High Voltage Microprocessor Controlled Power Supply

To Accompany the MCTracerTM

Summary and Conclusion: To get the most out of the MCTracer a precise low noise power supply capable of B+ and C- voltages of +400 and -50 is necessary. To use the MCTracer with least difficulty it would be helpful to control the supply with a microprocessor or PC. To do it inexpensively, we seriously POOGE a Heath IP-17 HV Adjustable Power Supply, removing the circuit board which controls the tube type regulators. Although we used an old IP-17 for this project, the circuitry described in this article can be adapted to a variety of other old adjustable power supplies. Voltage Control is directed via the processor in the MCTracer, or an outboard processor.

In searching for a solution there were several paths we could have taken. I had first seriously considered building a high voltage adjustable switching supply. By adjusting the duty cycle of the switching chip, a precise output voltage could be obtained without the need for heavy iron transformers. This is the technique that Sorensen uses in their high voltage adjustable power supplies. Since the supply was intended for a project which would be published, and probably (hopefully) built by people with varying skill levels however, I tossed this idea pretty quickly. High current switching transients, EMI, RFI, the need to use a PCB, wind your own transformer and the potential for disaster ruled this out. Next I considered a regulated supply with bipolar transistors, the type used in horizontal deflection circuitry. Not a bad idea, but I was worried about thermal runaway. Finally, the design shown here which uses high voltage MOSFET's was settled upon. Happily I have plenty of N and P-Channel MOSFET's to work with, making my task easier. The design shown here is loosely based upon a design in "The Art of Electronics" by Horowitz and Hill who show a regulated power supply in Chapter 6, "Voltage Regulator Circuits" which uses Motorola MOSFET's. As On-Semi, Motorola's successor in discrete products seems to have abandoned high voltage MOSFET's, International Rectifier devices were used in their place.

Instead of spending days spending a graphical user interface I use a simple 4X4 Keypad and LCD screen to allow the user control of the voltages, currents etc. NetMedia makes an device, the "LCD+" which incorporates a microprocessor with 8 10-bit channels, keypad decoder and relay driver for a very economical $59.95. The LCD+ is available from NetMedia at http://www.basicx.com/

The Circuit: Positive (B+) Regulator The Power Supply Derives its control voltages - for the Digital to Analog Converter, Voltage Reference, Operational Amplifier and LCD Display by ripping off the voltage from the filament supplies of the POOGE'd Heathkit IP-17. This power supply is common to the MCTracer and connection is made with an 8 pin 0.100" Molex connector. A 78L05 regulator is used are used to provide +5 volts to the MAX5250 DAC.

The output from the IP-17 high voltage transformer is doubled with a conventional diode-capacitor pair using 330uF, 450 VDC electrolytic capacitors. In the Heath IP-17 much smaller capacitors are used, forcing the regulator circuitry to do most of the work.

The B+ circuit employs a pair of International Rectifier HEXFET's to control the voltage under varying load conditions with N-Channel devices used in the positive regulator. The first MOSFET is simply a voltage amplifier whose gate is controlled by the error amplifier. The drain and source resistors allow the output of the voltage amplifier to swing between the threshold conduction voltage and the positive rail. The heavy lifting is done by the second HEXFET which will swing from 0 volts to the positive rail. Current limiting is provided by Q1 and R8. As current increases the voltage drop across R8 increases, essentially removing drive from the gate and bringing the output voltage down.

An op-amp serves as the error correction amplifier. In this case I used a Texas Instruments TLC2274 quad precision amplifier which I had on hand. You may wish to experiment with the gain of the error stage to provide the most linear output for the 0 to 4.1 Vdc output of the Digital to Analog Converter.

Negative Supply: For the negative supply we take advantage of the second high voltage secondary of the IP-17. The output of this winding is well above that necessary to generate C- of -100 V dc so a resistive voltage divider is used to produce - 50 volts after the rectifier and filter capacitor.

The negative supply is similar in design to the positive supply except that P-Channel (IRFP9240) HexFET's are used. The output from the DAC is inverted in one section of the error amp IC and summed with the error signal in the section. In this way, both the positive and negative regulators operate in the inverting mode. While the IRFP9240 HexFET is overkill in this application since very little current is demanded, I have plenty of them in stock from prior amplifier projects.

The schematic diagram for the B+ and C- supplies as well as the digital to analog control circuitry is shown below:

Control Circuitry (DAC) The voltage reference for the error amplifiers is derived using a Maxim MAX250 10-bit, quad digital-to-analog converter. A 4.1 Volt Reference (LM4040-4.1) is also used for Vref .

The MAX5250 requires three signals, Clock (CLK), Chip Select (/CS, active low) and Data to function. A16 bit data-word is shifted into the MAX5250, most significant bit first, to address the appropriate DAC register, control the loading of register data and transmit the voltage required in a 10 bit word. Since the transmission rates are slow and the distances short, TTL level outputs from the Basic Stamp II and the SHIFTOUT instruction are used to control the DAC. The LCD+ Display also transmits and receives its instructions as TTL levels so conversion to RS-232 is only necessary when the Stamp interfaces the host PC. Power for the DAC, voltage reference and opamp is taken from the filament windings of the IP-17, rectified and filtered. A 78L05 voltage regulator is used to power the DAC as its maximum Vdd is 5 volts.

The B+ control voltage from the MAX5250 is fed to the inverting input of Amplifier "B" of the TCL2274 which simply serves as a buffer. This signal is then fed to the inverting input of Amplifier "C" while the error signal is fed to the non-inverting input.

For the C- circuit, the DAC control voltage is fed into Amplifier "A" of the TLC2274 where it is inverted to provide negative voltage gate drive. Amplifier "D" of the TLC2274 acts as the Error Correction Amplifier for the C- voltage. Both error amplifier outputs are decoupled with 220 ohm resistors.

The B+ error signal is picked up via a three 470K ohm resistor string and a trimpot. Three resistors are necessary since carbon composition resistors have pronounced non-linearities at high voltages. Use ½ watt resistors in this application since ¼ watt units will fail. Because of the lower voltage present at the C- only one resistor and trimpot is necessary.

The Software: The most challenging part of the project was writing the software which makes the Power Supply and MCTracer work together. Data input by the operator from the 4X4 keypad is used to set the range and number of steps for B+ and C- voltages. There are routines which further allow the operator to correct mistakes and confirm the previous settings.

Keypad Entry Function 0 - 9 Numeric Data "*" (Star) Decimal Point (Moves to next set) "#" (Cross hatch) Finish Entering Data A Review (Summarize) Data Entries B Begin Analysis C Send Data to Host PC D End Analysis (or Emergency Stop!)

When the Supply is turned on a greeting screen is displayed and the operator is prompted to enter Grid and Plate Voltages and the steps between each, that is, after entering the data, press "#" to move to the next set. If you want to check the data press key "A" which will summarize your entries. Pressing key "A" again will start the process all over again. Once satisfied with the data entries, press key "B" which starts the process rolling. The data is stored (logged) in external RAM until you press key "C" which sends the data to your PC for display. (Remember to open StampDAQ before pressing "C").

Lastly, you can immediately (i.e. if you see smoke!) end the process by pressing key "D", or pressing the reset key. Either action will remove the B+ voltage via a relay controlled by the LCD+.

The Code

Construction and POOGING the Heath IP-17 The Heath IP-17 High Voltage supply has two transformers, one for filament voltages, the other for two high voltages. The IP-17 use a voltage doubler to provide the unregulated B+ voltage, vacuum tube (or Zener) voltage references for the error amplifier, and 6L6GC's as the PASS element. The problem which I have found with the IP-17 supplies is that the voltages tend to wander a bit with time and temperature. Further, the "C-" voltage control, essentially pulling a voltage from a resistive voltage divider with the supply determined by a gas regulator or zener string, really isn't satisfactory.

The Heath supply, pictured below, has meters for current and voltage and has more than enough real estate to fit a regulated Positive and Negative high voltage, microprocessor controlled supply. Importantly, you won't have to bend any metal to finish your project since Heath did such a nice job twenty or thirty years ago.

To remove the circuit board, clip all wires to the circuit board and unfasten it from the saddle on which it rests. You can drill out the rivets holding the circuit board in place as this will allow you to simply mount the new circuit board in its place. I also removed the octal sockets for the 6L6GC's and used a metal nibbler to fashion a circular opening on which I mounted a small computer fan to cool the MOSFETs.

A voltage doubler circuit was used in the Heath IP-17 and I use a similar setup for the microprocessor controlled supply, using a pair of 1N4007 diodes and 330uF/450V capacitors. Acknowledging that in any capacitative voltage multiplying circuit there will be a certain amount of ripple, I chose values that tradeoff ripple at the maximum load (100 ma) with initial current and expense. The unregulated portion of the C- voltage is conventional save for the resistive voltage divider.

Although I don't use the potentiometers on the front panel of the supply, (I prefer the PC mounted Bourns 15 turn pots) there is no reason why you can't modify the unit, "fixing" the voltage reference at 5.000 volts, eliminating the Digital-to-Analog Converter, and use the potentiometers to adjust the output directly.

The printed circuit board designed for this project fits right into the saddle used by Heathkit. I mounted a Molex Connector on the back of the IP-17 to accept the control signals (and ground) from the MCTracer.

The printed circuit board, reflected, shown below:

The arrows point out the 4 jumpers on the PCB.  The jumpers are shown as a broken lines.  

I suggest that before wiring the entire board you first put the DAC, voltage reference, opamp and associated components into place. Make sure that you can connect between the MCTracer and the Power Supply, and check that the DAC puts out 0 to 4.1 Volts.

If you just want to pooge your IP-17 Power Supply, you can eliminate the Maxim MAX5250 DAC and its associated wiring. Just connect the positive output of the 78L05 regulator to the inverting input resistors of the operational amplifiers. Instead of using the trimpots shown below, you can use the potentiometers of the appropriate value on the IP-17. This power supply will be far better regulated than the stock IP-17 and the voltage will be easily adjustable with the turn of a front panel knob.

The LCD+ from NetMedia The LCD+ provides a 4X20 character LCD with software controlled backlight, keypad encoder, relay driver and 8 channel, 10-bit analog to digital converter. I chose not to use the ADC in the unit, at least for the moment, although this will reduce the cost and complexity of the MCTracer. The LCD+ is available directly from NetMedia at www.basicx.com or through various distributors.

POOGE - an audio-logism, verb transitive meaning the art of Progressive Optimization of Generic Equipment. Horowitz, Paul and Hill, Winfield "The Art of Electronics", 2nd ed., Cambridge University Press, Cambridge, 1989.