Mains zero crossing. - Page 1
Forum section: electronics / beginners

author: orolo
author_rank: Frequent Contributor
timestamp: October 26, 2017, 02:53:35 pm
content: I've been reading the depletion mode mosfet section on The Art of Electronics, and decided to place some of these devices (LND150) in my next order to Mouser. As a first experiment, I'm thinking about a zero cross detector for mains:Here the mosfets play the two roles from TAoE, input protection for the comparator and discharge for the mains capacitor. Additionaly, they provide a neutral referenced bipolar power supply which, if I pulled it right, is an interesting twist.Is there something I've overlooked that could result in pirotecnics? I haven't found any spice models for small depletion mosfets, so I'm basing myself in calculations and a very limited experience with these devices. Of course, in case I build the circuit, I'll use an isolation transformer. Given the nature of the power supply, all the components should be very low power. With the comparator there's no problem; I wonder if the optocoupler could be replaced by magnetic coupling, to reduce power.

author: ebclr
author_rank: Super Contributor
timestamp: October 26, 2017, 11:59:05 pm
content: You can add 2 more ic's on your mouse list for research zero crossinghttp://www.mouser.com/ds/2/427/il4116-241328.pdfhttp://www.mouser.com/ds/2/149/MOC3043M-195994.pdf

author: BrianHG
author_rank: Super Contributor
timestamp: October 27, 2017, 01:42:54 am
content: If you want a 0 crossing detection circuit for timing or directing a microcontroller, why not just use an AC input optocoupler with 2 back-back series zener diode to control the 0 crossing window size with a series resistor and pull-down on the opto-input. The output is still a transistor out. You wont need any op-amps or mosfets or anything else.Something like a LTV-814 or FOD814.

author: Dubbie
author_rank: Unknown
timestamp: October 27, 2017, 01:44:19 am
content: Quote from: BrianHG on October 27, 2017, 01:42:54 amThats how I did it on a dimmer bank I built once.Worked fine.

author: orolo
author_rank: Frequent Contributor
timestamp: October 27, 2017, 08:47:08 am
content: Wow, there seems to be an IC for every conceivable task. I'm doing this to learn about depletion mosfets, not to build a zero crosser, but I'll get some of these optocouplers as reference to test how precise the circuit is, if it works. Thanks for the info!

author: james_s
author_rank: Super Contributor
timestamp: October 27, 2017, 08:46:35 pm
content: In many cases a simple zener diode and resistor is sufficient.When I built a DMX dimmer pack several years back I used a comparator and got a very clean and dependable zero cross signal. I don't recall the exact circuit offhand but I can dig out my schematic. No need for a special purpose IC.

author: frog
author_rank: Contributor
timestamp: October 28, 2017, 02:26:17 am
content: Interesting use of depletion-mode there. So as I see it the FETs are switched off when significant current starts to flow, so at high voltages you have more or less a current source/sink. If the resistor between the two FETs is the right value then you should be able to dissipate most of the excess voltage here so the FETs aren't working too hard. Depending on your priorities it might be preferable to grab all the charge you need when the AC voltage is relatively low and then switch off altogether for the rest of the half-cycle, this could give you better efficiency for some extra complexity in the design.Oddly you don't see depletion mode FETs that often. I suppose an Rds of 1000 ohm could be off-putting if you're used to something less than an ohm.As a cautionary note, I'd look at the startup conditions if you happen to switch on in the middle of an AC cycle.

author: Cerebus
author_rank: Super Contributor
timestamp: October 28, 2017, 02:59:26 am
content: Quote from: frog on October 28, 2017, 02:26:17 amYou don't see depletion modeMOSFETs all that often; of course, youonlysee depletion modeJFETs.

author: orolo
author_rank: Frequent Contributor
timestamp: October 28, 2017, 08:28:31 am
content: Quote from: frog on October 28, 2017, 02:26:17 amThat's the idea, exactly. The resistor between the FETs is to define gate bias, and could be maybe omitted; most of the dissipation work will be by the FETs themselves. As long as it's only 1-3mA, I guess it should be ok. I'm beginning to like the idea of the power supply, in a Big Clive kind of way: transformerlessandcapacitorless. My idea was to "improve" the design, poundland style. Turning off the FETs at the right moment is a magnificent idea, though: if the voltage of the supply rails is high enough, the FET is turned off for the rest of the half cycle, saving a lot of power. Very good.QuoteOops, I didn't think about this, and it's a classic failure mode. I didn't think about the turn-off trick, neither. Thanks a lot for the ideas!