# Need of Constant o/p voltage????

Discussion in 'Homework Help' started by darshan.14, Mar 14, 2010.

1. ### darshan.14 Thread Starter New Member

Mar 14, 2010
2
0
Hello all,

I am new to electronics and have some of doubts about voltage source.

I have read most of the times that we require constant voltage source (I mean constant voltage over wide range of load - current conditions). But I fail to understant that
1. why do we require it to be constant? though the current changes according to the load. what is the significane of keeping it constant.
2. what happens if it reduces with load. (still current requirement are met with the increase, isnt it?)

3. In otherwords voltage is the force and current if flowrate, so whats the secrent benind keeping the force constant with varous flow rate requitemts? whats the purpose getting solved?

waiting for replies ,
Darshan

Last edited: Mar 23, 2010

Jul 7, 2009
1,585
141
Please don't use abbreviations in your posts -- some people (like me) won't understand what o/p means.

The only reason that someone would want a constant voltage for a given load is that they would then know that the current was constant through that load. Of course, the current will change if the load resistance changes. Also, if a load was a constant resistance, then a varying voltage would of course produce a varying current.

There are many reasons why a constant voltage or constant current can be required. Here's one example: I want to measure the resistance of a piece of wire with connections in it to see if any of the connections have degraded. I can put a constant voltage across the wire and verify the current remains constant. Then I know the various resistances involved aren't changing. I can probe the various connections by putting my digital multimeter probes onto the wire on either side of the connection and read the voltage drop. Then I can calculate the resistance of the connection. If the voltage was known and constant, I could go back and repeat the same measurement years later and see if anything has changed.

This, by the way, is a handy technique (it's known as the Kelvin 4 wire method of measuring resistance, or a "Kelvin connection"). Since the input impedance to the multimeter is 10 MΩ or so, the contact resistance of the probe to the wire is irrelevant, as is the lead resistance of the probes. This lets me measure low resistances. For example, if I cause 1 ampere to flow through the wire and connections and my multimeter reads to 0.1 mV (my Fluke 83 meter does this), then I can read resistances down to 0.1 mΩ.

A couple of years ago I used this technique to find a degraded connection in the wiring of my utility trailer (I didn't even know the connection was there until I got a resistance 10 to 100 times larger than I expected).

Thus, if you have a DC power supply or a DC wall wart, you can use it to troubleshoot wiring. It's substantially easier if you have a wire-piercing type of probe. You can also get the Pomona 5913 probe, but they're more than twice the price of the Cal Test model (I bought the Pomona one over 20 years ago and it was just under \$20). If you work on cars or house wiring, these probes can save you lots of time.

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3. ### Bychon Member

Mar 12, 2010
469
41
Hmmm...why would anybody need a voltage source to be stable?

If you are amplifying an audio signal and the supply voltage is changing, the changes will affect the sound quality. If you're operating a light bulb and the voltage changes, the light bulb will get brighter and dimmer. If you're measuring a part and the voltage keeps changing, it will look the same as if the part was bad. If you're trying to start your can and the battery voltage changes very much, the car won't start.

and I have no idea what an O/P voltage is.

4. ### Bychon Member

Mar 12, 2010
469
41
Oops. That was "start your car". Starting your can is a whole 'nuther matter.

5. ### darshan.14 Thread Starter New Member

Mar 14, 2010
2
0
Opss Sorry ..I meant Out Put by the abbreviation O/P. Thanks you ..I shall never use abbreviations.

Still my doubt is as it is. Because there is lot of stress on designing constant volatage sources for electronic desing and also the source should with stand the changing load consitions.

6. ### Bychon Member

Mar 12, 2010
469
41
Designing constant voltage supplies is one of the most basic things we do in electronics. Every circuit needs voltage to operate. Every voltage supply must be fairly close to the voltage the circuit requires. After you realize that, it becomes a matter of, "how constant is it?" Some circuits work well enough with only a transformer, a rectifier, and a capacitor. A volt or two of change as the AC supply goes through its 50 or 60 cycles per second while feeding a 24 volt relay won't stop the relay from working. A thousand volts of error on the anode of a 30 inch television tube (CRT) isn't much of a problem. Two volts of change in a pair of "C" batteries feeding a flashlight could be enough to make the flashligh useless. It's all a matter of how constant the voltage needs to be to keep the circuit working properly. If you still don't believe this, try applying a thousand volt error to a flashlight bulb.