Power Supplies

Introduction: If there is one indispensable component of any piece of electronic equipment it is the power supply. The reason that power supplies are so essential is that the methods of producing electricity easily produce voltages which are inappropriate for most appliances, whether they are microwave ovens or computers. Power supplies may be batteries or ‘‘wall-warts’’, those little black boxes which plug into one socket of an electrical outlet and block the other, but you will build a more standard, simplified supply.

Theory: The easiest way to produce electric current is to make a loop of wire turn in a magnetic field. The mechanical energy for the turning may come from falling water or steam, but the electrical output is either pulsating DC or AC. Alternating current is the best form for transmission to customers since it can be transformed to very high voltages. For a given power delivery high voltage means low current which means minimized energy loss to heating. At the node (customer) the voltage can be transformed to the familiar 120V for home or commercial use. Unfortunately, 120VAC is not suitable for driving most electronic circuits: it must be adjusted. The first stage in a power supply is the transformer. This is the bulk weight in a power supply, and is probably not going to be downsized by technology since its efficiency depends on its heavy iron core. This will take the 120VAC from the wall and turn it into 24V, 18V, 15V, etc. depending on the turns ratio, the number of turns of wire on the input (primary) versus the number on the output (secondary). (The mechanism of transforming voltage will be discussed in lecture). In your case it will give you 6VAC. The symbol for a transformer is shown below in figure 1:

Figure 1
Figure 2

Note the center tap. This allows for more than one voltage from the transformer; you will use only one tap. Alternating current must be changed into direct current for electronic instruments. The process is called rectifying and the chief device used is a diode, the symbol shown above in figure 2. A diode allows current to flow one way but not the other. Below is the input AC voltage from a transformer:

Figure 3

Next we insert a diode in series with one lead:

Figure 4

and the output looks like this:

Figure 5

This is called a half-wave rectifier, obviously since half the wave is rectified. When the current goes positive the diode allows it through, but when the current goes negative nothing gets through. This is a crude rectifier since half the time the power supply supplies no power, so we will NOT be making a single diode rectifier. A better rectifier uses four diodes connected like this:

Figure 6

and the output looks like this:

Figure 7

This output is called a bridge rectifier and produces a full-wave. This is much more efficient, but still insufficient for an IC. The voltage is varying too much to use, so we insert a large capacitor across the output of the rectifier:

Figure 8

Note the polarity. Shortly after power-up the cap charges to nearly the maximum value. When the voltage drops below the charge value the cap discharges to the circuit, and when the supply goes above the cap charge level the capacitor recharges. This has the effect of smoothing out the varying voltage, similar to the graph below:

Figure 9

There are many other additions we could make to further purify the power, but we will add just one: a voltage regulator. This is an solid-state device which further smooths out the voltage as long as the supply is above a certain level. When connected to the circuit like this:

Figure 10A
Figure 10B

The output is a very smooth DC. In theory you have assembled an ace power supply. It produces a clean, single voltage.

Testing of a power supply involves checking the output for the voltage you wish and rating its current capacity and power output. The first is easy: place a voltmeter on the output. The second test checks for a symptom called ripple. This effect causes fluctuations of voltage in the output when the supply is loaded, meaning putting out lots of power. Ripple mimics the input voltage before regulation. A regulator can maintain a constant voltage as long as the supply voltage is always higher than the regulated output, but once the supply drops below this value, ripple creeps in. The overall effect is to reduce the average output voltage, similar to starting your car with the lights on: the lights dim because the voltage drops when the battery is asked to supply large currents to the starter motor. It can be expressed as a percentage of the output voltage, but since it varies with time the ripple is usually expressed as an RMS voltage.

Task:

Equipment:

Figure 11

Figure 12
Figure 13
Figure 14

 

Procedure:

N.B.: No spreadsheet is needed for this lab, only your report in a Word .doc. Be sure that you have a Circuit Maker diagram and a waveform for each stage of the power supply.

You will find it easier to build one stage at a time then graph its output first before moving on to the next stage. The complete power supply is shown below:

Figure 15

Additions in this circuit will be explained below. To isolate the waveform at each stage you will need to disconnect the later stages from the one you are measuring. (Perhaps you've heard that it will be easier to build one stage at a time then graph its output first before moving on to the next stage.)

Figure 16

The VOM in figure 14 is for checking voltages and diodes if necessary.

The following will be the wiring steps, but maybe you've read somewhere that it will be easier to build one stage at a time then graph its output first before moving on to the next stage. You may fire up the PCI now (procedure 8) or practice your wiring then disconnect each stage for analysis. In any case, before you turn on the power, your instructor needs to check your circuit so that you don't smoke it.

  1. Connect the two blue terminals on the transformer (Figure 16) to the two red posts on the breadboard.
    1. Check and save the waveform at the red posts.
  2. Build the bridge rectifier circuit, but first test the diodes - use the Diode setting on the VOM.
    1. The input of the rectifier should be connected to the bus strips leading from the red posts.
    2. Observe the polarity of the diodes. The ringed end is the forward end.
    3. Check and save the waveform at the output of the rectifier.
  3. Add the 2200F capacitor across the output of the rectifier.
    1. Be sure to observe the polarity. It may explode if reversed!
    2. Check and save the waveform across the capacitor.
  4. Insert the voltage regulator after the capacitor.
    1. The pin-out is given in the diagram. Hold the LM340T-6.0 (Figure 10a) with the front facing you for proper orientation.
    2. Pin 1 connects with the + side of the capacitor, pin 3 (the center one) goes to ground, pin 2 is the output.
    3. Check and save the waveform at the output of the voltage regulator.
  5. Connect a variable load (the decade box set to 1000) across the output. This will prevent the power supply for becoming overloaded.
  6. Using the PCI first: Plug the Voltage probe into Input A of the PCI.
  7. Boot up the PCI.
  8. Drag the Scope onto the page.
  9. Scroll down under Sensors/Instruments to Voltage Sensor and choose it.
    1. Select Measure: Time on the x-axis and Voltage on the y-axis (this may be pre-selected for you).
  10. Insert the probe leads of the PCI at each stage of the power supply (after the transformer, rectifier, 2200 F capacitor*, and regulator) and observe the output.
    1. This means the red probe goes at each red circle and the black probe goes at each black circle below it.

    Figure 17

    1. *You will have a total of four probe points; test the middle probe position without and with the capacitor in place.
    2. Size and maximize the trace by adjusting the scales.
    3. The frequency of the AC source is 60 Hz; what value will you use for the sample rate?
  1. After each insertion and observation, save the waveform for your report.
    1. It is probably easiest to the Windows Clipping tool, but you can also take a Snapshot of the plot for the PCI journal then export it:
  2. Figure 18

  3. Finally, slowly reduce the load at the decade box and observe the output on the PCI. At some point the output goes from relatively stable to almost chaotic. Record the resistance where this occurs, as well as the voltage across the load. About what current is being drawn?

Questions:

  1. Considering that a diode allows one-way current flow, how does the bridge rectifier produce full-wave rectification?
  2. Why do the backs of electrical appliances say "Danger! High Voltage!", even when the unit is unplugged?
  3. What was the ripple in your supply just before the load was too much for the unit? Express your answer as a percentage of the output voltage.
  4. What power output would you rate your supply?