Setting the output to 6 volts with 12 volts input.
Using two 39 Ohms resistors in series.
This gives me 5.93 volts with 0.530 Amp load.
I encountered an unusual situation.
Needed to raise the voltage a bit, used two 150 Ohms in parallel, this dropped the output to 2 volts no load.
Tried again with a 47 ohms and a 27 Ohms in series...same output as above.
Can some one explain this to me?

 

Welcome to AAC!

LM318 can't source or sink more than a couple dozen mA. It would be helpful if you posted a schematic.

EDIT: The original title referred to LM318 and was changed to LM338 by moderation.

 

You did not say or show anything about where you connected the two resistors.

 

The title is a typo.
The device is a LM338T.

 

R1 is almost always 120 ohms and has 1.25V across it. Then the current in R1 is 1.25V/120 ohms= 10.41mA. R2 has the same 10.41mA in it so if you want the output to be 6V then R2 must have 6V - 1.25V= 4.75V across it and its value must be 4.75V/10.41mA= 456.3 ohms.
The 1.25V might be a little higher or be a little lower. You can use 100 ohms for R1 if you recalculate its current and the different value for R2.

 

R1 is almost always 120 ohms and has 1.25V across it. Then the current in R1 is 1.25V/120 ohms= 10.41mA. R2 has the same 10.41mA in it so if you want the output to be 6V then R2 must have 6V - 1.25V= 4.75V across it and its value must be 4.75V/10.41mA= 456.3 ohms.
The 1.25V might be a little higher or be a little lower. You can use 100 ohms for R1 if you recalculate its current and the different value for R2.

R1 is not the resistor in question.
It is R2. The resistor that varies the output voltage.
78 Ohms gives me the 5.93 volts with the 0.530 Amp load.
I needed to raise the voltage a bit.
Tried the combinations above, with the posted results.
The resistor combinations in theory, should have raised the voltage a small amount.

 

R1 is almost always 120 ohms and has 1.25V across it. Then the current in R1 is 1.25V/120 ohms= 10.41mA. R2 has the same 10.41mA in it so if you want the output to be 6V then R2 must have 6V - 1.25V= 4.75V across it and its value must be 4.75V/10.41mA= 456.3 ohms.
The 1.25V might be a little higher or be a little lower. You can use 100 ohms for R1 if you recalculate its current and the different value for R2.

I compared my calculations with yours.
Found an error in the E/I for R2.
Have a stable 5.97 volts at 0.530 Amp.
R2 = 440 Ohms.
Thanks for the help.

 

Have a stable 5.97 volts at 0.530 Amp.
R2 = 440 Ohms.

440 ohms should give you 5.65V. With a set current of 10mA, 470 ohms will give you about 5.95V.

 

440 ohms should give you 5.65V. With a set current of 10mA, 470 ohms will give you about 5.95V.

It’s a little over 5.7 no load.
With the load it’s 5.96.
Thanks again for your help.

 

It’s a little over 5.7 no load.
With the load it’s 5.96.

Load regulation should be much better than that and I'd expect output voltage to drop under increased load.

 

For best regulation, the top of R1 should be connected directly to the LM338 output pin (not the circuit output terminal as you might expect).
That's because any resistance between that pin and the connection for R1 will significantly degrade the load regulation (due to the fact that the output pin is one of the internal voltage reference points).
And thus the connection from the LM338 output pin to the circuit output terminal should have low resistance.

Also the bottom of R2 should be connected directly to the output terminal common.
That minimizes the effect of the circuit ground resistance on regulation.

But normally the no load voltage is higher than the load voltage.
Did you mix up those two values?

 

I bet the resistance of the rows of contacts on a solderless breadboard are messing up the voltages.

 

For best regulation, the top of R1 should be connected directly to the LM338 output pin (not the circuit output terminal as you might expect).
That's because any resistance between that pin and the connection for R1 will significantly degrade the load regulation (due to the fact that the output pin is one of the internal voltage reference points).
And thus the connection from the LM338 output pin to the circuit output terminal should have low resistance.

Also the bottom of R2 should be connected directly to the output terminal common.
That minimizes the effect of the circuit ground resistance on regulation.

But normally the no load voltage is higher than the load voltage.
Did you mix up those two values?

No. The 120 is in it's correct position as is R2. I'm happy it works as needed.

 

I'm happy it works as needed.

The point we're trying to make is that it isn't working correctly.

 

The point we're trying to make is that it isn't working correctly.

The resistors are connected directly to the device terminals and in the proper places.
The voltage goes up to 5.97 from 5.72 when the load is applied.

 

The voltage goes up to 5.97 from 5.72 when the load is applied.

That's the point. The voltage shouldn't change that much and it should go down with increasing load; not up.

What is the input voltage and source?

 

That's the point. The voltage shouldn't change that much and it should go down with increasing load; not up.

What is the input voltage and source?

A three amp variable output power supply set to 12.6 volts.

 

Hello,

I have changed the title to LM338T.
I hope this will take away the confusion.

bertus

 

A three amp variable output power supply set to 12.6 volts.

Measure the voltage at all three terminals for no load and loaded conditions.

 

Measure the voltage at all three terminals for no load and loaded conditions.

Will do.