Linear Power Supplies

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engr_joni_ee

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I would like to understand how do the linear power supplies actually work.

I know that there are two main types of lab power supplies.

1- Linear PSU (Power Supply Unit)
2- SMPS (Switch Mode Power Supply)

The linear power supply can have various options and features available for example.

1.1 Voltage Limit - also known as Constant Voltage (CV)
1.2 Current Limit - also known as Constant Current (CC)
1.3 Voltage Readout
1.4 Current Readout
1.5 Remote sensing
1.6 Remote programming

I have attached a circuit diagram that represents the basic design of linear power supplies.

The electronic components in the attached circuit diagram are:

1.1.1 Input power transformer, T1
1.1.2 Bridge rectifier, D1-D4
1.1.3 Reservoir (or smoothing) capacitor, C1
1.1.4 Pass transistor, Q1
1.1.5 Output voltage sense and feedback, Q2
1.1.6 Voltage limit setting, VR1
1.1.7 Voltage reference, ZD1

There are few things I know regarding the attached circuit.

- The input is 220 V AC which is connected through a fuse to the input of the Transformer T1.
- The ratio of number of turns of primary and secondary coils in Transformer T1 decide the output voltage level at the secondary coil of the Transformer T1. For example if the ratio is 22:1 then the 220 V AC will be down to 10 V AC.
- The bridge rectifier consisting of diodes D1 to D4 will rectify. The ripples will be smoothen by the filter circuit or a large capacitor C1 (1000 uF) as mentioned in the attached diagram.

I am wondering what is the formula to calculate Vout provided the values of resistors R2, R3, R4, Potentiometer, and reference voltage 5.1 V are given ?

I understand that Q1 is pass transistor and Q2 is feedback transistor in the attached circuit.

Something more I know is how it work in linear regulator in which we also has a pass element (a transistor) and error amplifier (an OpAmp).

As the output voltage of the linear regulator drops the voltage divider's mid point voltage will also drop that will be compared with the reference voltage, making the error amplifier output low which will turn on the pass transistor, making the output voltage level increase.

As the output voltage level increase beyond the set value. The mid point voltage of the voltage divider will also increase that will be compared with the reference voltage, making the error amplifier output high which will turn off the pass transistor, making the output voltage level drop. This is how the linear regulator work.

Now I need to understand how the linear power supply's attached circuit work. I guess the operation of the attached circuit diagram is also similar but I am not getting it completely.

Can someone please explain how the circuit works ? and how do we calculate the Vout with the values of resistors R2, R3, R4, Potentiometer, and reference voltage 5.1 V ?

Thanks in advance.

 
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engr_joni_ee said:
I am sorry I do not have any specific design goal.
Click to expand...
Most power supplies don´t have sensing capabilities.
Many power supplies are also meters away from the load.

So if you ask for special "accurate voltage regulation methods" ... (voltage sensing) .. it is quite straight forward (for me) to ask why you think you need this special method.

Usual reasons are:
* your load is very sensitive to supply voltage change
* load that changes the drawn current in a wide range
* there is something special we don´t know yet.

We simply don´t understand why you accept a drift of the output voltage by ambient temperature...
while you are worried about output voltage drift caused by voltage drop on the wires.

You ignore the recommendation to adjust wire gauge to get low voltage drop ... but you are willing to install extra wires for sensing.
It does not make sense to us.

****
If we professions do this job with numbers and calculations ... we don´t do it just out of mood.
If we can expect an output voltage drift by temperature of 100mV max ... and we expect an output voltage drift by 30mV max caused by the load current... then we focus on the highest expectable error reason--> the thermal drift.
This are just example numbers. We don´t have realsitic numbers, but you should have. You are the designer.
You should have numbers for your use case and you should have numbers for the maximum error you allow.

***
An example: If I want to bulid a house ... and I ask for an earthquake proof foundation .. but the house is not meant to be built in an earthquake region .... then someone may ask for the reason behind it. And if I don´t have a valid reason ... it´s quite expectable that someone recommends me to forget about the extra solid foundation. It´s just a waste of anything.

***

Klaus
 
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Klaus ST filled in all the gaps perfectly above about that circuit..

As i tried to asses Your electronics skill level so i can give You information that might help You on Your level, i came to conclusion (and D.A.(Tony)Stewart above seemed to note the same..) that You would benefit greatly if You would go out in the google and really get the Kirchoff's law under Your belt and also connect it with Ohms law

..meaning..do like few of those 'resistor puzzles' that are sometimes in exams succesfully..You would have instantly recognized the voltage divider had You practiced such puzzle..(there are resistors connected seemingly randomly together and you have to figure out how to calculate some missing value..etc..).

That must not be regarded as some newbie stuff..but stuff that absolutely everything revolves around.. and sooner it is Your other nature, the sooner You'll start getting a real grip on the circuits that You look and need to understand..! Learn these and i see nothing standing in Your way in a lifelong process with electronics..
 
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engr_joni_ee said:
I am sorry I do not have any specific design goal. The purpose was to understand the basic liner power supply circuit and how it function.
Click to expand...
Then the purpose of your experiment should be to define the measurable node voltages in simulation, compare with math and logic with your understanding of Kirchoff's Laws and transistors then list test results so you understand how to design by specs here reverse engineering in lab tests and/or simulations.

Try harder.

Critical specs you would be expected to know in a day-to-day design job and fix are;
Adjustment range or sensitivity.
No/Full-load noise. (Vpp < 20MHz with 10:1 probe or AC coupled into 50 Ohms.) Then understand why.
Load regulation error % at various loads or worst case.
Power Loss vs load (Ohm's Law) and temperature rise vs theoretical thermal resistance in free air with case temperature.
Max current. wire guage and mOhms per meter thus voltage drop. This is critical for laptop chargers that have fine flexible cables and sense wires at the plug on a 1m cable. But not likely on a backplane or 2 m AWG20 with < 1 A.

Have you specified the wire ohms/m and current yet?
Would you expect it to affect anything on this regulator with remote sensing or cause more issues with sensitivity to nearby noise (parasitic) like a soldering iron relay clicking on/off transient?

With practice you will be able read plots and schematics as fast as cartoons. ')

Then ask a better question.
 
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