter which are available
almost everywhere ($20-$25). About
the smallest is 50 watts. (You can use
it for other things, too.) Then get a 24
VAC transformer and run off it for
your camera supply; 24 VAC control
transformers can usually be
scrounged off old central HVAC units
which are also everywhere. Or, if all
else fails, RadioShack has them for
Schematic for LM2586
Scott City, KS
October 2007 101