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2016 Extra Class study guide: E7D - power supplies and voltage regulators

Posted: 04 Feb 2016 12:00 PM PST
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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) This would be Q1 in the circuit in Figure E3-7 below. The device
typically used as a stable reference voltage in a linear voltage regulator
is a Zener diode. (E7D03)

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)



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 is 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
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.

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