Current limiting for constant voltage polarity reversal power source using LM334 - Page 1
Forum section: electronics / beginners

author: papagino
author_rank: Newbie
timestamp: October 14, 2024, 06:48:41 pm
content: Hello people, I need to make a circuit to limit current of a fixed voltage polarity reversal power source using two LM334 current limiter IC.Let's say I need a 28V constant voltage at the output and need the current to be limited at 8mA, can I use two LM334 IC with a 1N5819 Schottky diodes connected at their inputs/outputs to direct current depending on the polarity of the power source? See images below that shows current direction depending of polarity direction...Also, the polarity reversal is not fast, maybe once every 5 minutes...Will this work?

author: inse
author_rank: Super Contributor
timestamp: October 14, 2024, 08:45:00 pm
content: Why not take a bridge rectifier and place one constant current circuit in the center?But apart from that it should work

author: PGPG
author_rank: Frequent Contributor
timestamp: October 15, 2024, 12:16:40 am
content: You write about constant 28V but you don't have any voltage regulator.May be you assume the regulator is before your circuit but then you would not write 0-40V but rather write 30V (from 30V after LM334 and diode you will get about 28V).Also see in datasheet - LM334 max forward voltage = 30V while according to what you write the source is up to 40V.

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 04:11:42 am
content: Good point!

author: papagino
author_rank: Newbie
timestamp: October 15, 2024, 12:50:30 pm
content: My original question is if I could use two LM334 with diodes to limit the output current in both polarity.The voltage source is a Buck/Boost DC to DC converter and the voltage will be set between 24 - 30V taking under consideration the voltage dropout caused by the diodes in the circuit. I would like the output voltage of the circuit to be constant regardless on the load connected to it. The load will be variable to maybe 30K to 2M ohms.Is this possible? Or the output voltage will drop as the load resistance decrease?

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 12:57:10 pm
content: With minimal load of 30k, why would you need the limiter?But if you short the output, the LM334 might get destroyed if you supply more than ~30V

author: papagino
author_rank: Newbie
timestamp: October 15, 2024, 12:57:48 pm
content: No sure what you meant with "Why not take a bridge rectifier and place one constant current circuit in the center?"I want the polarity to switch at the output with current limited to what I set with the LM334.Can you provide a circuit with your idea!Thanks

author: papagino
author_rank: Newbie
timestamp: October 15, 2024, 01:01:54 pm
content: Not sure about what the load resistance will be, may be lower than 30k, but I would need to limit the current to maybe 5 to 10ma...

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 01:12:39 pm
content: Apart from the voltage issue: simply draw a bridge rectifier around the circuit, plus top, minus bottom and the AC in line with your power supply.So inside the rectifier voltage will always be in one direction regardless of the outside polarity.

author: papagino
author_rank: Newbie
timestamp: October 15, 2024, 03:15:17 pm
content: Like this:

author: inse
author_rank: Super Contributor
timestamp: October 15, 2024, 03:39:17 pm
content: You nailed it

author: papagino
author_rank: Newbie
timestamp: October 15, 2024, 04:21:25 pm
content: Probably make more sense to use one LM334 and one potentiometer to set current limiting for both polarity.I will use 4 schottky diodes do reduce the voltage drop.Cheers