# Thread: Power Supply Question ?

1. ## Power Supply Question ?

What is the difference between a linear or an unregulated power supply? What are the pros and cons to each one?

2. hello
i would say that linear is kind of a slang word used when switchers were fairly new on the market. people started to call transformer, bridge type supplies " linear "
linear means in a strieght line and applied to power supplies it would be a figure of speach, in my oppinion.
regulated means the the voltage and or current remains the same reguardless of the load.

3. Originally Posted by smarbaga
regulated means the the voltage and or current remains the same reguardless of the load.
I think maybe you mean Voltage stays the same as current will always vary with load.

A linear power supply can be regulator or non-regulated, it is usually not considered neccessary to provide a regulated type to feed servo drives etc, A regulated supply is usually only necessary when it is critical to the supplied circuit that the voltage remain constant and damage or poor performance would result if not regulated.
Al.

4. 060414-1509 EST USA

psch3:

First, go back to the vacuum tube radio days. There were three types of amplifiers defined, class A, B, and C.

Class A would be defined as a linear amplifier because without feedback its ouput would be a close approximation to the input. Meaning the transfer function from input to output is defined by a straight line.

Class B, considering one tube, is non-linear because it is biased to amplify only the positive half of the waveform or alternately the negative half. These would be used in push-pull to provide a full waveform. Feedback would also be used to reduce distortion. More generally a class called AB would be used that had both the + and - tubes on near zero.

Class C was used in RF amplifiers for power amplification and is essentially a pulsed excitation of an RF resonant circuit and did not have any need for linearity. It was simply a means to pump energy into the resonant circuit. Basically the tube was an on-off switch, binary, but with a high internal impedance in the on state compared to solid-state devices.

In more recent times Class D has been created for audio or similar applications where a pulse width modulation is performed at a frequency much higher than the signal bandwidth and filtered to allow the output to be a linear represenation of the input modulation.

For power supply regulators the term linear is used for regulators where the series pass element, a transistor or vacuum tube for example, is on continuously, basically an adjustable resistor, and its control element, base or grid, is not pulsed but continuously adjusted to maintain the output at some specified value. In a linear regulator you have power dissipation in the series pass element equal to the voltage drop from input to output times the load current. There is typically more power dissipated in the series pass element than in the load. A very simple linear series pass regulator is an emitter follower with a reference voltage applied to the base.

A switching power supply is a Class D amplifier (pulse width modulation). Here the series pass element is switched on and off at a high rate, thus less power dissipation because the series pass element is near zero resistance when on, near infinite when off. It is a non-linear control element. A non-dissipating device is required to store energy between pulses. Control circuitry is used to appropriately control the output.

An unregulated power supply is one where there is no feedback to control the output, voltage or current depending upon the goal, and the output voltage or current is determined by the equivalent source voltage of the power supply and the internal impedance of the supply.

If you build an unregulated power supply it would be typical to see a 10% or more change in the output voltage from no-load to full-load at constant input voltage. This is just a practical reality. Also the output voltage will be approximately proportional to input supply voltage for a constant resistive load.

A good voltage regulated power supply might hold output voltage within +/-0.1% from zero to full load, and 95 to 135 V input change.

As was previously said your application will determine whether you need a regulated supply. You generally do not need one for a switching type motor control because this is essentially a switching regulator itself. If you had severe input line voltage changes you might use a Sola regulator at your input.

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