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| LM317 digital control |
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| danadak:
Actually min load worst case to stay in regulation is min of 10 mA. This compounds the design. But graph of load regulation shows min load going to 0, not quite consistent. Here are some notes on min load - http://electronics.stackexchange.com/questions/11919/is-it-normal-for-lm317-to-give-a-higher-voltage-without-a-load What you might do is terminate the LED to the - supply you are going to use to get output to go to 0 so that it still draws current when output goes to 0. If you look at this datasheet fig 9 notice the protection diodes. Regards, Dana. |
| Maobuff:
--- Quote from: jose347 on May 19, 2016, 05:00:29 pm ---you could also use a digital pot for the resistances that control the output voltage and use spi or ic2 to control the pot --- End quote --- They are pretty expensive --- Quote from: StillTrying on May 19, 2016, 04:55:56 pm ---What are the supply rails for the LM358? --- End quote --- They are connected directly to input |
| Maobuff:
--- Quote from: StillTrying on May 19, 2016, 04:55:56 pm ---What's wrong with it? Without it's voltage setting resistors it will need a minimum load of 3.5mA - else it's output will float a lot higher than 1.25V. --- End quote --- I added a 249Ohm load to output, and get 1.25V out. And its didn't change at all. I checked op amp output and its looks normal (0-5V). Then i tried to use "classic" circuit for LM317 with two resistors, I chose 10k. From datasheet Vout = Vref*(1+R1/R2) so i expected to get ~2.5V but i get close to input voltage |
| Zero999:
--- Quote from: Maobuff on May 19, 2016, 02:30:16 pm ---I used a potentiometer as a voltage divider that give output from 0 to 5V. So this circuit will work? --- End quote --- That circuit won't go down to 0V. The LM317 keeps the voltage on its output 1.25V above the adjust pin so it will vary from 1.25V to 5V 6.25V. That circuit has no advantages over the traditional LM317 circuit, using two resistors and the disadvantage of requiring an additonal minimum load of 10mA to regulate properly. If you need it do go down to 0V, then the adjustment pin of the LM317 needs to be connected to -1.25V. The simplest way to do this is to use the LM337L. Another way is to use an op-amp to invert the voltage between the LM315's output and adjust pin. It's more complicated but the most accurate. |
| Maobuff:
--- Quote from: Hero999 on May 19, 2016, 08:43:08 pm ---That circuit won't go down to 0V. The LM317 keeps the voltage on its output 1.25V above the adjust pin so it will vary from 1.25V to 5V. --- End quote --- I know, its not a problem for my supply, and i think if i feed 0-5V to ADJ I will get 1.25-6.25 on the output |
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