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LM317 control by PWM
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soldar:
If I understand how this IC works then a bias voltage like in the attached diagram should control the LM317 output voltage.  Then it is just a matter of filtering the PWM so that it results in a stable voltage. The two diodes help bring the output closer to zero but they can be omitted if not needed.

With R1 = R2 roughly Vout = - Vcontrol.

Increasing the value of R1 would make roughly Vout = -(R1 / R2) * Vcontrol

Maybe somebody can go over it and check.
Zero999:

--- Quote from: soldar on February 02, 2019, 01:16:34 pm ---If I understand how this IC works then a bias voltage like in the attached diagram should control the LM317 output voltage.  Then it is just a matter of filtering the PWM so that it results in a stable voltage.

--- End quote ---
Yes, that's the general idea. The trouble with that configuration is the op-amp has to sink all of the current.
Yansi:

--- Quote from: Zero999 on February 02, 2019, 12:38:50 pm ---
--- Quote from: Yansi on February 02, 2019, 09:58:43 am ---Once more - using a NPN darlington output stage instead of 317 solves all of these hassles. Only current supplied, not needed to be sunk!

--- End quote ---
That's all well, until it overheats, resulting in magic smoke. It also will have a higher drop out voltage than the LM317, as the saturation voltage of the op-amp's output stage will add to Darlington pair.

Just use the LM317 and current sink, to provide the minimum load. It's not complicated.

--- End quote ---

Who cares about the Vdrop? Was not in the requirement list.

Why should it overheat?  Your LM317 will anyway, if you will not use enough heatsinking.

That is like saying your car is going to crash, because it is an older model without airbags. It is the same kind of logic.

//EDIT: Using a compound darlington the Vdrop becomes even smaller

//EDIT2: Wanted to compare the voltage drops, and ... wtf is this?
soldar:

--- Quote from: Zero999 on February 02, 2019, 02:10:32 pm --- Yes, that's the general idea. The trouble with that configuration is the op-amp has to sink all of the current.
--- End quote ---
I am sorry but I don't understand. What op-amp? Are you referring to the LM317? In which case... yeah. Isn't that what it is supposed to do?

If you are referring to the voltage source I have included, then, no, it does not sink all the current. The main current returns via the common ground. The PWM signal is very small and can be generated using a 555 timer or anything else, completely independent of the rest of the system.

Maybe I am missing something.

I am not good enough with LTSpice yet but maybe someone can do it. I would start by just connecting R2 to a variable negative voltage. Then add the diodes. Then add LC filter and put a PWM signal there.
Zero999:

--- Quote from: Yansi on February 02, 2019, 02:23:43 pm ---
--- Quote from: Zero999 on February 02, 2019, 12:38:50 pm ---
--- Quote from: Yansi on February 02, 2019, 09:58:43 am ---Once more - using a NPN darlington output stage instead of 317 solves all of these hassles. Only current supplied, not needed to be sunk!

--- End quote ---
That's all well, until it overheats, resulting in magic smoke. It also will have a higher drop out voltage than the LM317, as the saturation voltage of the op-amp's output stage will add to Darlington pair.

Just use the LM317 and current sink, to provide the minimum load. It's not complicated.

--- End quote ---

Who cares about the Vdrop? Was not in the requirement list.

Why should it overheat?  Your LM317 will anyway, if you will not use enough heatsinking.

That is like saying your car is going to crash, because it is an older model without airbags. It is the same kind of logic.

//EDIT: Using a compound darlington the Vdrop becomes even smaller

//EDIT2: Wanted to compare the voltage drops, and ... wtf is this?

--- End 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.

2) Reading between the lines, it's probably for a bench power supply, so it will be abused and thermal protection is important.

3) Yes a compound transistor helps, but still no thermal protection.

4) An error.


--- Quote from: soldar on February 02, 2019, 03:42:25 pm ---
--- Quote from: Zero999 on February 02, 2019, 02:10:32 pm --- Yes, that's the general idea. The trouble with that configuration is the op-amp has to sink all of the current.
--- End quote ---
I am sorry but I don't understand. What op-amp? Are you referring to the LM317? In which case... yeah. Isn't that what it is supposed to do?

If you are referring to the voltage source I have included, then, no, it does not sink all the current. The main current returns via the common ground. The PWM signal is very small and can be generated using a 555 timer or anything else, completely independent of the rest of the system.

Maybe I am missing something.

I am not good enough with LTSpice yet but maybe someone can do it. I would start by just connecting R2 to a variable negative voltage. Then add the diodes. Then add LC filter and put a PWM signal there.

--- End quote ---
I don't see how that circuit is supposed to convert 5V PWM into 1.25V to 30V.
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