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Fixed voltage circuit

Heeran

Nov 13, 2012
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Hi, I am currently trying to build a fixed votage output circuit but i need some assistance, i am using the lm2940t but it does not help. My application is that i require an output of a maximum of 10 volts and minumum of 9.6volts, my input is a 24 voltage lead acid battery. The circuit must be able to supply the required output voltage. And it must remain constant even as the input voltage is droping until the input voltage drops below the voltage required to supply that amount of output voltage. Please assist me in building this circuit with the correct components required and diagram.Thanks
 

Harald Kapp

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The LM2940 is a suitable part for this. You will not find a linear regulator that works with Vin=Vout. Every regulator needs a voltage differential (voltage drop) between input and output. The value of this drop depends on the type of regulator. The LM2940 has a min. drop voltage of 0.5V.

i am using the lm2940t but it does not help

What do you mean by "does not help"? What does your circuit like? What does ist do? What does it not do?

Harald
 
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Heeran

Nov 13, 2012
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Well when i connected the ic, the input voltage was 24 volts but the output voltage was less than 9v, and it did not work the way i expected.
 

Harald Kapp

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Show us your complete schematic. Obviously you have a mistake there.
 

Heeran

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The circuit is wired up like so, i got Vin going to input pole, a resistor connected between the adjust and output pole. The output pole is goin to the load . The ground of the circuit is direct from the source to the load.
 

Heeran

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The pitures are attached
 

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Harald Kapp

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That can't work. Who told you to connect the regulator like this?

Look up the datasheet. Page 2 shows a typical application. You need the LM2940T-12 for 12V output. And don't forget the capacitors.

Harald
 

Heeran

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If i use the lm2940 - 12, will my maximum output fixed voltage be 10 volts and will the circuit output remain constant even as the input voltage will be droping. Could you please provide me with a drawing diagram of the circuit and the components and component ratings.
 

CocaCola

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If i use the lm2940 - 12, will my maximum output fixed voltage be 10 volts

No it would be 12V, if you want 10V out use the LM2940 - 10... I think Harald had a brain fart :)

and will the circuit output remain constant even as the input voltage will be droping.
As long as the input voltage is about +1V over the output, in this case 11V or higher... Technically even 0.5V or higher will still be stable... And also as long as the drain is under 1A, anything more and the chip won't perform properly... You will also need a heat sink when using the high 24V in, unless your drain is very low... If the input voltage is say 12V - 14V you might be able to get away without an additional heat sink...

Could you please provide me with a drawing diagram of the circuit and the components and component ratings.
It's in the datasheet Harald linked, as well as a lot of other pertinent information on the chip...
 
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Harald Kapp

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I think Harald had a brain fart
Yeah, sorry, it's the LM2940-10.
I think I was a little bit fixed on 12 V.
 
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Heeran

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I have had a look at the data sheet but i did not find a diagram that will suit my requirement, if you could please draw a diagram with the required componants, I will be highly greatfull.Thanks
 

BobK

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What is wrong with the one labelled "Typical Application" on the first page of the datasheet?

Bob
 

CocaCola

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I have had a look at the data sheet but i did not find a diagram that will suit my requirement, if you could please draw a diagram with the required componants, I will be highly greatfull.Thanks

How about you help yourself a little? I mean seriously, a little effort goes a long way here... Did you even bother reading the datasheet, I don't mean 'look' it over I mean read it? If so skip the cover page and read the first page (2nd page of PDF) a few more times, there might something of interest to you on that page...
attachment.php


Seriously we don't mind helping people, but a little bit of effort on your part goes a LONG way... Especially when we have already told you were to look for what you want...
 

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Heeran

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Ok, i see it. So i should wire up my circuit using that diagram and a lm2940t-10 ic, if i dont use the capacitors, will there be a major differance, and with the circuit wired up liike that diagram, i should get my require output
 

CocaCola

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Ok, i see it.

How may I asked did you not see it before? Let me guess, you didn't bother actually looking?

So i should wire up my circuit using that diagram and a lm2940t-10 ic, if i dont use the capacitors, will there be a major differance, and with the circuit wired up liike that diagram, i should get my require output
Did you read the datasheet??? Honestly did you, or are we back at square one with you not applying any effort of your own?

Look at the two paragraphs under the picture you just discovered, same page 1 (you read it right?) let me know what those two paragraphs say...

Again we love to help, but please try to actually put a little effort of your own forward...
 
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BobK

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Summary of this thread:

"I really need some help, but I don't want to do any work, so give me the answers."

"They are in the datasheet, and don't forget the capacitors".

"I don't see them."

<post of datasheet with answer circled>

"Okay, after you circle the obvious answer in the datasheet, I get it, but I don't like capacitors, can I just leave them out?"

"Arghhh"

Bob
 
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