hi n advice. - Page 1
Forum section: electronics / beginners

author: drm
author_rank: Contributor
timestamp: June 08, 2013, 02:17:05 am
content: Hi All!This is my first post.Am looking for advice about a reliable zero cross detection, for a SCR DC motor drive.If someone can help with this issue, will tell the full story.By the way. here is lil picture of the notch that is disturbingmy zero cross detector made with a simple NPN transitor.Thanks in advance.

author: Paul Price
author_rank: Super Contributor
timestamp: June 08, 2013, 02:36:33 am
content: Can you draw a hand-drawn schematic and scan it into your computer, be sure to show power supply voltages and attach as a p icture.Tell mel what you mean by zero-crossing...in relationship to the mains? Do you need a mains AC 60-Hz zero crossing point reference for triggering the triac?

author: drm
author_rank: Contributor
timestamp: June 08, 2013, 03:20:50 am
content: hi Paul, thanks for your replayRight, need a way to detect zero crossing on a 50hz line, to fire the scrin syncro with mains.When firing point is near 0 crossing, notch distorts input signal to0 cross detector.Now its passed 0:00 AM here, tomorrow will draw the circuit and add more infoin my utube channel, theres more videos about the drive.

author: drm
author_rank: Contributor
timestamp: June 09, 2013, 01:11:49 am
content: Heres the circuit diagram.Am making a video showing important TPs, that upload when ready.

author: drm
author_rank: Contributor
timestamp: June 09, 2013, 07:37:37 pm
content: Video with TPs

author: drm
author_rank: Contributor
timestamp: June 09, 2013, 07:44:10 pm
content: TP LOAD, is 50 v-div, sorry.

author: David_AVD
author_rank: Super Contributor
timestamp: June 09, 2013, 09:56:47 pm
content: What happens when you remove the R/C network ("delay") ?

author: drm
author_rank: Contributor
timestamp: June 09, 2013, 11:39:47 pm
content: Hi David.It was added to override diode barrier of the first transistor, and make zero cross pulse, closerto real cero voltage. Shifting it, a bit to the¨right¨if I take it out, circuit looks a lil more prone to sporius firing near 180 degrees, but not big deal.Was reading that a 0 cross detector with OP AMP and histeresys, and PIC micro programmed to rejectout of time 0 crossing, can make it more reliable .May beKeep watching ...

author: David_AVD
author_rank: Super Contributor
timestamp: June 09, 2013, 11:43:19 pm
content: Actually, that base resistor looks really low in value. I'm sure I've used 1K in a DMX AC dimmer project.

author: drm
author_rank: Contributor
timestamp: June 10, 2013, 12:06:36 am
content: Thanks to mention, watching now can see that I made a mistake about that part of the circuit diagram.68 ohms R, goes to the union of 300k and 1uf.Like correction.jpg shows now.It should take 3,2 amp !! connected to 220 vac, as I drawn it before.

author: David_AVD
author_rank: Super Contributor
timestamp: June 10, 2013, 12:17:53 am
content: Oh, it's a feed direct from that mains! Mine was via a transformer secondary.

author: David_AVD
author_rank: Super Contributor
timestamp: June 10, 2013, 12:19:37 am
content: Also, please resize your pictures down to around 800 x 600 max or thereabouts. Having to scroll around all over the place really turns people off from looking and replying.

author: drm
author_rank: Contributor
timestamp: June 10, 2013, 12:42:57 am
content: OK.

author: drm
author_rank: Contributor
timestamp: June 10, 2013, 12:56:08 am
content: motor ready to fly...

author: Paul Price
author_rank: Super Contributor
timestamp: June 10, 2013, 06:17:33 am
content: Double Crossing Circuits that produce a clean zero-cross output.

author: miceuz
author_rank: Frequent Contributor
timestamp: June 10, 2013, 06:48:09 am
content: I've used an optocoupler-based zero crossing detection circuit for a recent project. It's just an "AC input" optocoupler with two LEDs in antiparallel on the input and two power resistors to drop the voltage. Works a treat and not much mocking with mains voltages. The drawback is that those two resistors are getting really hot - around 95C.

author: David_AVD
author_rank: Super Contributor
timestamp: June 10, 2013, 07:06:24 am
content: You should be able to increase the value of those two resistors to dramatically reduce the heat. Picking an opto with good transfer characteristic should help a lot with input current requirements.EDIT: That 200R pull-up could be way higher. That's probably the reason you have to drive the opto input so hard.

author: drm
author_rank: Contributor
timestamp: June 10, 2013, 04:35:33 pm
content: Thanks for those circuitsWill simulate them , and discover if Msim is able to show the notch produced by a 220ufcondenser, connected to 310v peak via a 2,5 ohms R ...

author: miceuz
author_rank: Frequent Contributor
timestamp: June 10, 2013, 06:37:14 pm
content: Quote from: David_AVD on June 10, 2013, 07:06:24 amoh, yes, the pullup is 10k, it's just a mishap in schematics. I'm using two 8.2k resistors. Those give me 340V/16.4k=20mA peak. Maybe I could go somewhere to 10mA peak CTR will be only 5% lower, but this would give me a more narrow ZC pulse and much lower power dissipation. Will try in a moment, thanks!

author: miceuz
author_rank: Frequent Contributor
timestamp: June 10, 2013, 09:32:33 pm
content: With double the resistors (32.8k) and 20k pullup I'm getting 400uS wide pulse (the bottom of it, the digital pulse is more narrow) and they are rising 16 degrees above ambient. It's a win!But now I have 4 resistors in series - they all are sharing the load. With two 18k resistors I would get somewhat 240V^2/36k = 1.6W -- 0.8W dissipation per resistor, I would guess that would effect in quite substantial rise anyway?

author: David_AVD
author_rank: Super Contributor
timestamp: June 10, 2013, 10:42:48 pm
content: For 0.8W dissipation I'd probably use a 2W resistor. Try it out and measure the temperature.

author: drm
author_rank: Contributor
timestamp: June 11, 2013, 01:41:13 am
content: So Msim can show a notch over ac mains.u can press space bar, and watch violet trace on virtual oscilloscope,simulating attached file: making a notch.

author: drm
author_rank: Contributor
timestamp: June 19, 2013, 05:33:29 pm
content: was no luck to get a reliable zero cross detection (didnt try with PIC even) , because of the big notch caused by load conmutation.So i decided by now, to replace the scr by a 6 amp and 200v mosfet that got at hand.And feeding the output stage with 110 vdc.Load voltage is modulated by pulse with 555.Look the inrush starting currrent of my DC motor, and how its decreasing, as rpm and rotor back emf goes high and get stable.

author: drm
author_rank: Contributor
timestamp: June 23, 2013, 05:08:02 pm
content: good info about notches.

author: drm
author_rank: Contributor
timestamp: June 26, 2013, 11:31:51 pm
content: very useful app notes about scr, triacs and power mosfet from philipshttp://www.4shared.com/file/x_IWLEAY/scr_triac_pmosfet_app_notes.html

author: drm
author_rank: Contributor
timestamp: June 29, 2013, 09:37:24 pm
content: A new kid appeared, on the power quality block.They call it,line impedance.Look what happens, when 1000 watts load, is switched on and off.

author: drm
author_rank: Contributor
timestamp: July 08, 2013, 11:56:00 pm
content: Was using PC parallel port, to generate pwm output with a rqbasic app.http://www.4shared.com/archive/4FiSvnmE/RQ_basic_PWM.htmlIts pwm signal, shows a phase shift, that makes output voltage flicker.Look:This rqbasic app, dont control P Port directly cause win xp and seven OS, dont let it.So i must make it calling a DLL called inpout32.DLL.How can i do to find out, if its a soft or hardware related issue?

author: drm
author_rank: Contributor
timestamp: July 09, 2013, 01:37:18 am
content: a video from usb logic analyzer point of view.