The LM317 is quite the hot head ...

[EasyGoing1]

Re: The LM317 is quite the hot head ...

http://www.edn.com/design/analog/436...h-a-PWM-signal

So I actually built this circuit using my Arduino Due to control it ... which only does PWM up to 3.3 volts. HOWEVER, it does offer 12 bit PWM resolution as opposed to the regular 10 bit, so I have 4095 different levels of PWM to play with vs 1024.

So I arbitrarily picked a 1k resistor for R4 and R2 ... and with my PWM at 0, the output voltage was like 3.2 volts ... I was not able to get it to go higher until I kicked the PWM up to 3000 or roughly 2.something volts ... and then it hit the ceiling of around 10 volts on the LM317 before I reached the end of my PWM abilities .... so in other words, I don't have a very wide "sweep" of adjustment with this design...

How do I go about figuring out which resistors to use? I swapped in and out dozens of different resistors with different results, none of which gave me what I needed...

Also, the potentiometer had little effect on things, except once or twice with different values for the other two resistors, at which point adjusting the 10k pot would move the output on the 317 about 2/10ths of a volt but then would swing back where it was as I reached the opposite end of the wiper on the pot ... kinda weird if you ask me.

Ideas?

[EasyGoing1]

Re: The LM317 is quite the hot head ...

http://www.edn.com/design/analog/436...h-a-PWM-signal

From that link

You can change the op amp's gain by choosing proper values for R4 and R2

I wouldn't even know where to start choosing values for these resistors...

[eccerr0r]

Re: The LM317 is quite the hot head ...

Oh not for your problem of a battery charger, it was a question I posed as a brain teaser.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

It's the only way to get the LM317 to get to 0V. Can't get around it any other way other than perhaps something to regulate down externally, duplicating the function of the LM317 except handling the 0 to 1.25V region.

Well actually my question was WHY is it necessary that the circuit be capable of 0V ... i didnt doubt the reasons for actually making a 317 produce 0V ... I just wanted to know why 0 was required is all...

[eccerr0r]

Re: The LM317 is quite the hot head ...

It's the only way to get the LM317 to get to 0V. Can't get around it any other way other than perhaps something to regulate down externally, duplicating the function of the LM317 except handling the 0 to 1.25V region.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Yeah once again it requires a negative supply of some sort, there's lots of ways to skin the cat, but the cat needs to be skinned.

Why is this mandatory?

[eccerr0r]

Re: The LM317 is quite the hot head ...

Yeah once again it requires a negative supply of some sort, there's lots of ways to skin the cat, but the cat needs to be skinned.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

At the very least you still need the LM358 for using that EDN article design as if you used the LM741 it won't even go below 2.5V most likely.

Check out this design for 0 to 3V using a 317.

... when I tested the link I just provided in preview mode, it seemed to have inserted %20's in the link ... dont know why ... so here it is again just in case (this one actually worked for me when the other didn't)

http://www.edn.com/design/analog/432...able-regulator

[EasyGoing1]

Re: The LM317 is quite the hot head ...

do whatever you want to get a somewhat low impedance, regulated, negative 1.25V to 2.5V rail.

k, not quite sure what all that meant or what a “rail” is ... but I only need to be able to make somewhat small adjustments in a voltage range of roughly 3 to 5 volts... the more resolution I have available to me in my adjustments, the better.

For example being able to go from 3.032 volts to 3.039 would be much better than if I could only go to 3.131 or something like that...

[eccerr0r]

Re: The LM317 is quite the hot head ...

I don't have a requirement for 0V in my project ... cool circuit though ... send me a 337 and a 358 and ill test it

At the very least you still need the LM358 for using that EDN article design as if you used the LM741 it won't even go below 2.5V most likely.

The LM337 is not absolutely necessary, do whatever you want to get a somewhat low impedance, regulated, negative 1.25V to 2.5V rail.

[EasyGoing1]

Re: The LM317 is quite the hot head ...

Now, the trick is how would one make a digitally adjustable LM317 that will regulate down to 0V?

I don't have a requirement for 0V in my project ... cool circuit though ... send me a 337 and a 358 and ill test it

[EasyGoing1]

Re: The LM317 is quite the hot head ...

http://www.edn.com/design/analog/436...h-a-PWM-signal

See this is why I seek out friends like you guys ... thats good stuff! I could potentially do this project without the digital pot that I spent a whole $5 for three.... lol

And I just happen to have a couple of 741s laying around ... hmmmm....

[eccerr0r]

Re: The LM317 is quite the hot head ...

Now, the trick is how would one make a digitally adjustable LM317 that will regulate down to 0V?

I suspect that the same trick of using some sort of regulator to make sure the negative rail doesn't go too far down else you'd have a lot of dead region.

In any case some offset trickery is needed as now a 0 to 3.3V signal needs to be -1.25 to +11 or whatever...

Plus the 3.3V signal needs to be regulated too else an RC low pass filter is not going to give you an accurate output...

Now question is: will this work: (note that I cheated with a LM337 to regulate negative voltage. LM358 is not RRIO so it will have trouble getting to the high voltages but should get close to zero output in this configuration. I also did not account for input leakage which will be significant with the values of the resistors used here.)

[mariushm]

Re: The LM317 is quite the hot head ...

You can find voltage regulators that have voltage references inside with much lower voltage, like 0.6v or 0.25v even , but generally these are designed for max 6v in , max 5v out or something like that.

For ex. LT3021 (max 10v in , max 9v out, 0.5a max, 200mV reference, 300mV max dropout voltage) : http://www.linear.com/product/LT3021

or LT3086 (max 40v in, 32v out, 400mV reference, 2.1a max out, programmable current limit ) : http://www.linear.com/product/LT3086

These are both kinda expensive (6-10$ each) and in general low voltage references are much harder to make, so chips are more expensive.

Another trick I've seen often used is to use a diode in front of the linear regulator, or the classic use of a voltage reference/zener diode and a negative power supply :

TI: AN-181 3-Terminal Regulator is Adjustable



Here you have a 6.9v voltage reference - you can probably find some cheaper or more common zener diode or a lower voltage one ... you basically need anything above 1.25v to make it possible to output 0v

You can make negative power supplies easily with voltage doubler ics (they can be configured to be voltage inverters as well or voltage half-ers) but the majority of the cheap ones only work with up to 5v (still you only need a few ma of current on the negative psu so you could use a 7805 or something cheap to get 5v out of a big tranformer winding, then use a voltage inverter to get -5v and use a zener with voltage lower than 5v ... ex 2.5v AN431 or a lot of TL431 (lots of companies make them) with default reference below 5v

[eccerr0r]

Re: The LM317 is quite the hot head ...

Yes, as said, with a RRIO LM6142 and a negative supply will give a negative output, which will get the LM317 down to 0V.

Or a LM741 with a negative ~3V supply which will both deal with the 741 not reaching negative rail plus get negative enough for the LM317. Either way, a negative supply is necessary.

But that edn article seems to have a design error.

[budm]

Re: The LM317 is quite the hot head ...

LM317 is not made to go down to 0V output, 1.25V is the min output, look at the spec sheet, and if you want it to be 0 then look at the Apps notes.
https://cdn.badcaps-static.com/pdfs/...4077910403.pdf

[eccerr0r]

Re: The LM317 is quite the hot head ...

http://www.edn.com/design/analog/436...h-a-PWM-signal

Woah wtf. Is that really going to work with a LM741? Having the null is not that big an issue, I'd think either a negative supply is necessary (~ -1.25V) or a LM6142 or other RRIO op amp is needed to get a minimum voltage of 1.25V. Or a negative supply with the LM6142 to get the output to 0V...

[budm]

Re: The LM317 is quite the hot head ...

http://www.edn.com/design/analog/436...h-a-PWM-signal

[stj]

Re: The LM317 is quite the hot head ...

maybe an opto-isolator could be used

[EasyGoing1]

Re: The LM317 is quite the hot head ...

the duo is ARM cpu based,
your i/o pins have a 3.3v limit.

I'm aware of this ... and I'm also aware (after running some calcs anyways) that the range in output that I could achieve with this is rather small ... but my point in bringing it up, is that it had literally just occurred to me that the LM317 uses a reference VOLTAGE to determine the output level ... I thought there was some other magic happening between adjustment pin, output pin and the brain of the device which was somehow also dependent on resistance values being detected between pins 2 and 3 etc. ... until I thought about it and realized that a voltage reading is really all you need to design a device like the 317...

I just thought it was interesting and wondered if it could actually work (applying a voltage directly to the adjustment pin with respect to ground instead of using a voltage divider...