# Understanding how this variable DC power supply works.

Discussion in 'Homework Help' started by Big_Earl619, May 17, 2015.

1. ### Big_Earl619 Thread Starter New Member

Jan 9, 2014
5
0
Hi everyone,

I am a student learning about variable power supplies. I am learning about linear and switching power supplies. I found a schematic online on a variable power supply but didn't understand the full circuit layout.

Complete_schematic_1_2, and Complete_Schematic_2_2 attached is the schematic of the variable DC power supply.

PS_Image_Q1/Q2/Q3/Q4 attached have boxes and/or arrows with a description of what I am confused about.

The main questions are:

1) Why are there 2 ground symbols in the schematic. I understand the concept between earth ground and common. They use an analog ground signal between two series capacitors at the output, however this analog ground is not used anywhere else in the circuit.
2) I don't understand how the return current from the output is measured with the IC1A op amp; how can you measure the return current if the return is not tied to the ground such as R57(0.1 ohm) resistor is.
3) What is the purpose of the op-amp IC3B? Does this limit the output current? If so, how? It is a voltage follower, the output of the op amp follows the reference voltage on the non-inverting pin, however how does this limit the output current?
4) According the the connector CON2, the signal OUT is connected to ground. This doesn't make sense(please see PS_Image_Q2 ).

I appreciate all the help!

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2. ### MikeML AAC Fanatic!

Oct 2, 2009
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1,066
My take:

The open triangle symbol is the chassis (and likely connected to earth ground-Green wire in the AC power cord).
The four bar symbol is short-hand for "connect all of these on the schematic together with a fat wire". This is an internal electrical node in the power supply which has no DC path to the open triangle, described above...

Jul 18, 2013
10,507
2,367
Per Q1 It would appear apparent they have the symbols backwards.
A common mistake, unfortunately.
Max.

Last edited: May 17, 2015

Feb 17, 2009
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5. ### Big_Earl619 Thread Starter New Member

Jan 9, 2014
5
0

Jony130 thank you for your help. It sort of makes sense now. However I am still confused on a few things:

1) What is the purpose of the 3rd op amp IC3B? Please see PS_Image_Q3. This is a voltage follower circuit whos output matches the non-inverting voltage. Seems to me that this would regulate the current coming out from the -5V supply? Is this correct?

2) Why do they label the output +Vs and -Vs on the load when it's clear the top part of RL is common and the bottom of RL is -Vs? Does the naming convention even matter?

3) what is the benefit of this type of configuration besides being a floating power supply(all variable DC supplies are floating, only becomes floating if their common is connected to earth ground).

6. ### Big_Earl619 Thread Starter New Member

Jan 9, 2014
5
0
" only non-floating if their common is connected to earth ground" - sorry for the mistake.

7. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,957
1,097
No. To be honest for me IC3B has no useful functions in this circuit except loading (unnecessary) IC6. So we should remove it from the circuit.

Vs+ is simply positive the output terminal for our variable power supplies.
And -Vs is a negative terminal. Name does not matter here.

And as for this GND at VS+. This ground is only a internal ground (a reference point) for IC5 and IC6 voltage regulators, which provide power for op amps nothing more. Our "real" output ground for the output voltage is -Vs as always.

Output voltage can be larger then op amp maximum power supply voltage. And we have a simple and elegant solution for CV/CC bench PSU.

And I upload a correct diagram. And as you can see the output node is a reference point for Vcc, Vee and for Vref also. But this dose not change the reference point for the Rload.

Last edited: May 21, 2015