Cant Understand Current Adjustment Function in Lab power supply??? - Page 1
Forum section: electronics / projects

author: Gurumurthy
author_rank: Regular Contributor
timestamp: August 22, 2017, 03:17:54 pm
content: Hi to all!!!!I cant understand the function of LM117 present accross the output. Why it should be connected to negative supply voltage to the output terminal.what is the function of 150 ohm variable resistor?

author: Cerebus
author_rank: Super Contributor
timestamp: August 22, 2017, 04:24:29 pm
content: The LM117-adj spends its life trying to do whatever it needs to to keep the voltage between the Voutterminal and the ADJ terminal at 1.25V.In the case of the second LM117 in your circuit, that means that it is trying to keep 1.25V across R3. So the combination of the two parts, wired that way, is a constant current sink. 1.25V/120ohms = 10.42 mA. Now, look at the data sheet for the LM117 and hunt for something like 'minimum load'. On page 4 of the original National LM117 datasheet you'll find 'Minimum Load Current' specified as 5 mA and you'll find that if am LM117 has no load it will not perform within specification.So this part of the circuit ensures that the first LM117 is kept within its operating parameters even when there is no external load on the power supply. The reason it's connected to a negative rail is that this constant current sink requires a voltage drop of 1.25V across R3 to work (and some drop across the LM117 itself, around 2V), so of the power supply output was below 3.25V the constant current sink would stop doing its job.Now knowing from above how you turn an LM117 into a constant current source/sink, perhaps you can revisit what R2 is doing and figure that bit out for yourself. Hint, the top LM117 is going to be trying to keep Vout(attached to R1) at 1.25V more than ADJ (attached to R2's wiper) and there is always going to be 10.42 mA flowing down R2. Check the data sheet to see how much current the ADJ pin is going to source or sink, or for a crude first pass at analysis, just ignore it.Give it a try and, if you can't figure it out for yourself ask again, if you can figure it out, tell us how it works as a way of testing your understanding.

author: Gurumurthy
author_rank: Regular Contributor
timestamp: August 22, 2017, 04:36:09 pm
content: Dear Sir,Check this schematic and correct me if i have understood wrong. thank you sir...

author: Cerebus
author_rank: Super Contributor
timestamp: August 22, 2017, 04:41:09 pm
content: Yup, don't forget to split R2 into two resistors (R2A+ R2B= R2) to preserve the original topology.Whoops, on a second look, change your V1 to ~2 volts more to allow for the drop across the 2nd LM117.BTW, note that if you try to simulate that in LTSpice with an ideal current source there's a good chance that you'll get some very strange results in edge cases with voltages in the order of GV showing up. This always seems to bite me when I'm trying to do something quick and dirty, and I always end up going back and substituting more realistic current sources/sinks to get the simulation to work at all.

author: Gurumurthy
author_rank: Regular Contributor
timestamp: August 22, 2017, 04:45:55 pm
content: Dear sir,Thank you sir.