In previous Instructables tutorials we have described how to make equipment used for DNA electrophoresis and imaging. These include a mini-gel electrophoresis tank, a UV transilluminator for ethidium bromide gels and a blue LED transilluminator for sybr-safe gels.

An additional piece of hardware required for electrophoresis & gel imaging is the electrophoresis power supply.  This high voltage power supply connects to an electrophoresis tank setting up an electric field between the two electrodes. DNA samples loaded into an agarose gel move through the gel towards the anode (+ve) with the agarose gel matrix separating the DNA molecules by size (see Step 5 for an example).Electrophoresis power supplies typically have a variable output voltage allowing the user to set the output voltage for different size gel tanks and modify voltage for optimum results and convenience.

In this Instructable we describe how to make a variable electrophoresis power supply suitable for mini-gels. The design is based on the Maxim high efficiency MAX1771 step-up DC-DC controller and boosts a 15V input, from an external wall-wart power supply, to an adjustable 25-100V output. The power supply uses a switch-mode design in a step-up (boost) topology -  as shown in figure 2 of Maxim's "An Introduction to switch-mode power supplies". An important source of inspiration for this design was the open source 150-220V Nixie tube power supply designed by Nick de Smith.

A schematic of the power supply design is  shown in the images above. The fundamental components of the design are the MAX1771 DC-DC controller (U1), a MOSFET switch (T1), an inductor (L1), a diode (D1) and an output capacitor (C5). In the design the controller regulates the output voltage via a Pulse-Frequency Modulated (PFM) signal applied to the gate of the MOSFET. When this signal is high the MOSFET switch is turned on and when this signal is low the MOSFET is turned off. The controller adjusts the pulse rate of the PFM signal, based on feedback from the output, in order to maintain a constant output voltage. The PFM signal divides the operating cycle of the power supply into charging and discharging phases. During the charging phase, the MOSFET is turned on and  energy is stored in the inductor, the diode is reverse biased preventing current flow, and the load is supported by energy stored in the output capacitor. During the discharging phase the MOSFET is turned off, the diode is forward biased and energy is transferred from the inductor to the load and the output capacitor. 

The magnitude of output voltage of the power supply is set using a voltage divider on the feedback from the output to the DC-DC controller.  In the schematic this voltage divider consists of resistors R1, R3 and RV1. Using the values of these resistors the output voltage of the supply can be determined via the formula Vout = Vref (R2/(R3 + RV1) + 1) where Vref  has a value of 1.5 V. For this design we used fixed resistors for R2 and R3 with values of 1M ohm and 15k ohm respectively. For RV1 we selected a variable resistor with 0-50k ohm range. Plugging these values into the formula above gives a theoretical output range of roughly 25-100 V. 

When determining the required voltage output, we followed the recommended guidelines of 5 V/cm, where cm refers to the distance between the two electrodes. For our mini-gel system, electrode distance is 17 cm, so ideally we should run the gel at 85 V.

Open source hardware - This is an open source hardware project licensed under the Creative Commons Attribution 3.0 License. The design files can be found on Bitbucket  here https://bitbucket.org/iorodeo/hv_switching_psu  and here https://bitbucket.org/iorodeo/hv_switching_psu_enclosure.

This Instructable is written in collaboration with willrodeo.

Step 1: Materials

To make the electrophoresis power supply you will need the PCB, electronic components and an (optional) enclosure. The PCB Gerber files, enclosure design files and list of all hardware and electronics are attached -- just download the zip file. We have also put together an Electrophoresis Power Supply Kit (Cat #IMG-08, $65) containing all of the parts necessary to build the power supply described in this Instructable.

Power supply Kit contents:
  • Electrophoresis power supply PCB v1.3
  • Electronic components: There are 20 through-hole components which are listed in the next step and in the BOM
  • Enclosure laser cut parts: 1/8" clear acrylic;
  • Enclosure hardware: Standoffs, machine screws and screwdriver
Additional equipment:
  • Wire clippers
  • Multimeter for testing
  • 15V, 1.6A DC power supply: 2.1mm plug, center +ve, e.g. Jameco Cat # 380173
  • Banana plug/banana jack cords: e.g. Pomona Electronics Cat # 4702-24-0 (black, 24") and 4702-24-2 (red, 24").
<p>Can we take this to a higher voltage? It would be useful to have higher than 100V for buffers that can take this (lithium borate, sodium borate, lithium acetate) or if we simply were making a longer gel slab.</p>
<p>nice instructable. As a side issue, is there anything about a typical electrophoresis PS that might preclude its use as a high voltage supply for an electrical discharge experiment requiring 400-500vdc at less than 500ma? I appreciate this is beyond your PS capacity, but thought you might have insights into those commercial units available on ebay.</p><p>Thanks</p><p>Doug</p>
<p>What are the specs for the inductor, such as frequency, etc?</p>
<p>You can find specs in this file:</p><p><a href="https://bitbucket.org/iorodeo/hv_switching_psu/src/4507dd4706fe2476aafa5ec8fe3668a799117026/BOM.txt?at=default" rel="nofollow">https://bitbucket.org/iorodeo/hv_switching_psu/src...</a></p><p>Take Digikey part number and search it, e.g.:</p><p>http://www.digikey.com/product-detail/en/5250-RC/M8271-ND/774811</p>
<p>What EXACTLY type of DC power supply I should use? It doesn't work on PC 12V power supply, doesn't work on laptop 15V adapter. I have unstable high voltage output on old, train toy transformator...</p>
<p>splendid! I like it, simple and efficient!</p>

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