Voltage regulator using lm338

[rock94]

I need help with the LM338 variable voltage regulator circuit.https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEoQFjAG&url=http%3A%2F%2Fwww.uni-kl.de%2Felektronik-lager%2F416059&ei=9SPCUcPCMNGrrgeiroE4&usg=AFQjCNFy8x6dLp9vWmCYI1HakR9ZkIWVlw&bvm=bv.48175248,d.bmk&cad=rjaI'm building the one with the improved ripple rejection.

I'm using a 24v 3A transformer for the input and I'm having trouble deciding the values of R1 and R2.I would like an output of around 2-20v.The circuit in the datasheet uses a 5k pot.However the potentiometer I have is marked 4.7k,but when measured is close to 7k.So if I use a 10k pot what should be the value of R1.Also can the tantalum capacitor at the output be replaced by an electrolytic capacitor?
Thanks.

 

[tpetar]

Adjustable Power Supply 1.2 – 30V 5A using LM338
https://www.eleccircuit.com/adjustable-power-supply-1-2-30v-5a-using-lm338/

LM338 / LM350 / LM317 VOLTAGE REGULATOR CALCULATOR
https://www.eleccircuit.com/lm338-lm350-lm317-voltage-regulator-calculator/


Look to use LM338K TO-3.



Best regards,
Peter

;-)

 

[rock94]

Thanks.Can a 4700µF capacitor be used in place of a 10000µF capacitor?I don't have a 10000µF right now.

 

[tpetar]

Of course. You can use two 2200uF its no problem.

Additional put 100nF capacitor on output.


This circuit is interested to be done in digital fashion with digital potentiometer, Up and Down for voltage control.

I will post circuit, pictures and code for uC if I find free time to make photos.



Best regards,
Peter

;-)

 

[Audioguru]

The datasheet for the LM338 says that its output current is 5A ONLY when the voltage from its input to its output is 10V or less.
But since your input voltage is extremely high at +32VDC then the output can produce 5A ONLY between about +22V to about +30V.
When your output voltage is less than +22V then the output current will be less than 5A, maybe only 1A.

The program resistor should be 120 ohms for the LM338 but it can be 240 ohms for the more expensive LM138.

If the output voltage is +22V and the current is 5A then the LM338 will get extremely hot trying to dissipate 50W.

- - - Updated - - -

The datasheet for the LM338 says that its output current is 5A ONLY when the voltage from its input to its output is 10V or less.
But since your input voltage is extremely high at +32VDC then the output can produce 5A ONLY between about +22V to about +30V.
When your output voltage is less than +22V then the output current will be less than 5A, maybe only 1A.

The program resistor should be 120 ohms for the LM338 but it can be 240 ohms for the more expensive LM138.

If the output voltage is +22V and the current is 5A then the LM338 will get extremely hot trying to dissipate 50W.

- - - Updated - - -

The datasheet for the LM338 says that its output current is 5A ONLY when the voltage from its input to its output is 10V or less.
But since your input voltage is extremely high at +32VDC then the output can produce 5A ONLY between about +22V to about +30V.
When your output voltage is less than +22V then the output current will be less than 5A, maybe only 1A.

The program resistor should be 120 ohms for the LM338 but it can be 240 ohms for the more expensive LM138.

If the output voltage is +22V and the current is 5A then the LM338 will get extremely hot trying to dissipate 50W.

 

[tpetar]

120R is just typical value for LM338 but can be other value such as 240R without problem (tested 100%).

http://diyaudioprojects.com/Technical/Voltage-Regulator/

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

Rock94 mention transformer 24V 3A, this will tell us that he count on current under 3A.

Full current can be reached in case of Vi - Vo <= 10V.

I use LM338K TO3 on aluminium heatsink for old PC CPU Athlon/duron and works ok.



Best regards,
Peter

;-)

 

[rock94]

I built the power supply today.I used a 560Ω resistor for R1 and a 10k pot for R2.However the voltage does not change.It remains constant at around 32 volts.Also how do I check the output current safely?When I tried to check the current the IC automatically shut down.Also it heated up a little.
Thanks.

 

[tpetar]

I built the power supply today.I used a 560Ω resistor for R1 and a 10k pot for R2.However the voltage does not change.It remains constant at around 32 volts.Also how do I check the output current safely?When I tried to check the current the IC automatically shut down.Also it heated up a little.
Thanks.

Use resistor values according to circuit of calculation based on formula.

What do you mean under "how do I check the output current safely" ?

Current can be measured with ampermeter instrument in serie with some load.



Best regards,
Peter

;-)

 

[rock94]

Thanks.I asked that because one of the resistors I used in series got burnt when measuring current.the problem is this R2 seems to be having no effect at all.I made R2 open and the voltage did not change at all.I even shorted R2.Even then the voltage remains constant at around 34-35 volts which is the same as the input voltage.Also the current at the output of the bridge rectifier is about 5 Amps .However the current at the lm338 output is 1.8 Amps.All the circuit connections seem to be right.All the regulator seems to be doing is limiting the current.Any ideas?
Thanks.

 

[tpetar]

Nothing special, just make all according to circuit.

If you use resistor as load for measuring current, that resistor should have adequate power rating.