Electronics > Beginners
PWM for LM317 - how to increase resolution?
joeyjoejoe:
Gotcha. I did some measurements, and with my mediocre power supply (Gophert 3205 mk2) I am getting about 60mV peak to peak from the regulator. I think most of that is probably from my PSU itself! :) Should be acceptable for the charger.
exe:
> I think most of that is probably from my PSU itself
That's easy to check: just measure noise before and after regulator. I'd also measure ripple after PWM filter just to be sure filter is working.
Zero999:
--- Quote from: joeyjoejoe on September 03, 2018, 03:15:07 pm ---Ahhh, that first picture for the voltage divider blew my mind a bit, so now it is more clear what you were referring to! :) Thanks so much!
So the values I've come up with is R3 = 933ohm, and R1 = 7.6k, which should change this to work from 3.5v to 4.7v. Perhaps you can double check the math?
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What value did you calculate for R2?
Also note that those are non-standard resistor values. It makes it easier to stick with standard E24 or E96 preferred numbers. There are some online tools which make finding optimum combinations for any ratio easy, but they're mostly designed for potential dividers so applying them to this circuit sin't self explanatory. When I get time, I'll show you how.
--- Quote ---Just watched a very cool EEVBlog video this weekend on Capacitance Multiplier, and one of the use-cases was for smoothing PWM values! I'll check the ripple on my PWM to see if that's something I need to look at, although he mentioned in his video a bit about RC causing too big a voltage drop for a large R value, but since all I'm doing is adjusting the LM317, which is 50-100uA, I'm probably ok with a simple but large RC filter...
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yess, as mentioned previously, it's not relevant to this circuit because it has an emitter follower.
The best way to reduce ripple with this circuit, is to use two RC filter stages. The simplest to calculate is to split R1 into two resistors and connect another capacitor to 0V. For example, using the values in my circuit, the 5k6 resistor could be replaced with 1k3 and 4k3 in series, with another 100μF capacitor to 0V. I don't have time to post a schematic at the moment, but will do later, if this isn't clear.
ogden:
--- Quote from: Hero999 on September 03, 2018, 10:27:00 pm ---The best way to reduce ripple with this circuit, is to use two RC filter stages. The simplest to calculate is to split R1 into two resistors and connect another capacitor to 0V. For example, using the values in my circuit, the 5k6 resistor could be replaced with 1k3 and 4k3 in series, with another 100μF capacitor to 0V. I don't have time to post a schematic at the moment, but will do later, if this isn't clear.
--- End quote ---
Could you please tell what you use to draw neat circuit snippets like this?
Seekonk:
If you want to save some heat, the same thing can be done with a buck converter. My whole solar system, several KW, are controlled this way with off the shelf cheap buck converters. You set the converter at the maximum voltage you would like to see and feed additional voltage into the feedback pin. The voltage has to be monitored as there is no direct relation to D/A counts and volts.
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