LM217 voltage regulator question - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 19, 2019, 10:12:40 pm
content: I was looking for around 6 to 7V out from 12V in and I had a 5k ten turn pot to hand. using a cadence resistor box I found R1 at 1k gave a nice adjustment range but the spec sheets cite R1 as usually being a lot lower resistance, circa 240 Ohms. Is there anything bad in using 1k for R1 in the divider? Thanks.

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 19, 2019, 11:23:37 pm
content: In order for it to regulate properly, the datasheet says it needs a minimum of 10mA (worst case, with 3.5mA typical) of load current. A resistor of 125 ohms or lower guarantees that without any additional load. If you use 1k, then you will need to be sure that you are always sinking additional current to meet the minimum load current specification.

author: magic
author_rank: Super Contributor
timestamp: May 19, 2019, 11:27:24 pm
content: My datasheet (ST) specifies 5mA worst case and that's where the 240Ω comes from.

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 19, 2019, 11:30:28 pm
content: Quote from: magic on May 19, 2019, 11:27:24 pmCorrect. I was looking at a TI datasheet (actually for the 317).

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 19, 2019, 11:36:18 pm
content: Thanks for the very fast replies! Will an HEF4013 flip flop (which it is powering) draw that much all the time or do i need to redesign the divider? It *SEEMS* to work OK on the breadboard...EDIT, hmm it seems the HEF4913 only draws Ua levels maybe I need to change thisIDD supply current all valid inputcombinations;IO = 0 A5 V - 1.0 - 1.0 - 30 - 30 uA10 V - 2.0 - 2.0 - 60 - 60 uA15 V - 4.0 - 4.0 - 120 - 120 uA

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 19, 2019, 11:41:15 pm
content: Quote from: Chris Wilson on May 19, 2019, 11:36:18 pmAdd a dummy load to make up the difference.

author: magic
author_rank: Super Contributor
timestamp: May 19, 2019, 11:45:33 pm
content: Funny, mine also says 10mA for the LM317 but still recommends 240Ω without specifying it's for LM217 only. I always assumed 240Ω is enough, good to know.

author: Chris Wilson
author_rank: Super Contributor
timestamp: May 20, 2019, 12:00:14 am
content: Quote from: Wimberleytech on May 19, 2019, 11:41:15 pm240 ohm 1/2 watt resistor OK? Although using R1 at 240 Ohms and adjusting R2 to around 900 Ohms should give circa 6V. Thank you again.

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 20, 2019, 12:06:45 am
content: Quote from: Chris Wilson on May 20, 2019, 12:00:14 am1.252/240 = 6.5mW so 1/2 watt is overkill but it is fine.

author: Zero999
author_rank: Super Contributor
timestamp: May 20, 2019, 08:35:10 am
content: Quote from: Chris Wilson on May 19, 2019, 10:12:40 pmAs mentioned above, the resistance needs to be lower because the LM217 has a minimum load current requirement of 5mA.The reason for this is it's actually an op-amp and voltage reference connected to a big Darlington pair, along with some protection circuitry. The negative rail of the op-amp is also connected to the output terminal of the regulator, so in order for the op-amp to work properly, the current flowing through the output needs to be high enough. At higher currents, the op-amp still draws around 5mA, it's just the output transistor turns on to allow more current through the output terminal.