Chapter 6. A High-Voltage Power Supply for Systems Biology

Introduction

I am sharing the design, along with the project files for home-lab manufacturing, of a low-current, high-voltage power supply that may be used for various experiments in Systems and Synthetic Biology. The schematic, board layout, and a suggested enclosure is provided. This circuit outputs up to +1,866 VDC at under 1 mA or can be tapped at various points to yield +622 VDC or +933 VDC. All components are easily obtained through common, hobbyist-friendly distributors.

The cost of this supply if built using high-quality components is easily under $100. The building blocks of the design are also reusable in other projects to further reduce total cost. This is in contrast to equipment used in typical journal research papers, which recommend scientific supplies costing more than$5,000.

This power supply may be useful for either DIY biology or institutional research experiments, such as:

• Digital microfluidics using electrowetting-on-dielectric[6]
• Electroporation
• Electrokinetic experiments, such as dielectrophoresis[7]
• And, lastly of course, generating large sparks that blast with a PAHHH-POP!

An important aspect of this design is the built-in current limiting of the components. Never connect the printed circuit board directly to a wall outlet.

Warning Regarding High Voltages

This circuit creates high voltages with enough circuit current to mandate a serious warning. Any contact with high voltages may cause serious physical harm. Build this project at your own risk. Please read and reread the paragraphs on potential misuse of this circuit contained in this article. Make note that the circuit charge remains even after the circuit is unplugged from the input voltage, even after a significant time; always discharge the final output after disconnecting the input voltage. Under no circumstances should the circuit board be connected to a wall outlet (which are typically rated at 20 A or 30 A), or to any power source which does not inherently perform current limiting. In practical use, avoid using two hands near the circuit and near the outputs, to avoid an accident where the shortest path for current could pass through the arms to the user’s chest and heart. All electrical components should be kept away from or shielded from the liquids in the wet lab during operation.

This article follows an engineering format by listing specific usage requirements for the project, followed by discussion of the possible design options that fit the requirements, and a theory of operation for the winning design. Projects are best designed by doing a bit of homework first.

This supply is intended to be used with a simple, computer-controlled, high-voltage switching circuit, to be published as a separate paper. Comments and suggestions regarding the construction and use of this project are welcomed on the DIYbio mailing list. The figures in this article can be used to fabricate the circuit board using circuit board etching techniques; see the instructions in “Using These Figures to Etch a Printed Circuit Board”.

This power-supply circuit is not intended for and does not supply sufficient output current for a typical tray-style gel electrophoresis setup. The power-supply output is purposely current constrained. For running gels, a different circuit board is needed that reuses some components of this project. For example, a typical agarose gel electrophoresis requires >100 mA; this current varies depending on the gel’s cross-sectional area.

Requirements

Scientific power supplies are well regulated and yield very stable output, especially if regularly calibrated. However, a survey of published results has not revealed experimental differences when ripple or electrical noise is either purposely or accidentally applied (this is an area where more research is needed). We assume, therefore, that if ripple and electrical noise is of little consequence, it is possible to design and use a simpler, less expensive high-voltage power supply. Many biological operations seem to activate from a wide range of electromagnetic field strengths, usually dependent on the particular organism or strain being studied.

The design should be easy to reproduce by others, to allow labs or individuals to build their own homebrew supplies. The desired circuit must be simple enough to allow multiple high-voltage output levels, in case voltages need to be much higher or much lower than the reference design. Where possible, the design should use off-the-shelf components that are reusable in other projects.

The laws of physics and today’s scale of technology dictate the costs and trade-offs of today’s power-supply designs, and these trade-offs factor into the requirements. Simultaneously producing both high voltage and high current is more difficult, expensive, and complex. Producing high voltage and low current, or vice versa, is simpler, lower cost, and fits the need for a range of scientific experiments.

Any power-supply design must allow the circuit to limit the current at the final output. It is important to limit the current available to the user to prevent mishaps in case of accidental shorts. The low current output in this design may be treated as a benefit.

Design and Theory of Operation

High output voltages, in excess of +400 VDC, are more easily obtained if starting with a high input voltage. There are several design choices:

• Starting from a typical AC outlet, a step-up transformer could be directly used, with the AC rectified to DC just prior to the final output. These transformers have marginal cost, and the output would be fixed.
• Starting from a typical AC outlet, a common low voltage AC-to-AC power adapter could be used, such as a 120 VAC to 24 VAC wall adapter, and this low AC voltage run through a secondary step-up transformer with high ratio (such as 1:200) and finally rectified to yield DC. These transformers can be found in some appliances, such as televisions, as flyback transformers. These transformers may be large, heavy, and either expensive or require a combination of reverse engineering and scavenging to minimize cost. Sharing a design that requires scavenging typically means the design is not reproducable by others.
• A high-voltage AC outlet source could be directly used, with current limiting. This requires some safety components and is typically not recommended for a homebrew build due to the possibility of manufacturing mistakes.
• A high-voltage AC outlet source could be converted to DC with a typical benchtop power supply, then converted to a high DC voltage with a DC-to-DC converter. This requires a benchtop power supply at some expense, and an expensive circuit. An example of this type of design can be seen in the popular home-built nixie-tube high voltage switching power supplies, which output a maximum of approximately 200 VDC at very low maximum current (50 mA).
• A high-voltage AC outlet source could be converted to a low-voltage, current-limited DC with a common wall wart, such as a 120 VAC to 12 VDC wall adapter, then the DC converted to AC with a commonly found DC-to-AC inverter, for example, an automobile 12 VDC to 120 VAC inverter, followed by a smaller voltage step-up circuit to further boost the voltage. These automobile inverters are mass-produced, so they are inexpensive, easy to find, and relatively small.

Based on the prior requirements, the winning design is the latter, use of a common AC-to-DC wall wart, followed by a 12 VDC automobile inverter, with a twist: by using a European automobile inverter, it is easy to obtain a safe, current-limited 240 VAC as power input to a custom circuit. Sourcing this inverter also provides a ready, current-limited supply for 240 VAC directly. The 240 VAC is then connected to a custom circuit to further boost and rectify the voltage. Note that a typical low-cost 12 VDC automobile inverter does not output a pure sine wave, thus the AC output will have additional noise characteristics.

The design of the custom circuit is straightforward (following the “Villard cascade voltage multiplier” circuit; see “Brief Overview of Cascade Voltage Multipliers”) and uses the principle of switched capacitors. Each capacitor will only have Vpk voltage across it (the half wave of the AC input) as the diodes are forward biased. The charge pump creates a doubling effect after the first stage of 2 × Vpk and a tripling after the second stage, to 3 × Vpk, up to 6 × Vpk in this circuit. Since the capacitors are “small,” the charge capacity and hence the output current is not high. Also, since there are losses in the components, the efficiency will decrease for each stage added, so practically speaking, after a multiplier of 6, a large drop-off is expected—this depends on the components used. The losses are expended in heat and vibration. The resulting HVOUT voltage potential is measured between the last stage and the AC-VIN ground. Larger capacitors will allow for larger current capacity, at the trade-off of circuit cost.

Cost and Components

Table 6-1. Bill of materials
 12 VDC to 240 VAC European automobile inverter $15 120 VAC to 12 VDC wall wart with 12 VDC automobile plug output$9 Power box for custom circuit $3 Custom circuit components (see schematic)$10 Double-sided copper clad board $4 Homebrew etching materials$8 Three-prong AC computer power socket $1.50 Computer power cable$1 European AC plug to USA plug adapter \$1

Building the Project

The schematic is pictured in Figure 6-1. The schematic includes sourcing information and part numbers for all of the electronic components needed.

The board may be etched using the positives or negatives of Figure 6-2 and Figure 6-3. Figure 6-4 is used for the final board marking. Either toner transfer or direct inkjet printing may be used on a copper PCB to prepare the board for etching. If using toner transfer, a tabletop laminator is recommended. For etching, a solution of hydrogen peroxide and hydrochloric (muriatic) acid is a good choice, as this solution is reusable and more environmentally safe than ferric chloride. Muriatic acid for use in the etching solution is available from a home improvement store or a pool supply center. Both solutions will etch faster if slightly heated above room temperature. The spent H2O2 solution is easily disposable, unlike ferric chloride, although refreshing the solution by bubbling or stirring for reuse is preferred to disposal. Dispose of the H2O2 solution, after 20x dilution, in the toilet (if this statement causes any doubt, contact the local city water treatment and/or waste management office). Spent ferric chloride solution, which is not reusable, must be handled as toxic material and disposed of only at a certified hazardous waste disposal site.

If etching equipment is unavailable, the circuit board may also be assembled using a perforated board using point-to-point wiring. Use AWG 12 wire or heavier for construction.

Assemble the circuit board using standard prototyping techniques, as discussed above. Verify continuity between major components of the circuit; this verification is an important step. Insert two fuses into the fuse holders. Fit the board into a power box with cover. A snapshot of a roughly completed assembly is shown in Figure 6-5.

Connect power leads as desired from the output points of the circuit to the experimental setup. Power leads should be kept as short as possible. If the circuit is used frequently, consider using stranded high voltage transmission wire (an ignition cable or spark plug wire) on the output connection.

Usage

Prior to using the supply for the first time, or after a period of disuse, perform a continuity check on the major components. The outputs must register as an open circuit.

Power the circuit and use a voltmeter to measure the output voltage at each output tap. Disconnect and discharge the supply, then connect the desired output voltage tap to the experiment. Power the circuit and again measure the voltage at the output voltage tap while connected to the experiment with a set of control or calibration reagents. Under load, the output voltage may drop. A significant voltage drop indicates that the supply is not providing enough current to the experiment, and a change to the experimental setup may be needed.

When powered and fully charged, the printed circuit generates a nice BZZZZ sound. This sound is normal and expected. The sound is created by the circuit board components internally discharging as the rated voltage potentials are met.

After the supply is no longer needed, disconnect the input power and discharge the outputs through a 1/4 W, 1 Megaohm resistor or larger. The supply should be allowed to discharge for a minute prior to disconnecting the outputs.

It is handy to perform output voltage switching operations through an automated, software-controlled circuit.

Conclusion

This high-voltage DC power supply design is deemed appropriate for various systems biology experiments where some voltage and current instability can be exchanged for simplicity and low cost. Of course, no claims can be or are made for either results or safety in practice.

[6] R. B. Fair. "Digital microfluidics: is a true lab-on-a-chip possible?" Microfluidics and Nanofluidics. June 2007, 3:3 (245–281), http://bit.ly/1qgTGSg. doi: 10.1007/s10404-007-0161-8.

[7] Peter R. C. Gascoyne and Jody Vykoukal."Particle separation by dielectrophoresis." ELECTROPHORESIS. July 2002, 23:13 (1973–1983), doi: 10.1002/1522-2683(200207)23:13<1973::AID-ELPS1973> 3.0.CO;2-1.