Question about a circuit with cap blocking DC. - Page 1
Forum section: electronics / beginners

author: uk.casmith
author_rank: Contributor
timestamp: June 25, 2020, 10:15:02 pm
content: Hello,Could someone please walk me through what is happening in this circuit in reference to the 1uF capacitor circuit (point 2) and the comparator.1. I do not understand why when I measure the voltage at point 1 there is no voltage? Isn't the comparator's non-inverting input lower than the inverting input thus the collector should be allowing current to flow through it?2. I kinda understand that the 1uF capacitor is blocking DC, and when something changes it allows current to flow for a second to allow the 555 timer to reset, but I just don't understand how it's working? I'm guessing the current is flowing from point 3 to point 1 allowing a drop in voltage?Sorry it's a bit vague these questions i'm kinda lost.Thanks,Cameron.

author: Cubdriver
author_rank: Unknown
timestamp: June 25, 2020, 10:33:21 pm
content: What are you measuring point 1 relative to - the negative side of the power supply, or somewhere else? Could you post a pic of the entire circuit schematic?Pin 2 is the output of the LM339; if the voltage on pin 4 is greater than that on pin 5 then pin 2 should be pulled to ground and would thus show little or no voltage.-Pat

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2020, 11:37:19 am
content: It appears to be a comparator triggering a monostable 555 timer. It's quite likely the circuit could be simplified by replacing the 555 timer, with an unused section of the LM339.I don't know for sure, because the schematic is poor quality and ambiguous. Are those packages viewed from the top or bottom? I'd have to look at the data sheets to be sure, as I don't know the pinouts off the top of my head.The circuit would be easier to understand, if proper symbols were used, rather than just showing the packages, which isn't much use, without referring to the data sheet.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 27, 2020, 11:59:13 am
content: From the top I believe, pins 4 and 8 on a 555 would normally be connected to V+

author: Zero999
author_rank: Super Contributor
timestamp: June 27, 2020, 12:09:01 pm
content: Quote from: AVGresponding on June 27, 2020, 11:59:13 amThen there's the LM339, which only one section is used. The LM393 could easily be used to save cost and space. Unused inputs shouldn't be left floating like that. It's bad practise.

author: AVGresponding
author_rank: Super Contributor
timestamp: June 27, 2020, 02:41:17 pm
content: Quote from: Zero999 on June 27, 2020, 12:09:01 pmAye, in certain conditions they can start to self-oscillate?And as you point out, the 555 could be done away with and a comparator section used instead.It looks like an example circuit in some kind of electronics textbook, maybe it mentions in the text about tying unused inputs to a rail.

author: uk.casmith
author_rank: Contributor
timestamp: June 27, 2020, 03:39:01 pm
content: Hi, guys,Thanks for your response. Yeah, the book mentions about not leaving the pins floating. I think the textbook is trying to teach through a process so i better understand electronics.After going back and reading more I think i've got a better idea. I had to build a small circuit just with a capacitor to understand that it causes the voltage to drop for a few milliseconds to allow the timer to start.Thanks for all your replies.Cameron

author: Zero999
author_rank: Super Contributor
timestamp: June 28, 2020, 10:17:32 am
content: Quote from: AVGresponding on June 27, 2020, 02:41:17 pmI don't know about oscillation. The LM339 data sheet recommends tying unused inputs to ground.https://www.onsemi.com/pub/Collateral/LM339-D.PDF