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Portable DC Bench Power Supply

Anila

Feb 4, 2016
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In the Portable DC Bench Power Supply circuit given, as per the data sheet of LT3081 output current can be controlled by varying the Ilim resistor, but the current cannot be changed by the resistor. What is the reason, give a solution?
Iout=Vout/Rload, Vout=Rset*1.45mA, Iout can be changed by the Resistor between Ilim and out pin, as 360mA/kΩ.

Portable DC bench Power Supply.JPG
 

Harald Kapp

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Welcome to electronicspoint.

The output current is given by I=Vout/Rload. You can control this current by either changing Vout or by changing Rload. Nothing else.
Current limiting is used to protect the power supply (or the load) from overload (too high current). When the output current rises above the limit value, the output voltage is automatically reduced such that by I=V/R the current is below the limit value.
 

Harald Kapp

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But when i simulate the circuit it didn't work so.
Then what did you expect, what was the behavior simulated? How did you simulate? Where or what are the differences?

I did the simulation with load resistors 100Ω, 50Ω, 20Ω, 10Ω, 5Ω, 2Ω and 1Ω. This is my result:
upload_2016-2-5_6-53-11.png
One can clearly see that the voltage is regulated to 8 V as long as the load current is less than ~3.8 A.
When the load current reaches 3.8 A, the output voltage is reduced such that by I=V/R the current is limited to 3.8 A.

The problem may not be with the circuit but the way you simulate it.
 

Anila

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The circuit output voltage is 8V and current limit is 3.8A, if i want to change the current limit to 2A with Vout=8V, According to the data sheet of LT3081 Ilim adjust resistor can be used to change the current limit , typical ratio of current limit to resistor value is 360mA/kΩ. Then to change the current limit to 2A,Ilim adjust resistor =5.5K, but if I put this value, I didn't get the Ilim as 2A.
 

Harald Kapp

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The value for Rlim in the downloadable .asc file differs widely from the one in the article: 104kΩ vs. 5kΩ.

  1. It seems that the value in the schematic is way too high. Therefore the internal current limit of the LT3081 sets in at ~2A per IC which adds up to ~4A (3.8A) with two ICs used.
  2. The equation from the datasheet is not directly applicable as two ICs share a common limit resistor in this schematic. Using values from 100Ω...5kΩ for Rlim you get different current limits. Simulate a few and figure out how Rlim relates to Ilim using linear interpolation.
Regards,
Harald
 

Anila

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lt3081.JPG
In this ckt single linear regulator is used, but if the Ilim resistor value changes, reduces the output vtg also, what is the reason ?
 

Harald Kapp

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To limit the current the voltage needs to be reduced to fulfill Ohm's law: I=V/R or V=I*R (which is the same).
 

Anila

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current limit.JPG


The image is from data sheet of LT3081, as per this graph current limit can be varied by varying RIlim and output voltage remains constant, how does it work?
Vout=Rset * 50uA
 

Harald Kapp

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You need to learn how to read a diagram. Voltage does by no means stay constant. Due to I=V/R it cannot!
Look at e.g. the red line for Rilim=1.5k.
Voltage is 1 V for current less than ~0.4A. At I=0.4A the voltage drops very steeply (almost vertically) to zero because any increase in current needs to be countered by a fall in voltage to keep the current at the limit.
 

Anila

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For RIlim=4.53K, the diagram shows that voltage is 1V for current less than 1.45A, ie regulate to 1V when load changes from 0A to 1.45A, at 1.45A the voltage drops steeply. And If RIlim=3.01K, the voltage is 1V for current less than 0.9A.

(Vout= Rset * 50uA -------------------(1)
Rset
is the resistor connected at the set pin of IC & RIlim is the resistor connected at the Ilim pin of the IC
Rset is used to change the output voltage and RIlim is used to change the output current limit)

If I reduce RIlim from 4.53K to 3.01K , Rset=20K (remain constant) then as per the eqn(1) Vout remain at 1V for current upto 0.9A, But the Vout also reduces. ie, for current less than 0.9A, vout is less than 1V,it doesn,t regulate to 1V, why?
 

Harald Kapp

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If I reduce RIlim from 4.53K to 3.01K , Rset=20K (remain constant) then as per the eqn(1) Vout remain at 1V for current upto 0.9A, But the Vout also reduces. ie, for current less than 0.9A, vout is less than 1V,it doesn,t regulate to 1V, why?
Show me your simulation results. In my simulation the regulator works well. You have toskip the first 80µs of the simulation, during this time the regulator settles to the set output value. Afte ~80µs, the output voltage is stable - and 1V.
 

Harald Kapp

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This is my simulation:
upload_2016-2-9_8-52-12.png
Not as nice and clean as in the datasheet, but recognizable. Some tweaking of components may be required.
 
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