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
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
- 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").
Step 2: PCB Assembly
|PCB Position||Description||PCB Position||Description|
|1||L1||100 µH power inductor||11||*P3||Banana jacks|
|2||SW1||Slide switch||12||*U1||MAX1771 DC-DC controller|
|3||P1||DC Jack, 2-pin||13||*D1||ES2F Diode|
|4||P2||Jumper||14||R1||Resistor: 0.01 µF, 1 W|
|5||P2||2-pin header||15||R3||Resistor: 15 K, 1 W|
|6||C1, C3||Capacitor: 100 nF, 50V||16||R2||Resistor: 1 M, 1/2 W|
|7||RV1||Knob||17||C6||Capacitor: 100 nF, 250 V|
|8||*T1||Heat sink||18||*C4||Capacitor: 100 uF, 50 V, polarised|
|9||T1||Transistor||19||*C5||Capacitor: 4.7 uF, 250 V, polarised|
|10||RV1||50 K Rotary potentiometer||20||*C2||Capacitor: 10 uF, 250 V, polarised|
* Some notes on assembly:
- T1 - Heat sink and transistor. Lay the heat sink on the PCB and bend the legs of the transistor so that it lays flat on the heat sink with the holes lined up. Secure in place with the screw and nut.
- D1 - Diode. The silver stripe on the diode is indicated on the silkscreen by a matching line. Orient the diode on the PCB so that the silver stripe on the diode matches the orientation on the silkscreen.
- C2, C4 and C5 -Polarized capacitors. The long leg of the capacitor is positive (anode) and the short leg is the negative (cathode). Insert the capacitors with the anode (long leg) in the pad indicated by the + on the silkscreen;
- U1 - MAX1771 DC-DC controller. Note that this component has a notch on one end which is indicated by a similar shape on the silkscreen for U1 to help you orient the part correctly. Place the component on the PCB so that the notch is in the correct orientation.
- P2 - 2-pin header with jumper: After soldering on the 2-pin header place the jumper onto the pins.
- P3 - Banana jacks. These two components (one red and one black) snap together. Before placing them on the board for soldering press them together with the red banana jack on the left and the black on the right as shown in the images.
Step 3: Enclosure Assembly
- Mount the assembled PCB onto the four short (1/4") round standoffs using the four smaller (3/16") screws
- Mount the PCB onto the clear acrylic base with four of the 1/4" screws
- Place the four longer (1") round standoffs in each corner. These are used to secure the top and the enclosure
- Place the four sides onto the clear base
- Place the top onto the case and secure with the remaining four 1/4" screws
- Finally, mount the enclosure knob onto the 50K potentiometer using the hex key to tighten the 6-32 set screw onto the flat side of the potentiometer.
Step 4: Testing the Power Supply
- Prior to operation the power supply should be placed within the insulating laser cut enclosure as described in the previous step in order to isolate the user from high voltages;
- The operator should take care to avoid contact of any body part with the live output of the power supply
- Ensure that the power supply switch is in the off position and that the DC jack is disconnected.
- Attach the banana patch cords to the positive and negative outputs of the power supply as shown in the image.
- Connect the positive banana patch cord to the V input of the multimeter and the negative banana patch cord to the COM input as shown in the image. Note, ensure that the multimeter is set to measure DC voltages and has an input range of at least 100V.
- Insert the plug from the 15V wall wart into the DC jack of the power supply.
- Place the output of the supply to its lowest setting by turning the potentiometer as far clockwise as it will go.
- Turn on the power supply using the switch.
- Examine the output of the supply on the multimeter display. Change the output voltage by turning the potentiometer. The output voltage should vary from about 25-100V throughout the range of the potentiometer. Markings indicating the expected output voltage for a given potentiometer setting are etched on the laser cut enclosure and can be used as a reference. Note, as the power supply is only lightly loaded during these tests, the output voltage may take a small amount of time to settle when changing from a higher to lower voltage setting.
- When you are finished examining the output voltage. Turn the power supply switch off and unplug the 15V plug from the DC jack.
Step 5: Electrophoresis With the Power Supply
- Prepare your electrophoresis chamber with buffer and an agarose gel. This is described in other Instructables (Preparing a SYBR safe agarose gel and How to prepare an electrophoresis agarose gel by mmdeaton).
- Once you have loaded your DNA samples and are ready to begin, connect your power supply to the electrophoresis chamber with the banana cables.
- Select your required output voltage on the power supply. We recommend running the mini-gel system at around 85V or 5V/cm where distance (cm) refers to electrode spacing. The mini-gel tank has an electrode spacing of approximately 17 cm.
- Switch on the power supply. As soon as you switch on the power supply you will see bubbles on the anode and cathode electrodes.