Electronics > Beginners

Help me design a PSU

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not1xor1:

--- 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 is not a trick but a really dumb solution as, besides the additional 1-2W of wasted power,  the output voltage would drift according to the temperature of the diodes and the load current.

As I showed in the previous schematic, with just a couple of diodes and capacitors you can get an additional negative rail from the transformer (charge pump) and with a cheap voltage reference and a resistor you can get -1.25V as stable as the inner reference of the LM317.

And if the unregulated supply is coming from an AC-DC converter (switching PSU) you may use a 555 or even a cheap CMOS logic IC (which are usually cheaper than a dedicated IC) to get a negative rail via a similar charge pump circuit.

not1xor1:

--- Quote from: fsr on November 17, 2018, 03:09:10 am ---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):

--- End quote ---

That would not work.
You have to connect R1 to Adj and the potentiomenter to the negative of the rect. bridge.
It is anyway a bad solution.

not1xor1:

--- Quote from: spec on November 17, 2018, 05:47:07 am ---
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)

--- End quote ---

You can even use a cheaper 1.25V shunt regulator (LMV431, LM385, etc.). Of course you must properly bias it so that the total current of the negative rail, at the worst condition (lowest transformer voltage) should be greater than the minimum shunt regulator voltage + the quiescent current of the LM317 (aproximately 12.5mA in the schematic you attached).

spec:

--- Quote from: not1xor1 on November 17, 2018, 07:37:52 am ---
--- Quote from: spec on November 17, 2018, 05:47:07 am ---
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)

--- End quote ---

You can even use a cheaper 1.25V shunt regulator (LMV431, LM385, etc.). Of course you must properly bias it so that the total current of the negative rail, at the worst condition (lowest transformer voltage) should be greater than the minimum shunt regulator voltage + the quiescent current of the LM317 (aproximately 12.5mA in the schematic you attached).

--- End quote ---

I know that, but you can't get much cheaper and ubiquitous, than an LM337. Besides, it looks nice and symmetrical with two three terminal regulators. ;D

not1xor1:

--- Quote from: spec on November 17, 2018, 05:59:51 pm ---
--- Quote from: not1xor1 on November 17, 2018, 07:37:52 am ---
--- Quote from: spec on November 17, 2018, 05:47:07 am ---
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)

--- End quote ---

You can even use a cheaper 1.25V shunt regulator (LMV431, LM385, etc.). Of course you must properly bias it so that the total current of the negative rail, at the worst condition (lowest transformer voltage) should be greater than the minimum shunt regulator voltage + the quiescent current of the LM317 (aproximately 12.5mA in the schematic you attached).

--- End quote ---

I know that, but you cant get much cheaper than an LM337 and besides it looks nice and symmetrical with two three terminal regulators. ;D

--- End quote ---
:-+
You're right. I just checked. The price difference is less than what I remembered... just 20-30 cents and only if one carefully selects the cheaper shunt regulator.

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