Approximate values for voltage level with no load condition

Status
Not open for further replies.

rajaram04

Advanced Member level 3
Joined
Jun 17, 2012
Messages
871
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,298
Location
earth
Activity points
7,687
Hello sir

I want to know about approx maximum value of voltage with no load (open circuited) for a particular voltage level driven out from a power source . . . . .

I mean to say that if for example a simple USB/SDcard player common circuit operates on 5 volts exactly or it needs an exact 5 volts for its proper operation , then how much maximum value should there be in the source (with no connection to circuit) to drive the circuit properly ? ? ? please comment

I need to know values for

1.5 volts
3 volts
5 volts
6 volts
9 volts
12 volts
 

Each circuit is different and has a different maximum output level. If the circuit has voltage gain then the input must be at a low enough level. If the signal is audio then your ears will hear severe distortion when the circuit reaches its maximum level and begins to clip.
 

This question comes into play particularly with an unregulated power supply.

Suppose it is an ordinary: transformer > rectifier > capacitor.

With a light load the capacitor charge rises to 1.41 times the AC input (less diode drops).

As you add greater load, the output voltage drops. Where it finds equilibrium has to do with how much juice the capacitor stores during the crest of the waveforms, versus how much it discharges during idle times.

Another factor is the effective resistance of the transformer.

Somewhere there might be a graph showing how output V drops as more current is drawn. I don't remember ever seeing such a graph.
 

The magic word is "regulation", a good laboratory power supply might have regulation of .1%, that is its output voltage will change by less then .1%, from zero current to rated current. As explained a cheap "wall-wart" power supply might be as bad as 40% :-(
Frank
 

ohh kk thanks to all of you , actually i am designing voltage regulator with LM317 & using this configuration below

**broken link removed**

Here in above one i hv selected R1 as 1k & with 12 volts 1.5 amp input i am trying to configure R2 but with constant resistor value i am getting a diffrence . .

here with R2 as 500 ohms i observed 2.75v approx
with R2 as 1k i am getting 4.29v approx
with R2 as 1.2k output is 4.74v approx
with R2 as 1.5k output is 5.34v approx
with R2 as 2k output is now 6.14 volts

now i am confused to choose resistor as well as voltage levels with no load
i am using presets to configure but please tell me about what should be the reading on voltmeter for a particular level in no load condition in approximation values like 5.13v for 5 volts , 9.17v for 9 volts etc etc etc & like that ???

(the output voltage reading of the adapter which i am using is near about 12.15 volts in no load condition)
 

Your attachment does not work so I cannot see your schematic.
The datasheet for the LM317 shows 240 ohms for the resistor from the output to the ADJ pin for the more expensive LM117. It should be 120 ohms maximum for the LM317 or the output voltage will rise with no load.
There will be 1.25V across 120 ohms making the current 10.4mA. You want an output of 5.0V so the resistor to ground will have a voltage of 5V-1.25V= 3.75V. It will also have a current of 10.4mA so its value should be 3.75V/10.4mA= 360 ohms.

The datasheet says that an input capacitor and an output capacitor should be used mounted close to the pins of the LM317.
 






hmm thats a kind of info i need here sir . . achematic is just like what i have shown in the diagram & at input i am using 0.1u & 1000u in parallel & close to IC & at output end i applied a 10u , as i told earlier that 1k is connected in between output & adj.pin & i am varying resistor between adj. & ground thats it

But as you explained above with your best so i ve to change & divert myself in this way , its awsome . . thanks
let me move with that . .

well anything else ? please comment . . waiting
 

rajaram04 said:
okk surely i ll do that

& what for 1.5v , 3v , 6v , 9v & 12v ?

please tell
Click to expand...
Use 120 ohms for the resistor from the output to the ADJ pin then calculate its current. Then use Ohm's law to calculate the other resistor value like I did in my post #6.
 

Audioguru said:
Use 120 ohms for the resistor from the output to the ADJ pin then calculate its current. Then use Ohm's law to calculate the other resistor value like I did in my post #6.
Click to expand...


well sir as you mentioned 120 ohms & 360 ohms for 5 volts , practically its not working & i am getting output around 2 volts something . . so i connected diffrent 2k pot. . . would you please refer & tell me a sequence to make the practical from very begining including how to measure current & voltage across resistor etc ? ? ?
 

Maybe you got the pins of the LM317 mixed up. Look at its pins on its datasheet.
1.25V/120 ohms gives a current of 10.42mA. 10.42mA x 360 ohms is a voltage of 3.75V. 1.25V + 3.75V= 5.0V.
The datasheet recommends an input capacitor close to its pins of 0.1uf and an output capacitor close to its pins of 1uF to 10uF.
 





o ya ya sorry my mistake i choosed wrong pins for power supply & outputs now i am getting correct result so i ve to move in that way . . thanks . . let me proceed sir . .

- - - Updated - - -

too i got a calculator link here , please refer is it ok or not very much ??

http://www.electronics-lab.com/articles/LM317/

- - - Updated - - -

This above link sets the default value for R1 as 240 ohms at the moment we open the page link
 

Electronics Lab and many other websites are WRONG!
Electronics Lab did not invent the LM317, National Semi did but now Texas Instruments owns them.
The datasheet says that the more expensive LM117 uses 240 ohms but the cheaper LM317 needs 120 ohms.
 

Audioguru said:
Electronics Lab and many other websites are WRONG!
Electronics Lab did not invent the LM317, National Semi did but now Texas Instruments owns them.
The datasheet says that the more expensive LM117 uses 240 ohms but the cheaper LM317 needs 120 ohms.
Click to expand...




oooo i see hmm so if all these things are wrong then how could we proceed ? If faithful & biased results are not available on net & all the way then how could we design any of them sir ??? then i guess we should know each of the basic theory relating devices circuits etc like peeling out skin of hair . .
 

That Electronics Lab web page is not wrong, just maybe a bit misleading.

The circuit shown with an LM317 and R1=240 Ohms will work fine as long as the external load draws at least about 5mA from the output.
You need to reduce R1 to 120 Ohms if you want good regulation even when there is no load connected to the output.

Also, that page specifically says "Typically R1 is 220 ohms or 240 ohms, but it could be some other value. Check the Data Sheet for more information regarding this."
That's why the calculator lets you choose the value of R1. Sometimes a lower value is needed and sometimes a higher value is OK.

rajaram04 said:
... i guess we should know each of the basic theory relating devices circuits etc...
Click to expand...
Yes, learning some basic theory is a very good idea.
 

Many websites post WRONG information. I corrected many projects at Electronics Lab and Aaron Cake but I am not interested in other wrong ones there.
Datasheets published by the company that invents a device are usually correct. Copies sometimes have errors.

There is a website in India where all the projects "are tested" but most of them are wrong and they do not work.
Texas Instruments publish an article, "A Single-Supply Op-Amp Circuit Collection" where a non-inverting opamp is biased wrong.
Fairchild copied Motorola's MPSA56 transistor and in their datasheet they show wrong spec's and wrong graphs.
 

my god then we could not believe on such articles in future when texas published wrong one . .
In schooling we read about so many devices designed by them . .
Well this forum here to discuss such with really skilled people like you sir . . i am doing work for mankind & need your support

- - - Updated - - -

So again coming to the point , i found a datasheet posted on net by fairchild semiconductor

should this one explain the right info ? may i follow it for my LM317 ??


Here one more diagram i got showing LM317 as audio amplifier . . now whats that heck again ????

 

rajaram04 said:
Here one more diagram i got showing LM317 as audio amplifier . . now whats that heck again ????
Click to expand...
Its output power is fairly low but it uses a high current from the battery heating the LM317 and heating the 10 ohm resistor ALL THE TIME even when it is not playing.
 

Audioguru said:
Its output power is fairly low but it uses a high current from the battery heating the LM317 and heating the 10 ohm resistor ALL THE TIME even when it is not playing.
Click to expand...



okk so how to protect that ?
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…