High Voltage Microprocessor Controlled Power Supply
To
Accompany the
MCUTracer
By
Jack Walton and Martin Hebel
Web
Version: Modified March 2, 2004
Summary
and Conclusion:
To
get the most out of the MCUTracer an adjustable power supply capable of B+ and
C- voltages of +400 and –50 is
necessary. The digitally controlled
power supply described in Part II uses
off-the-shelf parts, including high
voltage MOSFETs as the “pass” element
for the B+ supply. For the
example shown here, we used a Heathkit
IP-17 High Voltage Regulated Power supply as the carcass – employing the
chassis, meters and transformers, but discarding all of the control
circuitry. The design of the power
supply is such that you can use it for the basis of upgrading the IP-17 without
the microprocessor if you want.
Co-author Martin Hebel wrote additional code which employs StampPlot,
dedicated supervisory control and data acquisition software (SCADA) for the
BASIC Stamp. The new code for the Basic
Stamp II allows you to control the microprocessor from your PC, specifying the
range of plate and grid voltages and maximum current for the device under
test. An evaluation version of
StampPlot can be downloaded from www.stampplot.com.
The Power Supply
The
power supply consists of 3 modules, a digital to analog controller connected to
the Basic Stamp in the MCUTracer via a 4 conductor ribbon cable, and separate
B+ and C- modules.
The high voltage B+ supply is a variant of that
shown in The Art of Electronics[1]
using instead a pair of beefy IRFPG40 HexFETS. I kept the topology of the Heathkit supply and used a voltage
doubler consisting of a pair of 1N4007 rectifiers, 330uF/450V electrolytic
capacitors and 0.47R current limiting resistors. Instead of a fixed reference as shown in the text, I used a
12-bit 0 to 4.1 volt signal from the
LTC1446 DAC as an adjustable reference to the inverting input of the
opamp. This voltage to the gate of the
first HexFET is biased with an adjustable reference using an LM317LZ linear
voltage regulator, while this isn’t the quietest bias source it was easy to
implement. The trimmer resistor can be used to set Q1’s gate threshold and will
serve to set the “Zero Intercept” for the supply.. The B+ supply in the example shown below is not regulated.
Because the signal to the first HexFET is biased with the LM317LZ and its
associated circuitry, the output of the power supply increases monotonically
with the input from the DAC.
The
C- supply is also a variant of The Art of Electronics design using the International Rectifier
IRFD9110 DIPFET. An Analog Devices AD825AR is used as the error amplifier, and
a Linear Tech LT1014 is used to buffer the error and bias signals used to drive
the gate of the first HexFET. I had expected to use one of the windings of
the Heath power supply transformer to power the C- supply but this proved to be
a little impractical without the use of a resistive divider to bring the
voltage down . Instead I borrowed the
transformer from a wall wart, cracked the case and mounted it securely on
perfboard and spacers on the rear of the IP17 cabinet. I attached the secondary of this auxiliary
transformer to one of the 6.3 volt windings on the IP17 filament transformer. The primary of the transformer now became
the secondary and delivers about 42 VAC to the C- supply rectifier.
The
Controller board houses a voltage doubler circuit, 78L15 and 79L15 regulators
for the operational amplifiers and a 78L05 regulator for the DAC. An LT1013
amplifier is shown buffering the output from the LT1446 digital to
analog converter, and inverting the voltage for the C- Supply. The Linear Technology LT1446 has its own
voltage reference which simplifies construction. The controller board, B+ and C- supplies shown here use a mix of
surface mount and through-hole devices, but through-hole parts could be
employed just as well.
The
schematics for B+, C- and Operational Amplifier/DAC supplies are shown below.
Implementation:
As mentioned at the outset, I chose to cannibalize a Heathkit IP-17 High Voltage Adjustable Power Supply as the carcass for this venture. These supplies routinely show up on Ebay for $30 or so. (If you can’t find an IP-17, there are high voltage supplies made by Eico and Lambda, as well as earlier Heath supplies which could be pressed into service.) The IP-17 has switchable meters for voltage and current, 5-way binding post connectors for B+, C- and 0.0-6.3-12.6 filament voltages. Use of the IP-17 will save a lot of time and metal bending, search for transformers etc. The IP-17 has a 12.0 x 3.7” saddle on which the original and new modular circuit boards rest. The new boards fit right into this saddle.
It’s a simple matter to snip the connections, remove the saddle, drill out the rivets and install the new circuit boards. Connections to the circuitry of the IP-17 are pretty much as they had been, except that the connections to the control potentiometers are removed along with the surplus wiring which went to the original IP-17 regulator board. Since Heath did not include a connection from the chassis ground at the rear of the cabinet to the binding post on the front panel, I added it in my version. In addition, the space once occupied by the 6L6GC regulator tubes has been “nibbled” out and replaced with a fan to cool the high voltage transistors.
The LTC1446 DAC, its power supply and operational amplifiers are mounted on one PCB (this version uses surface mount components), and the B+ and C- supplies on separate PCBs. Connections from the DAC to the Non-Inverting inputs of the Error Amplifiers and the error amplifier power supplies are made via small ribbon cables fitted with 0.100” Molex connectors. Connection between the microcontroller of the MCUTracer and the power supply is also made with a 4-conductor ribbon cable. The controller, B+ and C- supplies, mount right onto the IP-17 saddle as shown in the picture below. Note that in order to allow the filter capacitors (mounted on the bottom of the printed circuit board) to clear the bottom of the IP17 chassis I had to mount the B+ supply on ½ inch insulated spacers.
Where the 6L6GC regulator tubes were in the original were located I mounted a 12 volt fan to cool the pass MOSFETs. On the rear of the cabinet is a small power supply which borrows 6.3VAC, rectifies and filters the voltage for the fan. I also replaced the neon bulbs (they weren’t functional) on the front of the cabinet with a red and green LED to indicate whether Plate or Grid voltages were being measured by the front panel meters. These are powered from the fan supply. Note that heat sinks are used on the B+ supply but these were omitted for clarity:
Printed Circuit Boards:
The topside traces of the controller PCB are shown in green, the bottom side (jumpers) in red:
I make my printed circuit boards “topside
up” with traces on the top and jumpers on the bottom. Illustrated below is the top foil of the controller printed
circuit board:
Graphical User Interface (GUI)
The
most difficult part of the project was to provide an interface which would be
easy to implement and effective in use.
In Part I of this article we used Parallax's StampDAQTM interface between the Basic Stamp II and
Microsoft Excel. For Part II of the
article Martin developed a macro which implements the GUI using StampPlot®, a
more sophisticated program. StampPlot
and the Macro are freeware available on his his website, www.stampplot.com To use StampPlot, Copy the Basic Stamp II
code in the Appendix to your microprocessor, download
StampPlot, install and run once. Then
download the HVSupply macro, save to your desktop, double-click to open and
you’re good to go!
Several “Text Boxes” in which the user can input data are shown in
the screenshot of the StampPlot HVSupply GUI
shown below:
The Plate and Grid text boxes supply data to the Basic Stamp for the two 12-bit words used by the LTC1446 DAC to control the power supply. The PVStep and GVStep text boxes tell the Basic Stamp how to increment the plotting of data. Finally, I Max, highlighted in red, provides the user with a threshold which, if reached, causes the program to stop operation and brings the Power Supply to “StandBy”, bringing all voltages to zero.
Testing the Power Supply:
Do
not yet connect the B+ and C- power supplies to their regulator circuitry. The first order of business it to test the
DAC and Error Amplifier control circuitry to see that they are working
correctly. Enter 400 volts into the
Maximum Plate Voltage and 50 Volts into the Maximum Grid voltage text
boxes, 100 into the Plate Voltage STEP
box and 10 for the Grid Voltage Step Box. Attach oscilloscope probes to each of
the DAC outputs. Power up the press the
CONNECT check-box and Mouse-Click the Run button. Examine the resulting waveforms, they should appear as the ramps shown below:
The
chart above illustrates that with the grid voltage initially set at zero, the
plate voltage will be stepped from 0 to 400 volts (five steps including zero).
The grid voltage is then reduced 10 volts and the plate voltage stepped again.
TESTING THE HIGH VOLTAGE CIRCUITS: In working with high voltage circuits it’s always a good idea to keep one hand in your pocket, or at least out of harm’s way. This will prevent any inadvertent (and catastrophic!) application of current across your heart! The heat sinks should be attached to the transistors using fiber shoulder washers and an insulation pad.
Make the high voltage transformer connections to
their respective regulatory circuits.
Set the trimmer potentiometers to their midpoint positions. Put the midpoint supply voltages (200 Volts
for the B+ supply and 25 Volts for the C- supply) into the respective boxes on
StampPlot. By typing a “1” into each of the Grid and Plate STEP boxes the Power Supply goes into a
“Static Calibration Mode”, that is, the DAC will not ramp, but provide only one
voltage to the amplifier on the power supply board. Attach a digital voltmeter to the B+ and C- outputs on the power
supply. Switch on the power supply and
examine the B+ and C- voltages, click on the CONNECT and RUN checkboxes and the
voltage on the meters should approximate half of full scale. You will undoubtedly have to adjust the
trimmer potentiometers so that the range of voltages covered tracks linearly
the output of the DAC on the controller board. The potentiometer which attaches to the power supply output, the
“Error Amplifier” potentiometer adjusts the slope of the power supply
output. The pot which is attached to
the LM317 or LM337LZ bias supply adjusts the “Zero-Intercept”. A scope is very handy in making these
adjustments, but with a little patience they can be done by hand.
The
output of the high voltage supply versus DAC input, after twiddling with the
adjustment potentiometers is illustrated by the chart below:
Using the MCU Power Supply and MCUTracer:
With the MCUTracer and MCU Power Supply connected,
input the correct plate and grid voltages for the proper range of B+ and C-
voltages. Enter the increment of
voltage steps in the appropriate TEXT BOX. Enter the maximum plate current in
the text box where indicated.
To connect the HV Power Supply and
MCUTracer to the PC, mouse-over and check the “Connect” check-box. Next mouse-over the “Run” button and click
once to begin graphing plate current (Y-Axis) versus Plate Voltage (X-Axis). To
halt operation, mouse-over the STOP! Button and click.
If the Maximum Plate Current value is exceeded during operation the MCUTracer will halt operation, and bring the plate and grid voltages to zero. If this situation occurs, you will have to make the appropriate adjustment in one or more of the parameters try again. As an example, perhaps the Grid Voltage was too close to zero, in which case a different minimum value should be chosen.
You
can use StampPlot to take a JPEG screenshot of the data. You can also save the data and download it
into a spreadsheet program for later analysis or reference. More information on the operation of
StampPlot can be found on the website www.stampplot.com
.
The Basic Stamp Program
Data
from the text boxes for Plate and Grid voltages, increments and Plate current
are read by the PC using the “!Read” function in StampPlot. These values are transmitted to the Stamp
using the “SERIN” command.
The
number of iterations and lines drawn are calculated from user-supplied inputs “PV
Steps” for Plate Voltage steps and “GV Steps” for Grid Voltage Steps. Every time a new Grid Voltage is calculated
another line is plotted in a different color from the previous.
The
code, as written below, will first measure the Plate Voltage and Current for
Device 1, and then plot a small circle on the Chart at this location. When the plate voltage is incremented
another circle is plotted and a line is drawn between the two points. The Basic Stamp then sets the grid voltage
back to its initial setting and performs the same routine for another
device. For the second device, however,
only a circle is plotted. If no second
device is attached it will just plot a bunch of circles along the X-Axis. You can remove the code lines for a second
device if you want, snipping where necessary!
When the program has made one complete iteration for a particular grid
voltage, the grid voltage is plotted just to the left of the
Changes
and improvements in the code will be posted on the StampPlot website, http://www.selmaware.com/stampplot/pubs_products/MCU_Tracer/home.htm
StampPlot
Graphical User Interface Code:
Controller Code:
'StampPlot Graphical User Interface Code:
'Controller Code:
'{$STAMP BS2}
'{$PBASIC 2.5}
CLK
CON 0 ' Clock
Pin
CS1 CON 1 ‘ Chip
Select ADC LT1093
DPIN1 CON 2 ' Data
Pin ADC
MUX CON 3 ' MUX
of ADC
CLK2 CON 5 ' Clock
DAC LTC1446
DPIN2 CON 6
' Chip Select for DAC
CS2 CON 7 ' Data
to DAC
DATUM VAR Word
' Variable Holder for ADC and
DAC
butRun VAR Bit ' Holds
status of RUN Button
butStop VAR Bit ' Stop Button
PlateVMin VAR Word ' Plate voltage Minimum
PlateVMax VAR Word ' Plate Voltage Maximum
PlateVStep VAR Byte '
Plate Voltage Steps
PlateV VAR Word
GridVMax VAR Byte ' Max Grid voltage
from slider
GridVMin VAR Byte
' Min Grid voltage from slider
GridVStep VAR Byte ' Steps to
increment Plate Voltage
GridV VAR Word
Amps VAR Word ' Maximum
plate current from text
ChkRun VAR Bit
V_X VAR Word ' temp X to plot
I_Y VAR Word ' temp Y to
plot
LastX VAR Word ' Last X
LastY VAR Word ' Last Y
Color VAR Nib ' Color
to plot with
Start:
PlateVMin = 0
PlateVMax = 0
PlateVStep = 0
GridVMax = 0
GridVMin = 0
GridVStep = 0
GridV = 0
Amps = 0
DO
DEBUG "!READ
(ChkRun)",CR ' Request run checkbox value
SERIN 16,84,[DEC
ButRun] ' Accept data
PAUSE 100
DEBUG "!STAT NOT
RUNNING",CR
LOOP WHILE (ButRun=0) ' Wait
UNTIL checked
PAUSE 100
DEBUG "!READ (txtPVMax)",CR ' Request TextBox
Max Plate voltage
SERIN 16,84,[DEC PlateVMax] ' Accept data
PAUSE 100
DEBUG "!READ (txtPVMin)",CR ' Request TextBox
Min Plate voltage
SERIN 16,84,[DEC PlateVMin] ' Accept data
PAUSE 100
DEBUG "!READ (txtPVStep)",CR ' Request TextBox
Plate voltage Increment
SERIN 16,84,[DEC PlateVStep]
' Accept data
PAUSE 100
DEBUG "!READ (txtGVMin)",CR ' Request Grid
Voltage Minimum
SERIN 16,84,[DEC GridVMin] ' Accept data
PAUSE 100
DEBUG "!READ (txtGVMax)",CR ' Request Maximum
Grid voltage
SERIN 16,84,[DEC GridVMax] ' Accept data
PAUSE 100
DEBUG "!READ (txtGVStep)",CR ' Request Grid
voltage Steps
SERIN 16,84,[DEC GridVStep] ' Accept data
PAUSE 100
DEBUG "!READ (txtPlateMax)",CR ' Request maximum
Plate Current
SERIN 16,84,[DEC Amps] ' Accept
data
PAUSE 100
AMPS = AMPS * 10 '
Normalize Amps for 10 bit max ADC
output
LastX = 0 : LastY = 0 '
Clear last positions
IF (PlateVStep = 1) OR (GridVStep=1) THEN ' Entering a "1" causes the DAC
to enter static mode
GOSUB StaticRoutine
ELSE
GOTO Main
ENDIF
MAIN:
DEBUG "!STAT
RUNNING!",CR
COLOR = 1
FOR GridV = GridVMin TO GridVMax STEP GridVStep
'V_X=PlateVMin
GOSUB PlateStepper
NEXT
HIGH CS2
LOW CS2
SHIFTOUT
DPIN2,CLK2,1,[0\24]
HIGH CS2
DEBUG "!O butRun = 0",CR ' Clear the
run button
DEBUG "!BELL",CR '
Sound the bell
DEBUG "!STAT DONE!",CR
DEBUG "!O chkRun=0",CR
GOTO Start '
Start over
PlateStepper:
FOR PlateV = PlateVMin TO PlateVMax STEP PlateVStep
HIGH CS1
HIGH CS2
LOW CS2
PAUSE 10
DATUM = GridV * 80 '
Converts desired GV to BITS
SHIFTOUT
DPIN2,CLK2,1,[DATUM\12] ' Send first
12 bits to DAC
DATUM= PlateV*10
' Multiply Plate Voltage * 10 for 12 bit DAC
SHIFTOUT
DPIN2,CLK2,1,[DATUM\12] ' Send second
12 bits to DAC
HIGH CS2 ' Load and enable DAC
' Set
MUX of ADC to Channel 0
LOW CS1 ' Enable ADC
PAUSE 10
SHIFTOUT
CLK,MUX,1,[99\7] ' Sends ADC MUX Selection -- Channel 0,
Unipolar, MSBF
SHIFTIN
CLK,DPIN1,2,[DATUM\10] ' Read Actual Plate Voltage
V_X = DATUM
PAUSE 10
SHIFTOUT
CLK,MUX,1,[115\7] ' Sends ADC MUX Selection -- Channel 2,
Unipolar, MSBF
SHIFTIN
CLK,DPIN1,2,[DATUM\10] ' Read Plate Current
I_Y = DATUM * 250 ' Normalizes Current for 4.1 Volt Reference
V_X=PlateV
PAUSE 10
DEBUG "!fcir ", '
Plot a point at VoltPlat & IP
DEC V_X,
",",
DEC I_Y,
",.7A,",
DEC Color,CR
DEBUG "!LINE
", ' Draw a line between last and
current
DEC V_X,
",",
DEC I_Y,
",",
DEC LastX,
",",
DEC LastY,
",",
DEC Color,CR
LastX = V_X ' Update last values
LastY = I_Y
HIGH CS1
'Read Second Set of Values
LOW CS1 ' Enable ADC
PAUSE 10
SHIFTOUT
CLK,MUX,1,[119\7] ' Sends ADC MUX Selection -- Channel 1,
Unipolar, MSBF
SHIFTIN
CLK,DPIN1,2,[DATUM\10] ' Read Actual Plate Voltage
V_X = DATUM
PAUSE 10
SHIFTOUT
CLK,MUX,1,[103\7] ' Sends ADC MUX Selection -- Channel 3,
Unipolar, MSBF
SHIFTIN
CLK,DPIN1,2,[DATUM\10] ' Read Plate Current
I_Y = DATUM * 200
PAUSE 10
DEBUG "!fcir
", ' Plot a DataPoint
DEC V_X,
",",
DEC I_Y,
",.9A,", '
Size of circle is ".9 Absolute"
DEC Color,CR
IF PlateV = PlateVMax
THEN GOSUB GridText
DEBUG "!READ
ChkRun",CR ' Reads the
Run button and terminates if un-pressed
SERIN 16,84,[DEC butRun]
PAUSE 100
IF butRun = 0 THEN
Terminate
NEXT
LastX = PlateVMin
LastY =0
Color = Color + 1 ' Next color for next plot
RETURN
GRIDTEXT:
PAUSE 10
LOW CS1
SHIFTOUT
CLK,MUX,1,[105\7] ' Sends ADC MUX Selection -- Channel 4,
Bipolar, MSBF
SHIFTIN
CLK,DPIN1,2,[DATUM\10] ' Read Actual Grid Voltage
HIGH CS1
IF DATUM.BIT9 = 1 THEN
DATUM = ~DATUM + 1
'Examine sign
bit, if 1 value is negative
DATUM=DATUM &
1023 '
Mask off bit10 - bit15
DATUM = DATUM/10 ' Normalize output
DEBUG "!TEXT
", '
Plot the actual value
DEC V_X -10,
",", ' -10 to offset horizontal axis
DEC I_Y -4,
",.7A,", ' -5 to offset vertical axisfont
size
DEC Color,
",", ' set color
"Grid V =",DEC
DATUM,CR ' text to plot
RETURN
STATICROUTINE: '
Used to Calibrate DAC or Hold Voltage Constant
' No Data is sent to ADC
' Routine is run until "STOP" box is
checked
PAUSE 100
DEBUG "!STAT Static
Calibration Routine",CR
HIGH CS2
LOW CS2
PAUSE 10
DATUM = GridVMax *
80 '
Convert GridVMax to BITS
SHIFTOUT DPIN2,CLK2,1,[DATUM\12] ' Send first 12 bits to DAC
DATUM = PlateVMax *
10 ' Multiply Plate Voltage * 10 for 12 bit DAC
SHIFTOUT
DPIN2,CLK2,1,[DATUM\12] ' Send second
12 bits to DAC
HIGH CS2 ' Load and enable DAC
DEBUG "!READ
(ChkRun)",CR
SERIN 16,84,[DEC
ButRun] ' Accept data
PAUSE 100
IF ButRun=0 THEN
GOTO Terminate
ELSE
GOTO StaticRoutine
ENDIF
RETURN
FAULT: '
Used if Max Current Exceeded
DEBUG "!STAT Maximum
Current Exceeded!",CR
HIGH CS2
LOW CS2
SHIFTOUT
DPIN2,CLK2,1,[0\24]
DEBUG "!O
chkRun=0",CR
HIGH CS2
GOTO Start
TERMINATE:
HIGH CS2
LOW CS2
SHIFTOUT
DPIN2,CLK2,1,[0\24]
HIGH CS2
DEBUG "!STAT Running
Terminated by Operator",CR
DEBUG
"!BELL",CR:DEBUG "!BELL",CR: DEBUG "!BELL",CR
DEBUG "!O
chkRun=0",CR
PAUSE 4000
GOTO Start