LM338 Voltage regulator ck. Why this is not working?

Dodgydave

Joined Jun 22, 2012
11,284
HI Sir, I have checked the voltage around R1, R2, and R3. R1=9.92V , R2 = 12.03V and R3 = 15.68V.
R1, R2, and R3 are the one connection between the ceramic cap and electrolytic cap with LM388.
If you have 15.6V across R3, then you have wired it wrong or your regulator is blown, there should be only 1.25V across R3....
 

Thread Starter

Raj Pandey

Joined May 27, 2019
62
If you have 15.6V across R3, then you have wired it wrong or your regulator is blown, there should be only 1.25V across R3....
As in the Image I have attached and you can check it with the datasheet I have provided in Main Post. Please tell me if you Find anything wrong. I will check with another IC also in the meantime.
 

Dodgydave

Joined Jun 22, 2012
11,284
Can you take a video of your work with voltmeter readings, ideally start a fresh circuit with just the lm388, and then we can see what you're doing...?
 

crutschow

Joined Mar 14, 2008
34,281
HI Sir, I have checked the voltage around R1, R2, and R3. R1=9.92V , R2 = 12.03V and R3 = 15.68V.
R1, R2, and R3 are the one connection between the ceramic cap and electrolytic cap with LM388.
I asked for all the node voltages be added to the schematic and then post that schematic.
If that's too much work for you, then it's also too much work for me. :rolleyes:
 
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bertus

Joined Apr 5, 2008
22,270
Hello,

As said, measure the voltages on the pins of the LM338 and write them in the schematic:

raj_lm338k_voltages.PNG

The difference between Vout and Vadj should be about 1.2 Volts.

Bertus
 

Thread Starter

Raj Pandey

Joined May 27, 2019
62
Hello,

As said, measure the voltages on the pins of the LM338 and write them in the schematic:

View attachment 178415

The difference between Vout and Vadj should be about 1.2 Volts.

Bertus
Ok I will do it and post.

I asked for all the node voltages be added to the schematic and then post that schematic.
If that's too much work for you, then it's also too much work for me. :rolleyes:
I thought you have just asked about the resistor node voltages. Sorry, Sir, I will do it as soon as possible.

I asked for all the node voltages be added to the schematic and then post that schematic.
If that's too much work for you, then it's also too much work for me. :rolleyes:

Hi, Sir did with the schematic and posted. Please review.
 
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danadak

Joined Mar 10, 2018
4,057
Very suspicious drop across R1 150 ohm is almost as much as drop
across R2 685 ohm.

Also drop across R3 not equal to 1.25V approximately.

And that Vr1 + Vr2 is not = Vadj....?

Is your voltmeter working correctly ? If its portable its battery
OK ?


Regards, Dana.
 

Dodgydave

Joined Jun 22, 2012
11,284
You clearly have a fault at R1,R2 voltages, and you're loosing 0.7V from the regulator out to R3, i suspect that R3 is not grounded...
 

LesJones

Joined Jan 8, 2017
4,174
Are all of your voltage readings in post #33 with respect to ground ? If so there are readings that make no sense. At the bottom end of R2 you are showing 12.03 volts (I assume positive.) This should be zero volts. at the output pin of the regulator you are showing 16.35 volts (Again I assume positive.) but at the top end of R3 you show 15.68 volts. These two readings are the same point so they should be the same. It could be that you have some poor (High resistance connections between those two points.)

Les.
 
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RamaD

Joined Dec 4, 2009
328
I see two resistors of 150 Ohm, R3 & R1. Colors of R2 are not clear. Can you measure the values with a multimeter and verify that they are correct? Also, note the points by @LesJones & @danadak. Are you measuring the node voltages or the voltages across the resistors?

The min. load seems to be 10mA as per the datasheet. So, put the two 1k resistors in parallel as the load. If the output happens to be 15V, then the Pd on the resistor is 225mW. So use resistors of 1/2W of more for this 1k.

Some may not be so trivial points -
  • R3 should be connected closer to the regulator than the load end.
  • Output cap is at least 1uF solid tantalum or 25uF Al Electrolytic. Replace the 1uF cap with 100uF Al Electrolytic capacitor. Keep it close to the regulator.
 

Thread Starter

Raj Pandey

Joined May 27, 2019
62
Very suspicious drop across R1 150 ohm is almost as much as drop
across R2 685 ohm.

Also drop across R3 not equal to 1.25V approximately.

And that Vr1 + Vr2 is not = Vadj....?

Is your voltmeter working correctly ? If its portable its battery
OK ?


Regards, Dana.
yeah just I have bought a new one done checking with it.

Are all of your voltage readings in post #33 with respect to ground ? If so there are readings that make no sense. At the bottom end of R2 you are showing 12.03 volts (I assume positive.) This should be zero volts. at the output pin of the regulator you are showing 16.35 volts (Again I assume positive.) but at the top end of R3 you show 15.68 volts. These two readings are the same point so they should be the same. It could be that you have some poor (High resistance connections between those two points.)

Les.
I don't know Sir. What is the problem

I see two resistors of 150 Ohm, R3 & R1. Colors of R2 are not clear. Can you measure the values with a multimeter and verify that they are correct? Also, note the points by @LesJones & @danadak. Are you measuring the node voltages or the voltages across the resistors?

The min. load seems to be 10mA as per the datasheet. So, put the two 1k resistors in parallel as the load. If the output happens to be 15V, then the Pd on the resistor is 225mW. So use resistors of 1/2W of more for this 1k.

Some may not be so trivial points -
  • R3 should be connected closer to the regulator than the load end.
  • Output cap is at least 1uF solid tantalum or 25uF Al Electrolytic. Replace the 1uF cap with 100uF Al Electrolytic capacitor. Keep it close to the regulator.

Ok, I will replace output cap to 100uF and R3 Close means I don't get it.
@LesJones & @danadak I have checked there points.
I am measuring the voltage across each resistor like taking one point(+ve) end of the resistor and ground(-ve). I am not checking across it.
 
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crutschow

Joined Mar 14, 2008
34,281
I am measuring the voltage across each resistor like taking one point(+ve) end of the resistor and ground(-ve).
Then how can R1 have a lower voltage than R2?
You are either measuring wrong or there is in error in the connection of R1 and R2.

You need to think about whether the voltages make sense in the circuit.
 

Thread Starter

Raj Pandey

Joined May 27, 2019
62
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