About LM317 Dimmer Circuit  - Page 1
Forum section: electronics / beginners

author: catalyst
author_rank: Contributor
timestamp: October 08, 2017, 11:11:20 am
content: Hi everyoneThis is a circuit that either can work full version or can work dimmer version. However i can not understand how this circuit works. Is there anyone who can tell me circuit's detailed working principles and Q1's working principles?Sincerely regard.

author: Coceth
author_rank: Contributor
timestamp: October 08, 2017, 11:35:35 am
content: Where did you get the circuit from? I've never seen an LM317 circuit where the adjust pin is grounded with no feedback.Maybe try using the LM317 as a current source to dim the LEDs.Good luck!

author: Ian.M
author_rank: Super Contributor
timestamp: October 08, 2017, 11:47:32 am
content: Its the classic LM317 constant current circuit but with the LM317 + resistor between Out and Adj on the low side of the load, not the high side. It has an extra resistor switched in by MOSFET Q1 that increases the current to provide a choice of two brightness levels depending on whether +24V power is fed to the '+24V_DIMM' or '+24V_FULL' supply terminals.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2017, 05:50:11 pm
content: Quote from: Coceth on October 08, 2017, 11:35:35 amThe LM317 always maintains its output pin 1.25 volts above its adjust pin so this is just a configuration where the lower voltage divider resistor is reduced to 0 ohmsSince the output pin is maintained at 1.25 volts, any resistance between it and ground creates a constant current at the output which translates into a constant current at the input; the adjustment pin current is essentially zero. This circuit is advantageous because the current determining resistor is grounded at one end allowing easy switching and only the low 1.25 volt output has to be considered.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2017, 11:06:50 am
content: Look up LM317 constant current circuit. This circuit is the same as that but part of the current determining resistor is being switched in/out of the circuit, using a MOSFET.Q1 only needs to be rated to 1.25V but it needs to carry the full current.D1 is a bad idea. It will slow down the MOSFET's turn off time.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 06:40:31 pm
content: Another way to think of it is the 317 is operating like a common base NPN transistor. With suitable biasing, an NPN transistor could be used to do exactly the same thing but the 317 includes all of the biasing built in as well as protection. This is another variation of the single LED, two resistor, and NPN transistor temperature compensated current sink.

author: catalyst
author_rank: Contributor
timestamp: October 18, 2017, 11:29:45 am
content: Hi again,What is the purpose of R7 ? May it be a precaution?Thank you.

author: David Hess
author_rank: Super Contributor
timestamp: October 18, 2017, 06:00:20 pm
content: Quote from: catalyst on October 18, 2017, 11:29:45 amI do not see any reason to include R7. The 33 volt zener diode already protects the LM317 against high voltage and the LM317 and LED string against reverse voltage. R7 makes it easier to test if the fuses are blown but how often is that expected to happen?