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power supply schematic help

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ericgibbs

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Hi, I have this power supply schematic

Power Supply +13.8V 5A with LM338 | TehnikService

I don't need the external regulator, can I just remove it? Will schematic still work?

And I need to get 12V. so I need to change R1 and TR1 to values 370ohms and 3300ohms to get 12.40V.

hi,:)
Looks OK to run without the external option.
Follow the heatsink requirement as stated.

I would keep the R1 at 270R and adjust TR1.
 
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smilem

Member
Thanks, do I need to keep D2 too?

Also I want to drive a motor with PWM using this power supply and I want to power 1W LED (will use anohter LM338 for it). Is it OK?

Will the LED be stable if it is powered from the same power supply as motor?
 

ericgibbs

Well-Known Member
Most Helpful Member
Thanks, do I need to keep D2 too?

Also I want to drive a motor with PWM using this power supply and I want to power 1W LED (will use anohter LM338 for it). Is it OK?

Will the LED be stable if it is powered from the same power supply as motor?

hi,
D2 could be omitted, but keep D1 in.

Whats the current rating of the LED and motor.?
 

smilem

Member
LED is rated at 3W 700mA but I'll run at 1W 350mA to make shine not so bright.
The motor is 12V or 24V don't know as I salvaged it from a printer. I measured the current drawn by motor when connected to computer PSU 12V ant it was 0.3 to 1Amp including PWM controller.
 

ericgibbs

Well-Known Member
Most Helpful Member
LED is rated at 3W 700mA but I'll run at 1W 350mA to make shine not so bright.
The motor is 12V or 24V don't know as I salvaged it from a printer. I measured the current drawn by motor when connected to computer PSU 12V ant it was 0.3 to 1Amp including PWM controller.

hi,
In that case I would consider using just the one psu for the LED and motor.

The only point I would change is C5 10uF, I would fit a larger value as you are switching a motor, say at least 220uF to 470uF.

Motors tend to draw high switching currents and in somecases the LM338 is not fast enough to compensate, so the C5 cap will help to supply the switching currents.
 

smilem

Member
Is there a difference will I adjust TR1 or R1?

I have resistors for R1 250R (multiturn) and 2K2 for TR1. This gives me 12.25V


I mean can I adjust R1 instead of TR1?
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
Is there a difference will I adjust TR1 or R1?

I have resistors for R1 250R (multiturn) and 2K2 for TR1. This gives me 12.25V
I mean can I adjust R1 instead of TR1?

I would keep R1 value between 120R and 270R and use a variable resistor for TR1 to adjust the output voltage.
 

smilem

Member
Well I just fried my adjustable resistor TR1 0.25W 2.5K, the R1 is 0.5W and still OK.
It happened while I tried to measure voltage output.

At first I so no measurement, but then while I turned the TR1 it just smoked :(
There was no load connected.

Please help, do I need more powerful resistors?
 
Last edited:

smilem

Member
It seems I have mixed the Vout and Vin pins on LM338.
Now I have new LM338 chip and corrected the wiring.

Here is my diagram for PCB (Two LM338 power supply):


the resistor R1 is 270Ohm
the resistor POT1 is 1450Ohm (250Ohm adjustable and 1220Ohm not adjustable)

My problem is: I get 18V (I want 12V) out and it barely asjusts but acording to this formula here


My AC transformer is 15Volts.

I need max POT to be 1480ohms (my VREF measured between Vout and V adjust is 1.8V).
 
Last edited:

ericgibbs

Well-Known Member
Most Helpful Member
hi,
if you are getting 18Vout with those values,, either the pcb is incorrect or the LM338 is blown.??


Using the formula: assume the 250R is set to 0R

1220R/270R = 4.51
plus 1 = 5.51..... times 1.25V = 6.88V ... you can ignore the small Iadj part.!


EDIT:
for approx 12V , if R1 = 270R, the lower resistance should be approx 2300R

BTW: note that the minimum load current for the LM38 is greater than 5mA to 10mA.

Do you have a dummy load on the output.?
 

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audioguru

Well-Known Member
Most Helpful Member
Your value for R1 and for the pot are too high.

R1 must not be higher than 120 ohms for the LM317, LM338 and LM350. It must not be higher than 240 ohms for the more expensive LM117, LM138 and LM150.
Or the output voltage might rise without a load as described in the datasheets.

Then of course the value of the pot must also be reduced to go along with the lower value for R1.
 

smilem

Member
It seems I have 2 fake LM338T IC's. I made simple air wire setup using datasheet schematic for 1.2-25V (page7) voltage regulator.

I used socket to be able to change LM338 without soldering it. I then took my old LM338T from another project, and guess what with it the circuit works, and with my new LM338 it does not.

I'll post photos of fake LM338T and the one that works fine. Both have National logo.
 

audioguru

Well-Known Member
Most Helpful Member
If it is the one that you connected its pins backwards then it is blown up.
 
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