Comparator with hysteresis - Page 1
Forum section: electronics / beginners

author: PetarP
author_rank: Newbie
timestamp: January 17, 2016, 11:30:12 am
content: I have problem making work comparator with hysteresis, I use schematics and explanation from TechnologyCatalyst videobut what ever value I use fro R3 it act as comparator without hysteresis. Can someone help me what I am doing wrong?

author: CarlG
author_rank: Regular Contributor
timestamp: January 17, 2016, 01:08:41 pm
content: My guess is that you (in the simulation) have connected a generator to the input without adding a resistor between the generator and the noninverting input?

author: Simon
author_rank: Unknown
timestamp: January 17, 2016, 04:40:59 pm
content: 26 minutes to understand hysterysis ? good god.

author: Audioguru
author_rank: Super Contributor
timestamp: January 17, 2016, 06:09:39 pm
content: In Google I looked for Comparator With Hysteresis and thousands of schematics are shown. Haven't you heard about using Google?

author: MarkS
author_rank: Unknown
timestamp: January 17, 2016, 06:15:49 pm
content: Google is great, but it can never replace the interaction with knowledgeable individuals.

author: CarlG
author_rank: Regular Contributor
timestamp: January 17, 2016, 08:45:08 pm
content: BTW, it's not good practice to use an OPA as comparator. Just an advice to start you on the right track

author: jitter
author_rank: Frequent Contributor
timestamp: January 17, 2016, 08:59:54 pm
content: Yeah, use something like an LM393...

author: Simon
author_rank: Unknown
timestamp: January 17, 2016, 09:30:36 pm
content: To be honest that is poor and long winded explanation. Looks like we have a Dave immitator and all his videos seem to have the same thumnails.. hm. His explanation in my opinion is incomplete and innacurate and of little ptractical use. I would have started with hysterysis in the inverting comparator and then in the non inverting comparator. For someone claiming to close a huge gap in you tube video's he's doing a horrendous job. As far as i know you need a resistor in series with the non inverting input, just a feedback resistor and relying on the output impedence of the signal source is a very poor way to do it and requires a source that can sink as well as source current.

author: miguelvp
author_rank: Super Contributor
timestamp: January 17, 2016, 09:47:45 pm
content: It would help if you show your Simulation Command.Maybe you don't have one and that's your problem. There are plenty of LTSpice out there, So i'm linking the 1st one that popped up.http://denethor.wlu.ca/ltspice/

author: PetarP
author_rank: Newbie
timestamp: January 17, 2016, 11:10:16 pm
content: Quote from: CarlG on January 17, 2016, 01:08:41 pmI didn't use LTspice to verify it is working or not, I just draw it in it for this question, I try it on breadboard and it is not working as it should.edit: and yes I did use low impedance source in test.

author: Audioguru
author_rank: Super Contributor
timestamp: January 17, 2016, 11:31:41 pm
content: Your circuit did not work as expected because it is missing the important input resistor that is part of the hysteresis.

author: PetarP
author_rank: Newbie
timestamp: January 17, 2016, 11:48:15 pm
content: Thanks you all for replies, so if I understand correctly circuit in TechnologyCatalyst video is not correct.I am only puzzled because he demonstrate it work in second part of that video, and when I try it it dont work.

author: Audioguru
author_rank: Super Contributor
timestamp: January 18, 2016, 12:48:24 am
content: What video? The circuit and its video do not work:

author: PetarP
author_rank: Newbie
timestamp: January 18, 2016, 02:26:53 am
content: It look like TechnologyCatalyst removed video, which I suppose is improvement from before, because when I asked him why his circuit do not act as supposed he told me that I am talking boolshit and he removed my comment.Again thanks all for help. I will try circuit by Audioguru.

author: Audioguru
author_rank: Super Contributor
timestamp: January 18, 2016, 03:18:01 am
content: Do you know how to calculate the amount of hysteresis? Usually only a little amount of hysteresis is used. The datasheet of the LM339 shows a 10M positive feedback resistor and a 10k input resistor.

author: CarlG
author_rank: Regular Contributor
timestamp: January 18, 2016, 06:59:36 am
content: Quote from: PetarP on January 17, 2016, 11:48:15 pmI never bothered to watch the video so I don't know what he was up to. But for instance, testing with square wave doesn''t require feedback at all to give a decent output. (Best way if you ask me is a relatively slow triangle wave.) Or, with just 10k feedback, a few ohms in the leads connected to the input gives you some, maybe enough, hysterisis.

author: Simon
author_rank: Unknown
timestamp: January 18, 2016, 07:09:01 am
content: Quote from: Audioguru on January 18, 2016, 12:48:24 amLooks like he removed it. But yes it was incomplete and a poor explanation, I understood it only because i knew what he was driving at. His example worked because he had knowingly or unknowingly created the right conditions. An easier explanation is the inverting comparator with hysterysis.An inverting comparator with hysterysis just works regardless of the source, what he showed relys on the source impedence interacting with the comparator circuit and in his case a light sensitive resistor. If the source inpedence is wrong you use a resistor in series with the non invertinginput.

author: CarlG
author_rank: Regular Contributor
timestamp: January 18, 2016, 07:18:40 am
content: Quote from: Audioguru on January 18, 2016, 03:18:01 amI'm not sure if this is a question to PetarP or to any of usAnyhow, for this case (non-inv comp), the voltage at V+ (non-inv input) is determined by the voltage divider from input to output. Thus, the hysterisis is the difference between V+ when the output is high and low, resp. With small hysterisis the voltage drop across Rin may be neglected in the first approximation, which means that the input voltage in the equation is appr equal to the voltage at the inverting input and is cancelled in th equation.Uhyst appr= deltaUout*Rin/(Rin+Rfb)