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Building a DC Power Supply
The device shown here is ONLY for places with ALTERNATING CURRENT!

This device is commonly used for doing electrophoresis. Commercially available units that provide the necessary 120VDC usually cost $1,200 or more. The following one costs less than $20.

It is expected that this power supply would be calibrated using a volt-ohm meter, which you might own or can borrow from a physics teacher. The power supply model pictured here uses a common dimmer switch for room lighting. The dimmer is NOT a reostat that varies voltage; instead it varies amperage by being what some call a wave cutter. This is important because this type of power supply will NOT vary the voltage, but it will - even more importantly - vary the milliamperage.

Before you start electrophoresis, check out your system. How to do this checking is in the footnote.*

The heart of the power supply is the rectifier diode. Without it, you will not "rectify" or convert alternating current (AC) to direct current (DC). Surprisingly, these are quite inexpensive - less than $5. Every radio and TV has one inside as these appliances internally run on DC. They can thus be "stolen" from old radios and TV's, or purchased at an electronics parts store. For safety, their amperage should be low (1 amp or less) in event that someone's hands complete a circuit. If this shocking event occurs you want either the fuse or the rectifier itself to melt and break the circuit rather than "melting" the person. Since electrophoresis is rarely done at more than 100 ma (milliamps), you will never want a fuse much greater than that.

It is a good idea to thread the cord to the Power Supply through a rubber grommet (collar), which has been put in the hole you have made in the lid. Grommets are available at hardware stores. Also remember to tie a bulky knot in the cord just inside of the lid to prevent the cord's being pulled back out. This will prevent future damage to the interior electronics.

One of the neatest waterproof containers available for your construction is a plastic sandwich box. Install all the parts on the lid so that the intact bottom can sit on desks and protect the device in event of spills.

What doesn't this device have that the expensive ones do? Your "cheapie" device has an output that pulses your desired milliamperage at between 50 and 120 cycles per second depending on your region's electric utility company. This means that your molecules will be jerking along as they move. But remember that your molecules already have Brownian motion vibrations that far exceed a mere 120 cps. The expensive models have various capacitors, resistors and heavy transformers in parallel to "smooth" things out. This does not matter to you!


* Calibration and checking:

  1. To begin: First check out and calibrate the Power Supply. Leave the Power Supply unplugged until told to plug it it. (Unguided busy hands may lead to a fried student with bulging eyes and frizzy hair!) Snap the top of the device, with all its attached electronic components, onto the bottom of the sandwich box. Turn the dimmer switch all the way counter-clockwise and off. Turn the redundant toggle switch to "off"
  2. Turn your volt-ohm meter (VOM) to VDC, and insert the red probe into the red banana socket in your newly created device. Put the black probe in the black socket.
  3. Push the plug into the wall socket and then out as fast as you can. Hopefully, your VOM did not register a surge in power. If it did, you have a short somewhere inside - or the switches were not turned off.
  4. Push the plug into the wall socket and leave it there.
  5. Turn on the toggle switch. Little or no voltage should be apparent, because the dimmer is off. If you did get more than ten volts, the dimmer must have been "on". Push it on and off to make sure you are familiar with its operation.
  6. Turn the dimmer switch "on."
  7. Slowly turn the dimmer knob clockwise and watch the VOM. The voltage should almost immediately rise to 110 to 120V (or whatever is your region's "house" voltage). (There may be an initial pause because many dimmers have a dead spot at the beginning of the rotation.
  8. Just to be sure, now switch the VOM to VAC. The reading should vibrate around zero volts. This means that you do indeed have DC.

To calibrate your Power Supply FOR electrophoresis

Prepare a "practice" gel, and insert it into the electrophoresis chamber. Add the buffers to the appropriate levels. The dimmer and toggle switches are turned off. Plug the cord of the DC Power Supply into the wall and then insert the black lead from the electrophoresis chamber into the black hole of the Power Supply. Put your volt-ohm meter in series with with the chamber's red lead and the red hole of the Power Supply (do it this direction: the VOM's red lead goes into the red hole in the power supply, and the VOM's black lead is attached to the red lead from the electrophoresis apparatus. Set the meter to reading amps (preferably 1,000 or 500 milliamps, if possible. Turn "on" both switches. Slowly rotate the dimmer's knob so that the meter shows 90 ma. (See the bubbles rising from the two platinum or carbon electrodes.) Make a distinguishing mark on the lid next to that dial setting, which will henceforth be the voltage you will apply for that gel and buffer system. Other types of gels (and sizes of gels) and buffers will have their own special settings.

Warning! The milliamps in electrophoresis are far more important than the voltage. Milliamps means heat, and you do NOT want to overheat your system. With enzymes, this is particularly important. Otherwise, for non-heatlabile compounds you don't want to start boiling your system. (You might want to read about "high-voltage capillary electrophoresis".)


SUPPLIES NEEDED
ItemCost ItemCost
Plastic Sandwich Box1 x $1.85 Fuse holder1 x $0.15
Extension cord1 x $1.60 Fuses1 box @ $2.00
3/8 in rubber grommet1 x $0.05 Red and black banana recepticals1 x $2.10
On/off toggle switch
Radioshck 275-602A
1 x $2.00 #6 x 1/4" nut and bolt to secure fuse holder and dimmer3 x $0.15
Power Rectifier$1 x $2.50 Dimmer switch1 x $5.00
Box knife to cut holes in box cover1 x $2.35 Soldering iron kit1 x $8.50
Volt-Ohm Meter1 x $20.00   
Total cost (not including tools) = $17.50