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# LM338 Power Suply Current Regulator

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In lm338 data sheet asking
Features:
Guaranteed 7A peak output current
Guaranteed 5A output current

But problem is that in Electrical Characteristics:
Vin-Vout = 30V Than max current 1A
what is meaning that
lets Vin = 38V  and Vout = 8V
38-8 = 30

and Vin-Vout = 10V or less than 5A can handle
mean
38-28 = 10
mean 8 to 28V 1A handle
and 28 to above 5A handle

Attatch Diagram

2.    in Application Hints diagram all diagram is designed that can handle 5A

Attatch Diagram

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Exactly what are you wanting to do here?

You Must Heatsink the components for 5 amps each if not they will only handle 1 amp each.
There is another way to make a high current power supply.

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http://www.national.com/ds/LM/LM138.pdf

http://www.national.com/mpf/LM/LM338.html#Overview

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Exactly what are you wanting to do here?

You Must Heatsink the components for 5 amps each if not they will only handle 1 amp each.
There is another way to make a high current power supply.

That circuit will have very poor regulation because the base-emitter saturation voltage of the 2N3055 will vary depending on the current drawn and the 0.47R resistors will have a significant voltage accross them.

Suppose the current draw is 0A and you set the voltage to 12V.

Then you increase the current to 5A.

@0A the VBE of the transistors will be near 0.6V.

Each transistor will have 5/6 = 833mA through it, the same for the emitter resistor.

The voltage will drop by the 0.47R resistor will be 5/6

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The LM338 will only be able to output 5A if the difference between the input and output voltage is under 10V and a good heat sink is used (1

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Very Very Thanks Hero

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• 2 weeks later...

Attatch diagram
what is the role of emittr resistance 0.1 ohm
why using this resistor

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They help to ensure the current is shared equally between the transistors. The value isn't critical, I chose 0.1R because I originally designed it for high currents, for 5A you can use 1R which will be easier to get hold of.

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if i change the value of 1R instead of 0.1R than what change in circuit current
the resistor wattage
the voltage across R = ?
the current across R = ?

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Changing the value will not change the current, the circuit does not provide current regulation, it's just that if the voltage drop across the resistors is very low, the current sharing won't be very good.

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it's just that if the voltage drop across the resistors is very low, the current sharing won't be very good.

please explain it using examples
if 10R resistor use than
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what is the efficiency of these circuit
if this method using with lm317 and lm338 than thses ic better current and voltage or not

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please explain it using examples
if 10R resistor use than

There's no hard and fast rule, I just use current balancing resistors which have a voltage drop of a couple of hundred mV or so at full current. The voltage drop can be calaculated using Ohm's law.

what is the efficiency of these circuit

It's as efficient as an linear regulator.

if this method using with lm317 and lm338 than thses ic better current and voltage or not

?
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There's no hard and fast rule, I just use current balancing resistors which have a voltage drop of a couple of hundred mV or so at full current. The voltage drop can be calaculated using Ohm's law.

ok

Quote
if this method using with lm317 and lm338 than thses ic better current and voltage or not
?

my mean that if using transistor current increasing method for lm317 or lm338 regulator ic than this method ok or not because volatge vary from 1.2 to 30 v

7805 or 7812 is fixed regulator
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The variable voltage requirement doesn't make any difference, why would it?

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Attatch diagram
first circuit is use for current limiting
transiter is use for current increasing transistor current provide 5A at all voltage range 1.2-30V
this circuit is good
better
best
or not

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It's not a very good idea because the extra regulator will increase the drop out voltage by 4.2V.

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than any idea for increasing current

and tell me that this circuit provide good current limiting and
5A current provide at all voltage range 1.2 to 30V  and
good voltage regulation from 1.2 to 30V

if 0-30V instead 1.2-30V than modification on circuit

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It's not a very good idea because the extra regulator will increase the drop out voltage by 4.2V.

where 4.2V drop
2V drop across IC 1 that for current limiting/regulating
0.7V drop across BE resistor
2V drop across IC 2 that for voltage regulating
i am right or not

if input voltage 38V than 4.7V drop acrosss these the output will vary easily 1.2 to 38-4.7 = 33.3V
yes or not
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The LM338 has a drop out voltage of 3V at full current and when configured as a current regulator, the voltage reference of 1.25V across the current sense resistor, adds to it making 4.25V.

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This circuit provide good current limiting and
5A current provide at all voltage range 1.2 to 30V  and
good voltage regulation from 1.2 to 30V

if 0-30V instead 1.2-30V than modification on circuit

The LM338 has a drop out voltage of 3V at full current and when configured as a current regulator

if lm338 configured as voltage reglator than voltage drop across it = ?
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It looks like you did not read the datasheet for the LM338:
1) It shows a graph of the typical dropout voltage at various currents but does not list a max dropout voltage.
2) It lists a max output current of 5A only when the Vin - Vout is 10V or less (then your minimum output voltage is 23V. It lists its max output current to be only 1A when Vin - Vout is 30V.

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hello hero where are you
your site silicontronics does not open why
and you are angry with me why

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