E7D11 (D)
What circuit element is controlled by a series analog voltage regulator to maintain a constant output voltage?
A. Reference voltage
B. Switching inductance
C. Error amplifier
D. Pass transistor

E7D - Power supplies and voltage regulators; solar array control chargers

Linear power supplies are a type of power supply used in amateur radio stations. They are called linear power supplies because they use linear voltage regulator ICs to maintain a constant output voltage. One characteristic of a linear electronic voltage regulator is the conduction of a control element is varied to maintain a constant output voltage. (E7D01) The control element in the circuit in Figure E7-3 below is Q1.

The device typically used as a stable reference voltage in a linear voltage regulator is a Zener diode.

(E7D03) D1 in Figure E7-3 be1ow is a zener diode.

A series regulator is the type of linear voltage regulator that usually makes the most efficient use of the primary power source. (E7D04) A shunt regulator is the type of linear voltage regulator that places a constant load on the unregulated voltage source. (E7D05)

Figure E7-3

The circuit shown in Figure E7-3 is a linear voltage regulator. (E7D08) This is a series voltage regulator. Q1 in the circuit shown in Figure E7-3 increases the current-handling capability of the regulator. (E7D06) Q1 is sometimes called a pass transistor. A pass transistor is the circuit element controlled by a series analog voltage regulator to maintain a constant output voltage. (E7D11) C2 in the circuit shown in Figure E7-3 bypasses hum around D1. (E7D07)

An important analog voltage regulator specification is the drop-out voltage. The drop-out voltage of an analog voltage regulator is the minimum input-to-output voltage required to maintain regulation. (E7D12) For example, if an analog voltage regulator has a drop-out voltage of 2 V, the input voltage must be at least 11 V in order to maintain an output voltage of 9 V.

Power dissipation is also important when designing a power supply with a series-connected linear voltage regulator. Excessive power dissipation reduces the efficiency of the supply and could require that you use large heat sinks to dissipate the power. The equation for calculating power dissipation by a series connected linear voltage regulator is the voltage difference from input to output multiplied by output current. (E7D13)

Switching power supplies

Nowadays, you are as likely to find a switching power supply in an amateur radio station as you are a linear power supply. Switching power supplies use a much different method of regulating the output voltage than a linear supply. One characteristic of a switching electronic voltage regulator is the controlled device’s duty cycle is changed to produce a constant average output voltage. (E7D02)

Switching power supplies are usually less expensive and lighter than a linear power supply with the same output rating. Switching supplies are also generally more efficient than linear power supplies. The primary reason that a high-frequency switching type high voltage power supply can be both less expensive and lighter in weight than a conventional power supply is that the high frequency inverter design uses much smaller transformers and filter components for an equivalent power output.

(E7D10) One disadvantage is that the circuitry in a switching supply is more complicated than the circuitry in a linear supply.


High-voltage power supplies

Most HF transceivers and VHF/UHF transceivers operate at a relatively low voltage. This is normally around 12 - 15 VDC. Some devices, such as older tube equipment and linear amplifiers need higher voltages to operate. These power supplies are quite different than the low-voltage linear and switching supplies describe above.

One difference is that the unregulated supplies used in tube equipment often have what’s called a bleeder resistor. One purpose of a "bleeder" resistor in a conventional (unregulated) power supply is to improve output voltage regulation. (E7D14)

High-voltage supplies may also have a step-start circuit. The purpose of a "step-start" circuit in a high-voltage power supply is to allow the filter capacitors to charge gradually. (E7D15)

When several electrolytic filter capacitors are connected in series to increase the operating voltage of a power supply filter circuit, resistors should be connected across each capacitor:

  • To equalize, as much as possible, the voltage drop across each capacitor
  • To provide a safety bleeder to discharge the capacitors when the supply is off
  • To provide a minimum load current to reduce voltage excursions at light loads

All of these choices are correct. (E7D16)

Solar array charge controllers

Solar array charge controllers are voltage or current regulators that are used when charging batteries from a solar array. The main reason to use a charge controller with a solar power system is the prevention of battery damage due to overcharge. (E7D09) Most solar panels that are rated at 12 V actually output 16 to 20 V, and if that output is not regulated, batteries connected to the solar panel may be damaged from overcharging.