Brown University
Physics Department
PHYS 0060
LAB B - 200
A Low Voltage DC Power Supply
• Introduction A DC power supply is described and constructed, to exhibit the main features of a typical supply. The type discussed here is suitable for powering transistors and integrated circuits. The student may easily build this power supply with components readily available in the lab, or may choose to build a pre made kit (Elenco XP -­‐720k Power Supply kit). • Theory There are three principal sections to a power supply of the type we want to construct-­‐-­‐
the transformer, the rectifier, and the filter. Figure 1 sketches the components, and the waveform in and out of each section and out of each section. The transformer changes the amplitude of the 60 Hz AC according to the desired amplitude of the output DC. The input coil of the transformer is called the primary, and the output the secondary. Since the output voltage is less than the input, this kind of transformer is called a “step-­‐down” transformer. Because of the rectification and filtering that takes place at the later stages, the transformer generally has to be selected to have an output whose peak value is larger than the target DC value of the supply. Ideally, the ratio of the input and output currents would be the inverse of the ratio of input and output voltages. Since there are generally some power losses in the transformer, the current output to input ratio is generally less than the inverse of the voltage output to input ratio. These considerations fix the design parameters of the transformer used in the supply. Of course, in the practical case, there are only a finite number of transformer designs easily available, and they generally conform to the common applications. 140512 1 Brown University
Physics Department
PHYS 0060
LAB B - 200
Similar considerations would apply if the power supply were intended to deliver an output voltage that was higher than the input (for example, a power supply to operate photomultiplier tubes would require a transformer whose input was 110 VAC and whose output was one to five KVAC). This is called a “step-­‐up” transformer The next stage of the supply is the rectifier. This stage converts the AC, whose average value of current is zero, to pulsating but unidirectional current, which has a non-­‐zero average value. The heart of the rectifier is the diode, a semiconductor device to be considered in more detail in a future experiment. 140512 2 Brown University
Physics Department
PHYS 0060
LAB B - 200
The next stage of the supply is the rectifier. This stage converts the AC, whose average value of current is zero, to pulsating but unidirectional current, which has a non-­‐zero average value. The heart of the rectifier is the diode, a semiconductor device to be considered in more detail in a future experiment. For our present purposes, it is sufficient to represent it by the symbol shown in Fig. 2(a), and treat it as a device that presents zero impedance to the flow of (conventional positive) current front A to B, and infinite impedance to flow from B to A. This is of course an idealization to be examined in more detail in the future. The simplest use of the rectifier would be in as in Fig. 2 (b), in which the coil represents the transformer secondary, and we would obtain a waveform across the resistor R such as is shown in Fig. 2(c). For the sake of simplicity and economy of parts, this rectifier (a “half-­‐wave” type), is throwing away half the available power, and presents a severe filtering problem. The full-­‐wave rectifier shown in Fig. 3 draws pulsating unidirectional current on both halves of the AC cycle. This design uses only two diodes, but requires that the transformer secondary have a “center tap” (C) available as a common return on each part of the cycle. Showing that this configuration produces full wave rectification is left as an exercise. 140512 3 Brown University
Physics Department
PHYS 0060
LAB B - 200
An alternative full-­‐wave rectifier design that does not require a center-­‐tapped transformer is shown in Fig. 4. Four diodes are required instead of two. It is also left as a problem for you to show that this produces full wave rectification. Factors in the selection of the diodes include the amount of current that must pass through them, and the peak inverse voltage (PIV) that they can withstand. Thu latter is the amount of voltage that must drop across the diode in the “reverse” direction, when it passes no current. A third question left for you is whether the PIV is the same for each of the full-­‐wave rectifiers is shown. The final stage of the power supply is the filter circuit. This has the purpose of removing, to some designed degree that depends on the purpose of the supply, the AC component of the pulsating unidirectional current form the rectifier. Its effectiveness is represented by the ripple factor, which is the ration of the rms value of the AC voltage after filtering to the DC value. There are many forms of such filters, which are combinations of resistors, capacitors, and/or inductors. The form used in the particular application depends both on the purpose of filter and the expense, of course that one is willing to meet for a particular degree of filtering. In general, we can say that capacitors are less expensive than inductors, but also less effective at low frequencies (for example, when the pulsating AC comes from a 60Hz supply). On the other hand, if the supply is one in which little current is drawn, the ability of the capacitor to store charge may be more than adequate to compensate for the charge drawn off during the low voltage part of the pulsation. Another factor is the ability to produce integrated circuits that are combinations of resistors and capacitors (as well as transistors, of course), while inductors do not lend themselves to the miniaturization techniques required. 140512 4 Brown University
Physics Department
PHYS 0060
LAB B - 200
We will conduct only one kind of filter, the so—-­‐called “Pi” RC filter Shown in Fig. 5, and if time permits examine another type of RC filter that actually has a sharp degree of selectivity in the frequencies it passes and attenuates .This is the “twin—T” filter also shown in Fig. 5. • Procedure The ET3100 circuit board has a center tapped 30 VAC output. This comes from the secondary of a transformer whose primary is connected to the line voltage (120 VAC 60 Hz). You can connect either across the entire secondary, or between the center tap and one side, to obtain either 30V or 15V as the initial unrectified AC of the supply. Make a note of the following-­‐-­‐the “ground” connection of this section is electrically equal to that of the D supply section and of the frequency generator section. This has no importance in the present case, but should be remembered if you later experiment with other setups that use more than one section at the same time. Check that this section is operational by using the scope, before connecting any components. Lengths of number 22 wire can be inserted into the white terminal blocks to make circuit connections, and also to give you contact points for the scope probe. All the insertion points on one block are connected together. 140512 5 Brown University
Physics Department
PHYS 0060
LAB B - 200
Check the operation of a single diode such as is shown in Fig. 2. The resistor can be the 1K or l00K pot on the board. Do not use the center tap (known as the wiper arm) of these pots, which gives variable resistance, since for this check the resistor is used to protect the diode from excess current. By looking across the terminals of the pot with the scope, you should see the half wave rectified output indicated in Fig. 2. If you look across the diode you will see a similar pattern. You wi1l probably be able to see, by checking with the scope in the “DC” mode, that the diode does pass some current in the reverse direction. A time consuming search for burned out diodes results when any one in a network such as is shown in Fig. 4 is connected backwards. To avoid this, the four-­‐diode networks have been assembled ahead of time. Two yellow leads emerge for connection to the AC supply. A red and a black lead carry the high and low DC output, respectively, when the AC connection has been made. Make the AC connection, and connect the DC leads to the 1K or 100K resistors as before. You should see a full wave rectified signal across the resistor. The next step will be to add an RC filter of the type shown in Fig. 5(a). Use a resistance box and two capacitance boxes for the setup. To make connections that can be traced easily, bring fairly short wires from the boxes to the circuit board of the ET3100. Then the filter connections can be made there, and the output leads of the rectifier brought to the filter input (across the first capacitor). A suggested set of values for the filter would be R = 10K, and both capacitors equal to 1uf. With no load connected to the filter output, you should expect that the signal across the second capacitor is a very stable DC without ripple. The scope should be set to DC mode to see the DC signal -­‐ in the AC mode a capacitor in the scope blocks the DC component of a signal and shows only the AC component. Check the output of your “unloaded” DC supply. Finally, load down the supply using the variable 100K resistor of the circuit board. Connect the output DC that appears across the second capacitor to one side of the pot and its center tap, so you can vary the load. See if you observe a change in the DC level, or a rise in the AC ripple, as the load resistance is lowered, and so more current is drawn from the supply. You can also try other values of R and C in the filter. You will find their values are not particularly critical. 140512 6 Brown University
Physics Department
PHYS 0060
LAB B - 200
• Questions 1. Show that the center-­‐tapped full wave rectifier produces the indicated output of Fig. 3. 2. Show that the full—wave rectifier of Fig. 4 will also work. 3. If the same transformer were used in both applications would the output have the same voltage amplitude? What would the PIV across the diodes be in each case? 140512 7