Power supply combination problem

Click For Summary

Discussion Overview

The discussion revolves around a homework problem involving the combination of power supplies to achieve a specific voltage output. Participants explore various configurations of three 2V power supplies to test at 5V, discussing theoretical setups and practical considerations.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the problem of connecting three 2V supplies to achieve a 5V output and notes that their attempts have not been successful.
  • Another participant seeks clarification on the type of supplies being used and whether they are ideal, questioning the setup and the method of testing.
  • A participant corrects a previous misstatement regarding voltage outputs and clarifies their understanding of the problem context.
  • One participant suggests combinations of connecting supplies in series and parallel, noting the resulting voltages and currents but expresses confusion about achieving the desired 5V.
  • There is a discussion about the implications of connecting power supplies in parallel, with some participants arguing against it due to potential issues with unequal voltages and internal resistances.
  • Another participant raises the question of whether resistors are available for the circuit, hinting at their potential role in the setup.
  • Concerns are expressed about the safety and practicality of connecting equal value supplies in parallel, with references to real-life scenarios and risks involved.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility and safety of connecting power supplies in parallel, with some advocating against it while others question the reasoning. The discussion remains unresolved regarding the best approach to achieve the desired voltage output.

Contextual Notes

Participants note the lack of information about internal resistance and the specific context of the problem, which may affect the proposed solutions. There is also a mention of the complexities involved with real-life power supplies compared to ideal models.

nfactor
Messages
5
Reaction score
0

Homework Statement



If I have 3 2V and 1A supplies, and I want to test at 5V, how will I setup?

Homework Equations



The currents add up when connecting the supplies in parallel, and the voltages add u when connecting in series.

The Attempt at a Solution



I tried different combinations but none of them seem to work!
 
Physics news on Phys.org
Welcome to PF;
Do you mean you have 3x 2V constant voltage supplies and 1x1A constant current supply, or 3x 2V constant voltage supplies rated at 1A, or something else?

Are these to be treated as ideal PSUs - i.e. zero internal resistance?

How is the test performed? Just putting a voltmeter across it?

Which combinations did you try?
Note: PSUs do not have to all face the same way, and they do not have to be connected to each other.
The terminals supplying the +5V do not have to span the PSUs.

JIC: how is the test performed? Just putting a voltmeter across it?
 
Its the latter, i.e. 3x 2V constant voltage supply rated at 1A, the problem does not say anything about internal resistance of the supply and I believe that the test is performed just by putting the load across the output terminals of the supply combination. For the purpose of the problem we can assume that the we are measuring the potential difference across the output terminals using a voltmeter.
 
So which combinations did you try?
(I realize it may be a problem describing them.)
 
I tried combinations like connecting two supplies in parallel and one in series, here the total voltage is 40 V with 2A total current (although it not encouraged to connect constant voltage supplies in parallel, just for the sake of problem), if you connect them all in series, you'll get a total voltage of 60V with 1A current, but I couldn't seem to get 50V.
 
here the total voltage is 40 V
How do you get 40V with 2V supplies?

Lets set up a notation: the supplies are:
A--2V>--B
C--2V>--D
E--2V>--F

the arrow indicates the positive terminal, and the letters label the terminals.
connect a circuit by specifying which letters to join together.

i.e. two in parallel with one in series could be:
connect: A-C, B-D, D-E,
voltages
V(AF)=4V = -V(FA)
V(AB)=2V
V(AC)=0V
... see how that works?

What is the context for the problem?
 
I am sorry, I was working on another problem, wrote 40 instead of 4V accidentally, 50 instead of 5V and 60 instead of 6V. I am terribly sorry.
 
What is the context for the problem?
 
This is just a basic test circuit analyses problem, to help learn different ways of using power supplies. It is one of my homework problems, which just tells what I have already written.
 
  • #10
nfactor said:
This is just a basic test circuit analyses problem, to help learn different ways of using power supplies. It is one of my homework problems, which just tells what I have already written.
You have three identical power supplies, each fixed at 2V. You want to be able to connect them up somehow so that a voltmeter probe can show a reading of 5V. Correct?

Hint: do you have some resistors available?

http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon2.gif BTW, you never connect these supplies in parallel, not even for the sake of solving a homework problem.
 
Last edited by a moderator:
  • #11
Hint: do you have some resistors available?
That's what's bothering me ... just a length of non-ideal wire will do. Which is why I wanted the context.

BTW, you never connect these supplies in parallel, not even for the sake of solving a homework problem.
What's wrong with connecting equal value supplies in parallel?
You probably have devices with batteries in parallel...

(If they are unequal - then good idea also to have a resistor in the circuit too.)
 
  • #12
Simon Bridge said:
What's wrong with connecting equal value supplies in parallel?
You probably have devices with batteries in parallel...
Poster is not using batteries.
 
  • #14
Simon Bridge said:
What's wrong with connecting equal value supplies in parallel?
You probably have devices with batteries in parallel...

What's wrong is the batteries are never exactly equal in potential, so a current equal to the difference in V divided by the sum of internal resistances is running and at best wasted. This current can be very large in e.g. car batteries, could cause overheating and even a fire ...

I have never seen any appliance of any kind with batteries connected in parallel. If I did I'd freak out ...
 
  • #15
Curious - well I don't want to derail the thread...
 

Similar threads

Designing a linear DC power supply
  • · Replies 6 ·
Replies
6
Views
3K
Engineering Is my Power Supply Circuit correct in the recreated version?
  • · Replies 6 ·
Replies
6
Views
3K
Making a 12-24v DC variable power supply
  • · Replies 11 ·
Replies
11
Views
3K
Sizing an Isolation Transformer for a DC Power Supply
  • · Replies 8 ·
Replies
8
Views
3K
How Do You Design a 12V 2A Unregulated Power Supply?
  • · Replies 4 ·
Replies
4
Views
2K
High School Analog power supply output question
  • · Replies 52 ·
2
Replies
52
Views
4K
Manually charging batteries
  • · Replies 11 ·
Replies
11
Views
3K
Checking my answer.... finding the power of a current and a voltage source
  • · Replies 6 ·
Replies
6
Views
3K
Voltage query -- A power supply, resistors and a voltmeter....
  • · Replies 46 ·
2
Replies
46
Views
5K
Engineering Easy zener diode circuit questions
  • · Replies 33 ·
2
Replies
33
Views
5K