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Help needed with PSU

BITSNBOBS

Aug 21, 2022
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Hi to all
i have got the following psu it all seems to work ok but i know that you all gonna spot that the LM317 looks wrong, but it aint
anyway when i short out the output to set the current limit the led2 lights and adjusting RV3 i can make the led dimmer or brighter
so i think it is working.
Now i have got one of them led volt\current meter display things from china, the volts side works ok, but when i connect the
current side ( thick black wire to D and thick red wire to C) and then short out the output the led2 does not light, no mater how much
i adjust RV3, i do get a current reading of about 1Amp on the display while the output leads are shorted ( but no adjustment can be made with RV3) and things start to get warm (the heatsink for the 2x TIP35C and the 47r R9)!
Any idias as to what is going on ? and am i doing things right ?
Meny thanks for any help.
BitsnBobs
 

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  • 10 Amp PSU.png
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bertus

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Nov 8, 2019
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Hello,

You do not have feedback from the output of your supply.
Also R4 and R5 will cause a large voltage drop.
You will not get the wanted current with the given values of 22 Ohms for R4 and R5.

Have a look at section 4 , figure 7 of the following page:

Bertus
 

bertus

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Nov 8, 2019
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Hello,

Is R9 supposed to be the current sensing resistor?
Imagine 10 Amp through 47 Ohms.
That shoud give a voltage of 10 X 47 = 470 Volt, wich will never happen.
Also the power in the resistor will be huge:

Ohm's_law_formula_wheel.JPG

Bertus
 

BITSNBOBS

Aug 21, 2022
6
Joined
Aug 21, 2022
Messages
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Hello,

You do not have feedback from the output of your supply.
Also R4 and R5 will cause a large voltage drop.
You will not get the wanted current with the given values of 22 Ohms for R4 and R5.

Have a look at section 4 , figure 7 of the following page:

Bertus
Hi
so R4 and R5 would be better being 0.22R
bitsnbobs
 

BITSNBOBS

Aug 21, 2022
6
Joined
Aug 21, 2022
Messages
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Hello,

Is R9 supposed to be the current sensing resistor?
Imagine 10 Amp through 47 Ohms.
That shoud give a voltage of 10 X 47 = 470 Volt, wich will never happen.
Also the power in the resistor will be huge:

View attachment 55960

Bertus
Hi
Not sure about R9 being the current sensing resistor, but i think it is
bitsnbobs
 

BITSNBOBS

Aug 21, 2022
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can you please expand on "no feedback from the output of your supply"
bitsnbobs
 

BITSNBOBS

Aug 21, 2022
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Hi
Thank you very much for what you have done, i'll be able to test it out in the morning.
can you tell me why R2 is now connected to the output (for feedback ?) but i thought that
it should be connected to the output of LM317MT (ic4).
Do you think that the led display should work ok now ?
Thank you again
bitsnbobs
 

bertus

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Nov 8, 2019
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Hello,

Did you look at the page I gave you?
It uses PNP transistors to boost the LM317:
vreg-f7.gif
That way, the regulator can be used in the "normal" way.
You are using emittor followers, that will drop voltage to the output that will vary with the current.
By changing R2 to the output of the emittor follower, the voltage drop will be compensated.

Bertus
 

BITSNBOBS

Aug 21, 2022
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Hello,

Did you look at the page I gave you?
It uses PNP transistors to boost the LM317:
View attachment 55963
That way, the regulator can be used in the "normal" way.
You are using emittor followers, that will drop voltage to the output that will vary with the current.
By changing R2 to the output of the emittor follower, the voltage drop will be compensated.

Bertus
Hi
Thank you for you'r comments, yes i did look at that page.
Which of the two (npn,pnp) is the better way to do it ?
Do i need to put D1,D2 and D3 into the mix ?
Here is the psu with PNP

bitsnbobs
 

Attachments

  • 10 Amp PSU_PNP.png
    10 Amp PSU_PNP.png
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bertus

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Hello,

The three diodes are for protection.
When there is a voltage on the output (large capacitor), but the input is gone, the regulator may be damaged.

Bertus
 
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