# variable current limiting circuit help

Discussion in 'Homework Help' started by Maldorin, Aug 17, 2008.

1. ### Maldorin Thread Starter New Member

Aug 17, 2008
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0
Hi all

I have to build a variable voltage and current limiting linear power supply as a project and I have selected a specific circuit(which i attached to this post) which I got from the national semiconductor datasheet. There are just 3 things which I dont understand and was hoping sum1 could answer for me, In the circuit diagram there is a 0.24ohm resistor, what is the purpose of this resistor? can it be left out? and for the LM117, from my understanding it is used to make the current limiter go down to 0V but I dont quite understand how and why you need the negative voltage, is the negative voltage just to bias the transistors and internal circuitry inside of the LM117, There is a constant 1.25V reference between the output and adjust, why is the 120 ohm resistor not between adj and output instead in that case? and finally, if I have a voltage regulator stage before this stage, will that output be able to go down to 0V through the LM117 from this stage, Any help would be greatly appreciated. Thanks for your time

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2. ### JoeJester AAC Fanatic!

Apr 26, 2005
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Once you read the LM338 datasheet, you'll have the answer to the second question as well.

According to the diagram, you want 0-5A output.

3. ### SgtWookie Expert

Jul 17, 2007
22,201
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Well, it's not really obvious - even if you read the datasheet a number of times.

The 0.24 Ohm resistor sets the maximum current output.

Vref may vary from 1.19v to 1.29v; typical is 1.24v (see datasheet)
Iout = Vref/R1
Conversely, R1 = Vref/Iout, where ((minimum load current) <= Iout <= 5A)
Find minimum load current in the datasheet.

Next, get the datasheet for the LM117/LM317. Find out what happens when you connect a 120 Ohm resistor between the out and adj pins, and connect the adj pin to a current sink that is several volts lower than the input pin.

Following along with that same train of thought, what is the maximum actual current output you can expect to get from the LM338 and stay within the rating of the regulator?

What will be the wattage required of the 150 Ohm variable resistor R2? (think in terms of what happens when the resistor is set near minimum resistance)