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-
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this indicates the unit is in "Constant
Current" mode.
( 3) An LED Marked C.V. When on,
this indicates the unit is in "Constant
Voltage" mode.
( 4) An external terminal strip: "C1"-
is connected to C2, when used multi-external VR of voltage control. "C2" is
connected to C1, when used multi-external VR of voltage control. EXT "+"
is connected to the outside VR of voltage control. EXT "-" is connected to the
outside of voltage control. PL is the
connection terminal for parallel operation.
All of the above is directly from
the instruction manual for the
GP-303/305/503/505 power supplies.
If Mr. Lang would like a copy of
the manual he can email me with a fax
number or mailing address. There
would be no charges.
Merrill Hendrickson
wa2cll@aol.com
[#8064 - August 2006]
I want to build a 24 VDC power
supply using the LM723 voltage
regulator, at around 20-30 amps.
I can custom wind a large
transformer for this project, but I need
a schematic for the voltage regulation
section. I would also like to know if I
could use a power MOSFET instead of
a bipolar transistor like the 2N3055. I
believe that using a MOSFET with a
low on resistance should lower the
heat generated. Does anyone have an
idea or schematics to do this?
#1 A construction article and
schematic can be found in the ARRL
Handbook 2005 starting on page
17. 37. It uses the LM723 regulator
you wanted. The supply shown is
adjustable from 20 to 30 volts, so it
satisfies your 24 volt requirement, but
is only 10 amps. To modify it for
20 amps, you can halve the resistance
of R7 and double its wattage, and
double the pass transistor string
Q1-Q5 and R2-R5.
Figure 3 (on the next page) shows
the addition to the pass transistor
string, and the new value for R7.
If you go to 30 amps, you will
need to replace the bridge rectifier
with one rated over 30 amps, double
the value of C1, change R7 to .022
[#7065 - July 2006]
I quite enjoyed reading Russell Kincaid’s response on a simple current
sensing circuit. Another, tougher problem is on measuring the 250V plus DC
voltage. Can Russell or anyone else suggest a circuit that is isolated from the
microcontroller?
#1 Monitoring the
voltage and current of
your 230 to 270 volt
array with isolation to
the PIC was fairly simple with the voltage
monitoring because
a simple resistive
divider may be used.
The main problem is
getting a suitable
voltage into the PIC
so the PIC can
give a reliable and
stable display. I have
found from previous
experience the PIC
A/D converter works
best with voltages
between 1 and 4. 5
volts. Since the resistive divider furnishes . 23 to . 27 volts, it is necessary to
multiply those voltages by 10 to give 2.3 to 2.7 volts for use by the PIC. A
simple eight pin DIP op-amp (LM358) available from Digi-Key was used.
Monitoring the current with complete isolation from the PIC requires a Hall
sensor device (Tamura Open Loop Current Sensor). That device is also available
from Digi-Key (Part # MT7195-ND @ $14.18). Again, since the sensor output is
in volts (i.e., 11 amps = 11 volts out), it is necessary to divide the voltage down
to get it within the best range for the PIC. Using the simple resistive divider, 11
volts in gives 4. 4 volts out — an ideal range for the PIC. In the programming,
you can use some math manipulation to get the correct values displayed for the
voltage and current.
Figure 1
Charles Irwin
Hendersonville, NC
Figure 2
#2 Isolation means a transformer. T1 in this circuit (Figure 2) has a split
bobbin so a primary to secondary short is unlikely. The transformer is rated 230
volts primary and 9 volts secondary, 50-60 Hz. The LMC555 is a CMOS type to
minimize power. It is an astable running about 250 Hz to provide faster
response than if it was running 60 Hz. The secondary voltage is rectified and
September 2006 103
