Electronics > Projects, Designs, and Technical Stuff
LM317 control by PWM
<< < (6/9) > >>
soldar:

--- Quote from: Zero999 on February 02, 2019, 05:50:07 pm --- I don't see how that circuit is supposed to convert 5V PWM into 1.25V to 30V.
--- End quote ---
Yeah, sorry, just ignore it. It was a sketch of a half baked idea which I cannot complete. At least I have been playing around and learning a bit of LTSpice so something good came of it. :)
Yansi:

--- Quote ---1) Voltage drop is important because you won't be able to get 30V out, without exceeding the maximum voltage rating of the LM358.
--- End quote ---

You won't likely be able to do that anyway, with a mains-transformer based supply.  If you add into account mains voltage variation, which is likely +-10% at most places, you have extremely small margin there, if any at all.

So back to the drawing board anyway.  ;)
Zero999:

--- Quote from: Yansi on February 02, 2019, 08:20:57 pm ---
--- Quote ---1) Voltage drop is important because you won't be able to get 30V out, without exceeding the maximum voltage rating of the LM358.
--- End quote ---

You won't likely be able to do that anyway, with a mains-transformer based supply.  If you add into account mains voltage variation, which is likely +-10% at most places, you have extremely small margin there, if any at all.

So back to the drawing board anyway.  ;)

--- End quote ---
With the LM317 on the output, the op-amp only has to swing from 0V to 27.5V to cover a 1.25V to 30V output range, so it could easily be powered from a regulated 31V power supply.

A far bigger problem is the LM317's safe operating area protection kicking in when the output voltage is set low. A centre tapped transformer with a switch should be used minimise power dissipation.
Yansi:
Regulated dedicated 31V supply for opamp.  Then add negative bias voltage so you can lower the current limit of the 317 realiably even to short circuit operation. Then regulated this or that or that there... you end up with something pretty hilarious, as most 317 or 723 based power supplies are. Overly complex (junk)!   :)



Zero999:

--- Quote from: Yansi on February 02, 2019, 09:39:32 pm ---Regulated dedicated 31V supply for opamp.  Then add negative bias voltage so you can lower the current limit of the 317 realiably even to short circuit operation. Then regulated this or that or that there... you end up with something pretty hilarious, as most 317 or 723 based power supplies are. Overly complex (junk)!   :)

--- End quote ---
You get that anyway when designing a 0V to 30V PSU, whichever route you take.

If you opt for the traditional Darlington pair method you're proposing, the op-amp's power supply voltage needs to be even higher, ruling out most common op-amps or you can add a buffer circuit, but that leads to extra complexity.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod