Comparator circuits causing regulator to not start up. - Page 1
Forum section: electronics / projects

author: homeward
author_rank: Contributor
timestamp: February 26, 2025, 10:44:06 am
content: Hi everyone,I have four of these comparator fed LED strings amongst some other audio stuff being powered by a +/-15V regulator (Meanwell DCWN06A-15). Roughly half of the time the regulator won't properly start up and gets stuck around 3V.If I disconnect the comparator PCB from the supply then the regulator starts up everytime. If I send a signal and get the LEDs to light up then then regulator will 'wake up'. If I keep only one of the supply rails to the comparators connected on start up then manually connect the 2nd rail once the regulator is running, that works fine. There is a softstart IC added so I don't think it's inrush current locking the regulator.It definitely seems like this circuit is causing issues for the regulator but I've no idea what. I've tried 10r inline resistors to the supply rails, adding hysteresis, changing the series current limiting resistor value. (I am aware the LEDs are not configured in the usual fashion, it works though and similar circuits are in use in countless professional mixers).I'm fairly sure I can add a load switch to keep the comparators off until the regulator has started but that seems a bit of a hack, I'd prefer to work out what the issue really is.Any ideas please? Something to look for? Thanks.

author: moffy
author_rank: Super Contributor
timestamp: February 26, 2025, 11:00:52 am
content: According to the datasheet equivalent schematic, page 23 of:https://www.ti.com/lit/ds/symlink/lm339.pdf?ts=1740540629631&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM339The output is an open collector NPN transistor, I don't understand why that would be connected by an LED to -15V?

author: PGPG
author_rank: Frequent Contributor
timestamp: February 26, 2025, 11:18:26 am
content: Quote from: homeward on February 26, 2025, 10:44:06 amYour schematic is incomplete. We don't see how LM339 is powered.OpAmps and comparators typically have 2 supply pins and all voltages they operate (at input and at output) have to be between supply voltages (for OC output voltage can be higher than + supply).As your output operate from -15 to +15 and input from 0 to +? we can suppose comparator is powered by -15 and +15. Is it?

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 11:55:56 am
content: Quote from: moffy on February 26, 2025, 11:00:52 amAssuming the comparator is correctly powered i.e. has +15V and -15V, it should work. When the comparator's output goes low, it will pull its output to -15V, which will turn the LED(s) off, by shorting it to the negative supply. When the comparator's output goes high, the output transistor will switch off, allowing the current to flow through the LED(s).Your confusion probably comes from the fact the power supply rails to the output are drawn with -15V at the top and +15V at the bottom.Here's a similar design which uses a constant current source rather than a resistor, which results in less variation in LED brightness. To conserve power, the current source is disabled, when no LEDs are illuminated. This is why U1a is connected differently, to the other comparators. It will work with a negative supply: connect the bottom of the potential divider, the cathode side of LED chain and LM339's negative to -15V, rather than 0V.Bargraph LED CC.asc

author: moffy
author_rank: Super Contributor
timestamp: February 26, 2025, 12:03:54 pm
content: As PGPG stated the drawing is incomplete in that it depends on what the -ve supply of the comparator is, and the OP states that connecting the board causes a fault.

author: homeward
author_rank: Contributor
timestamp: February 26, 2025, 12:05:19 pm
content: Yes, the comparators are powered by +/-15V.

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 12:29:26 pm
content: Do you really need the negative supply?The input range can't go below negative because of the diode.I've converted it to single rail. The current limiting resistor has been reduced to reflect the power supply voltage.

author: homeward
author_rank: Contributor
timestamp: February 26, 2025, 12:40:17 pm
content: Quote from: Zero999 on February 26, 2025, 12:29:26 pmThank you, what would be there advantage of this?

author: Ice-Tea
author_rank: Super Contributor
timestamp: February 26, 2025, 12:51:25 pm
content: Quote from: homeward on February 26, 2025, 12:40:17 pmSimpler is always better ;-)

author: homeward
author_rank: Contributor
timestamp: February 26, 2025, 01:38:50 pm
content: Quote from: Zero999 on February 26, 2025, 12:29:26 pmI tried this and it does seem to fix the regulator start up issue, unfortunately the reduced voltage across the LED string means their brightness drops as more LEDs in the string turn on. It's not feesible at this point to add a CCS.This is progress though and thanks for the idea, hopefully this provides a clue towards getting them working with the dual rail supply.Edit: Forgot to mention before that all comparator supply pins have 100nF to ground decoupling with 10uF electros for each rail on the PCB.

author: PGPG
author_rank: Frequent Contributor
timestamp: February 26, 2025, 03:18:13 pm
content: Quote from: homeward on February 26, 2025, 12:05:19 pmI was sure that the reason of your problems is that you connected comparator - supply to GND.As your comparatrs are supplied correctly than it is not clear what was the source of problems. And such source should always be found and not like: "I have randomly changed something and it is gone - correct."

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2025, 06:34:59 pm
content: Quote from: homeward on February 26, 2025, 01:38:50 pmYes with 15V, the resistor no longer acts as such a good current source, so the current varies more, depending on the LED forward voltage.Why is it not feasible to add a constant current source?Can't you stretch to an LM317L, a TO-92 package regulator?LM317L CC.asc

author: mikerj
author_rank: Super Contributor
timestamp: February 26, 2025, 07:21:15 pm
content: Do you have any large value supply decoupling caps that aren't shown on the schematic in your OP?

author: 741
author_rank: Frequent Contributor
timestamp: February 26, 2025, 08:50:04 pm
content: What voltage power supply is the DC/DC running from?The DS states 12V input types need 8.8V minimum at start-up.

author: Andy Watson
author_rank: Super Contributor
timestamp: February 26, 2025, 09:33:14 pm
content: Do you have diodes across the regulator outputs to prevent reverse polarity bias?Some positive regulators will not start up if their output is pulled below ground level by, say, the negative supply becoming established first or quicker. (or vice versa for negative regulators).

author: homeward
author_rank: Contributor
timestamp: February 27, 2025, 08:53:42 am
content: Quote from: mikerj on February 26, 2025, 07:21:15 pmOnly a couple other 10uF pairs on the various PCBs

author: homeward
author_rank: Contributor
timestamp: February 27, 2025, 08:54:13 am
content: Quote from: 741 on February 26, 2025, 08:50:04 pm15V

author: homeward
author_rank: Contributor
timestamp: February 27, 2025, 08:56:20 am
content: Quote from: Andy Watson on February 26, 2025, 09:33:14 pmI haven't but I'll try it, thanks for the idea.