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# LM338 Power Suply Current Regulator

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in my mind that junction where transistor or IC leg sold
Case transistor or IC outer body
ambient mean air

am i right or not

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The junction is the actual piece of silicon inside the case.

I'd recommend using an old Pentium 3 CPU cooler as the heat sink because it will have a thermal resistance of under 0.5

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Hello hero and audioguru
how are you
my final power supply circuit diagram that attatch
Fig 1
the 40V dc supply make using transformer and diode and capacitor circuit
the first sec current limiter circuit

value of R2 vary from 0.24ohm to 12ohm (current varry 100mA to 5A) and wattage 10W

the wattage of R3 is V=0.8V and I = 400mA about 0.5W

the wattage of emmitter resistance is = ?

Please tell me that this circuit is right or any error in this circuit please under line those error

In fig 2 R5 replaced fixed resistors that control using CD4066 ic for open and close these resistors that right or not

very very thanks a lot

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The circuit is totally impractical.

Where are you going to buy a 12 Ohm pot. rated to 5A or 6.25W when it's set to 0.25Ω?

The maximum power rating for a pot. is always for the full track so when the wiper is in the middle it's halved, in this case 0.25Ω is just under 2% of the maximum resistance so the power rating will only be 2% of that specified by the datasheet.

The maximum output voltage with 40V in will be about 32V so why have you used a 5k pot which will give lots of dead band and no voltage regulation when the wiper is set too high? In other words with 120R and 5k the set voltage is 50V but the maximum output voltage is only 32V so when the pot is set for above 32V, there will not be any voltage regulation when 5A is drawn.

The CD4066 has a maximum voltage rating of only 18V and the maximum voltage it can switch is equal to its supply voltage so can't be used for this application.

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The voltage regulator uses many transistors for high power but the current regulator uses only a single LM338 that will get so hot and will shut-down when the current is higher than its setting then the circuit will not work.

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The voltage regulator uses many transistors for high power but the current regulator uses only a single LM338 that will get so hot and will shut-down when the current is higher than its setting then the circuit will not work.

The LM338 will only overheat after the current limit has kicked in, before then the voltage across it will be between 1.5V and 3V, depending on the current and ambient temperature.

When the current limit, the current regulator might shut down, depending on the load voltage and current setting, if it's a problem a diferent solution is required.
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Where are you going to buy a 12 Ohm pot. rated to 5A or 6.25W when it's set to 0.25Ω?

The maximum power rating for a pot. is always for the full track so when the wiper is in the middle it's halved, in this case 0.25Ω is just under 2% of the maximum resistance so the power rating will only be 2% of that specified by the datasheet.

i am using different fixed resistors and these resistors control with realy's

The maximum output voltage with 40V in will be about 32V so why have you used a 5k pot which will give lots of dead band and no voltage regulation when the wiper is set too high? In other words with 120R and 5k the set voltage is 50V but the maximum output voltage is only 32V so when the pot is set for above 32V, there will not be any voltage regulation when 5A is drawn.

i am using about 3K resistor whose cove 32.5V

The CD4066 has a maximum voltage rating of only 18V and the maximum voltage it can switch is equal to its supply voltage so can't be used for this application.

Ok if i use this circuit for 0-18V then this is right or not

The voltage regulator uses many transistors for high power but the current regulator uses only a single LM338 that will get so hot and will shut-down when the current is higher than its setting then the circuit will not work.

Quote
The voltage regulator and current regulator will have 30V or more across them sometimes. An LM338 limits its current when it has more than 10V from its input to its output. Then its max current is only 1A.
if lm338 use as current regulator and input voltage 40V
the voltage drop across ic at 5A equal to approxomately 3V
and at this condition lm338 can handle 5A
than P=3*5 = 15W
am i right or not

Yes but then its output voltage must never be less than 37V.
If you connect a load that tries to draw more than 5A then the current regulation will reduce the output voltage until the current is 1A. Then the LM338 current regulator will dissipate as much as 40W!

as above quote this situation lm338 handle 15W if i use heat sink then this circuit ok or not

The LM338 will only overheat after the current limit has kicked in, before then the voltage across it will be between 1.5V and 3V, depending on the current and ambient temperature.

When the current limit, the current regulator might shut down, depending on the load voltage and current setting, if it's a problem a diferent solution is required.

if i max current limit 5A then how increase wattage above 15W
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When the current is below the current limit , the voltage across the first LM338 (used as a current limiter) will be very low: between 1.5V and 3V, depending on the current drawn and IC temperature so the power dissipation will be 15W maximum. When the current exceeds the limit set, the voltage across the first LM338 will increase but the current will remain the same causing a higher power dissipation. If the power dissipated is too high, causing the first LM338 to overheat, it will shut down in order to protect itself and the current will fall. This sort of current limiting is no good if you want to use your PSU as a constant current power supply to run high powered LEDs of but it's fine for a general purpose constant voltage power supply, when all you want is adjustable current limiting to protect sensitive components.

Why not use a real digital pot. rather than making a poor one with the CD4066?

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When the current is below the current limit , the voltage across the first LM338 (used as a current limiter) will be very low: between 1.5V and 3V, depending on the current drawn and IC temperature so the power dissipation will be 15W maximum. When the current exceeds the limit set, the voltage across the first LM338 will increase but the current will remain the same causing a higher power dissipation. If the power dissipated is too high, causing the first LM338 to overheat, it will shut down in order to protect itself and the current will fall.

Ok

This sort of current limiting is no good if you want to use your PSU as a constant current power supply to run high powered LEDs of but it's fine for a general purpose constant voltage power supply, when all you want is adjustable current limiting to protect sensitive components.

Then please tell me that how change in circuit that solve my problem
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• 2 weeks later...

Hello Hero and audioguru
how r u
where are you in these days since about 2 weeks
i am waiting you
please come on internet and solve my problem

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Your problem is that your high supply voltage range and high output current will cause the LM338 current regulator to reduce its max current  to only 1A and overheat.

If the LM338 current regulator has an input voltage of only about 13.2V then it can supply a max current of 5A when the LM338 voltage regulator has an output of 1.2V. Even then, the LM338 current regulator will dissipate 50W which is a lot of heat to remove.

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Hello Hero and Audioguru
how are tou

The LM338 will only overheat after the current limit has kicked in, before then the voltage across it will be between 1.5V and 3V, depending on the current and ambient temperature.

if i add a circuit that prevent short circuit then this problem solved or not
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if i add a circuit that prevent short circuit then this problem solved or not

No because your input supply voltage is too high and the LM338 current regulator will overheat when it regulates the current between 1A and 5A and when the output voltage is set low.

If the input voltage is 38V, the current is set to only 1A and the output is set to 4.75V then the LM338 current regulator dissipates 30W. How will you cool it?
It probably will not supply a current as high as 1A when it has 30V across itself because that is its max current.
When it overheats then it shuts down and does not work until it has cooled.
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I don't know if your problem has been solved or not because I don't know what it is.

It depends on whether you need current regulation or just current limiting. Adding another LM338 is fine, if you just want to limit the current but is no good for regulation. Note there is a big difference between a current limiter and current regulation: the former just shuts down when the current limit is exceeded, the latter will keep the current constant, even if the output is shorted to 0V.

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• 5 weeks later...

Hello Hero and Audioguru
how are u
i sure that both are well
ok next
Hello Hero
in other threat named "Switch mode power supply" you tell me that if you buy a 12V 200W smps then you add a circuit for making power supply 0-30 and 0-5A variale voltage and variable conatant current power supply.

If i buy PC P-4 PSU then this psu use as a primary 12V 200W smps first tell me that

How idea

and how can add a circuit please if you have a spare time then send a diagram using IC (SG3524) beacause this IC is avail in our country easily.

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thanks
very very thanks a lot  ::)  ???

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Hero99, are you a moderator?

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Hello KevinIV
who are you

Hero99, are you a moderator?

Hero not a moderator but
is a brilliant man
and my friend
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• 1 year later...

i have read this post recently but i can't understand the comments (very interesting), because i can't see the circuit they are talking about.

where is it? is it possible to post again? I know the topic is old but the info is good for me, super newbie

Cheers

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i have read this post recently but i can't understand the comments (very interesting), because i can't see the circuit they are talking about.

where is it? is it possible to post again? I know the topic is old but the info is good for me, super newbie

The schematic is attached as a thumbnail in the first post. Click on the thumbnail to enlarge it.
The schematic is copied from the datasheet of the LM338 IC.

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