# Homework Help: Power supply combination problem

1. May 18, 2014

### nfactor

1. The problem statement, all variables and given/known data

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

2. Relevant equations

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

3. The attempt at a solution

I tried different combinations but none of them seem to work!

2. May 18, 2014

### Simon Bridge

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?

3. May 19, 2014

### nfactor

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.

4. May 19, 2014

### Simon Bridge

So which combinations did you try?
(I realize it may be a problem describing them.)

5. May 19, 2014

### nfactor

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.

6. May 19, 2014

### Simon Bridge

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?

7. May 19, 2014

### nfactor

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.

8. May 19, 2014

### Simon Bridge

What is the context for the problem?

9. May 19, 2014

### nfactor

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. May 19, 2014

### Staff: Mentor

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 [Broken] BTW, you never connect these supplies in parallel, not even for the sake of solving a homework problem.

Last edited by a moderator: May 6, 2017
11. May 19, 2014

### Simon Bridge

That's what's bothering me ... just a length of non-ideal wire will do. Which is why I wanted the context.

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. May 19, 2014

### Staff: Mentor

Poster is not using batteries.

13. May 20, 2014

### Simon Bridge

14. May 20, 2014

### rude man

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. May 20, 2014

### Simon Bridge

Curious - well I don't want to derail the thread...