Electronics > Beginners

Help me design a PSU

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james_s:
It has obviously been fried, the internal pass transistor is shorted. Also if you bought them from a random China source they are probably inferior counterfeit parts.

fsr:
You can make an LM317 go to 0v even without a negative transformer. The trick is to add two diodes between GND and the (-) output of the PSU (so, your outpur is aprox 1.4v above the real GND). The diodes must be able to handle the max output current and the power dissipation. The resistive divider that controls the LM317 output voltage is also referenced to this point.
You will need to consider the power dissipation of the LM317 in the worst-case scenario and use a suitable heatsink. The datasheet has a very good explanation on how to calculate this under the "Heatsink Requirements" section.

EDIT: i'm sorry, the circuit i posted before won't work, it was just a part of a circuit i did which had a current limit that could go all the way down to 0v, but for the CV control to go down to 0v, you need some way of getting 1.25v below your main diode bridge's negative output. I should have checked the circuit carefully, instead of sketching it and posting.

Anyways, if you use the LM317, remember to check the "Heatsink Requirements" section, and forget about using a 48v transformer. The power dissipation for the chip is about (input_voltage - output_voltage) * current. Try to use a transformer/bridge/capacitor combination that will result on a voltage that just goes over the requirements for the LM317 for the output voltage you need (check the dropout voltage), and whatever additional drop that the final circuit could introduce.

VEGETA:

--- Quote from: fsr on November 16, 2018, 01:19:47 pm ---You can make an LM317 go to 0v even without a negative transformer. The trick is to add two diodes between GND and the (-) output of the PSU (so, your outpur is aprox 1.4v above the real GND). The diodes must be able to handle the max output current and the power dissipation. The resistive divider that controls the LM317 output voltage is also referenced to this point.
You will need to consider the power dissipation of the LM317 in the worst-case scenario and use a suitable heatsink. The datasheet has a very good explanation on how to calculate this under the "Heatsink Requirements" section.

--- End quote ---

that act itself creates a negative voltage. Remember that voltage is relative, so it is negative relative to ground which is 0. There is no way lm317 could output 0v without negative voltage.

In your case, you did it by splitting the rail using diodes, and in my case by creating another rail using 7660. Same result.

fsr:

--- Quote from: VEGETA on November 16, 2018, 02:05:38 pm ---
--- Quote from: fsr on November 16, 2018, 01:19:47 pm ---You can make an LM317 go to 0v even without a negative transformer. The trick is to add two diodes between GND and the (-) output of the PSU (so, your outpur is aprox 1.4v above the real GND). The diodes must be able to handle the max output current and the power dissipation. The resistive divider that controls the LM317 output voltage is also referenced to this point.
You will need to consider the power dissipation of the LM317 in the worst-case scenario and use a suitable heatsink. The datasheet has a very good explanation on how to calculate this under the "Heatsink Requirements" section.

--- End quote ---

that act itself creates a negative voltage. Remember that voltage is relative, so it is negative relative to ground which is 0. There is no way lm317 could output 0v without negative voltage.

In your case, you did it by splitting the rail using diodes, and in my case by creating another rail using 7660. Same result.

--- End quote ---
I mean that he doesn't need a center-tapped transformer, or something like that for this to work, just a voltage drop greater or equal than 1.2v under the negative output terminal, reference the voltage divider to that point and that's it. Bob's your uncle ;D

Something like this (see attachment):
EDIT: Sorry, wrong circuit. Check my previous post for details.

spec:

--- Quote from: Efe_114 on November 13, 2018, 05:36:54 pm ---I want to make a small psu that could go up to 15 volts and DOWN TO 0 VOLTS (because i like hard projects) .no current limit needed. i want to make it out off jellybean parts and not copy it from a random website. And most importantly im a beginner(i have been in hobby for 3 years but since 90% of my projects fail i call myself beginner)Can anyone help me about this? Also this is my first post in this forum so forgive my mistakes please ;)

--- End quote ---

Hi Efe,

The attached schematic may be what you are looking for. It adjusts down to zero volts using the principle (already discussed) of generating a -1.25V line to offset the 1.25V reference voltage of the LM317, but without an additional winding on the transformer or voltage dropping diodes.

If you are interested in this approach I can post a practical schematic showing decoupling capacitors and describe the best layout.

(4m7 on the schematic is 4.7 milli Farads or 4700 micro Farads)

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