3-4) Power Suppler Filters-Converting AC to DC

Question 1

What type of wave is the output from a bridge rectifier?

Answer 1

The output is “ripple voltage,” defined as the departure of the waveform from pure DC. Ripple is the amplitude modulation from positive to zero and back. It produces a “bouncing” or “pulsing” DC signal.

Question 2

What is the purpose of using a power supply filter?

Answer 2

The purpose of a filter is to smooth out the ripple contained in the DC pulses that are produced by the rectifier while also increasing the average output voltage or current.

Question 3

What are 2 different types of filters used in power supplies?

Answer 3

Capacitor input and choke input

Question 4

How is the designation for the type of filter used determined?

Answer 4

The closest element electrically to the rectifier determines the basic type of filter being used. For example, if a capacitor is placed in parallel with the load resistor, then it’s a capacitor input filter.

Question 4

How is the designation for the type of filter used determined?

Answer 4

The closest element electrically to the rectifier determines the basic type of filter being used. For example, if a capacitor is placed in parallel with the load resistor, then it’s a capacitor input filter.

Question 5

How does a capacitor input filter operate?

Answer 5

• As the first pulse appears across the capacitor, changing it from negative to positive, bottom to top, the peak voltage is developed across the capacitor.
• When the first half-cycle has reached its peak and starts its negative half-cycle, the capacitor will start to discharge to the output, which maintains the voltage, thereby holding voltage at a higher value. Before the capacitor can fully discharge, the positive pulse of the next half cycle is nearing its peak, which recharges the capacitor.

Question 6

How does an inductor choke filter operate?

Answer 6

• The choke input filter uses an inductor in series with the load resistor. The inductor opposes changes in current to provide smooth output for the load.

Question 7

How does a multiple section choke input filter operate?

Answer 7

• Two or more choke input sections may be used as desired. Additional sections can reduce ripple output, they also result in reduced regulation because additional sections add more resistance in series with the power supply, which results in increased voltage variation in the output when the load current varies.
• Excess voltage at the power supply is needed to account for the loss of signal for filtering.

Question 8

How does a Pi filter operate?

Answer 8

• The Pi filter is so named because of its resemblance to the Greek Letter Pi (∏).
• It is a combination of a simple capacitor input filter and the choke input filter.
• The Pi filter is a capacitor input filter with the addition of an L-section filter.
• The majority of filtering action takes place across C1, which charges through the conducting diode(s) and discharges through R, L and C2.
• C1 charge time is very fast compared to the discharge time. The inductor (L) smooths out the peaks of the current pulses across C2, thereby providing additional filtering action.
• Voltage across C2, which is in parallel with the output, is the output voltage of the power supply.
• Voltage output is lower than if the output were taken directly off C1, but the amount of ripple is greatly reduced.

Question 9

What is a bleeder resistor and what is its function?

Answer 9

• A load resistor, known as a bleeder resistor, is also found in practically all power supplies. It is configured in parallel across the output to the load.
• There bleeder resistors has 2 functions:
  
  • When the equipment has been working and then is turned off, it provides a discharge path for the capacitor, preventing a possible shock to maintenance personnel.
  • It also provides a fixed load, no matter what equipment is connected to the power supply.

Question 10

How does an L-section input filter operate?

Answer 10

• A capacitor input filter will keep output voltage at a higher level compared to a choke input.
• The choke input will provide a steadier current under changing load conditions.
• If stable voltage is the prime consideration, a capacitor input filter would be used. If steady current is the prime consideration, then an inductor choke input filter can be used.
• Using a combination of choke inductor with a shunt capacitor, both voltage and current DC output will remain smooth.