Detecting AC VOLTAGE [INPUT (0-200) up to 1000 V AC] [OUTPUT: Any low DC is ok] - Page 1
Forum section: electronics / projects

author: AndyChriss123
author_rank: Newbie
timestamp: April 27, 2022, 05:20:14 pm
content: Hello!I need to build a circuit with a DC output that is ON whenever the AC input is active (a.i. voltage is present at the input) ideally from as low as possible but it could be as high as 200, 250 VAC and still be acceptable but needs to survive up to 1000VAC spike and some 600VAC constant. How do you voodoo?The only idea I got involves many transformers and many zenners which makes me feel like there should be a much much easier way to do this hence why I am here. I just want to detect the voltage, thinking about using then an optocoupler with the output in order to get the signal out or whatever isolation. I looked for Voltage Detection Relays but nothing is within my range and an AC sensor 20-1000VAC is way too overengineered/expensive for this application, I think at least.What is your idea? Thank you for your time!Summary:(0 (max 250) - 1000  INPUT RANGE)----| INSERT CIRCUIT HERE |----(SOME CONSTANT LOW VOLTAGE DC OUTPUT)EDIT: Or whatever idea you might have, i just need my PLC to know when there is tension present on those wires (many cases there will be tension but no current unless I connect something so wireless/magnetic detectors are no good unless some small load is used)

author: pigrew
author_rank: Frequent Contributor
timestamp: April 27, 2022, 05:33:47 pm
content: Does the input need to be isolated from the output? Is the DC output circuit's common tied to one of the AC inputs? How high impedance does the input need to be? Could it draw 100uA?What DC power supplies are available on the output? What is an acceptable DC current draw when the AC is on and when the AC is off?I'm imagining that a opto-coupler would work nicely.

author: AndyChriss123
author_rank: Newbie
timestamp: April 27, 2022, 06:07:36 pm
content: The output of the circuit goes to a digital input of the controller so yes I was thinking about using an optocoupler which is the cheapest and easiest way to do it.The DC current draw depends on the components from the circuit itself, as I will use on optocoupler on the output there is a lot of slack there as the current will be set using a resistor on the controlled line of the optocoupler going to the digital input.If the input of this circuit would be of constant voltage it would be simple but I don't really know how to approach this problem as the circuit needs to operate in a large range up to 1000(V) which is the maximum of the generator.Also the DC available is 24 volts.

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 06:09:01 pm
content: If are OK with a few mA of load on the AC line, just use a capacitor as a dropper and a AC-input wide current range optocoupler like the TLP182. This device can work with a range of inputs from less than 0.5mA to 50mA. If you used a 1000V 0.022uF capacitor, you'd have about 8mA @ 1000VAC and 0.5mA @ 60VAC, both of which should result in the phototransistor being 'ON' enough to be sorted out with some simple circuitry on the other side.https://toshiba.semicon-storage.com/info/docget.jsp?did=14414&prodName=TLP182

author: AndyChriss123
author_rank: Newbie
timestamp: April 27, 2022, 06:29:00 pm
content: Awesome! Thank you very much, it sounds like a very good simple way to do this as I can handle the current loads you specified but how low of a voltage will bee too low for the circuit to trigger, do you have any idea about this or should I just experiment with it after it's done?something like 200, 250 VAC is more than enough as I don't care about values lower than that.EDIT:Wow almost 5000 posts Haha, I guess a lot of people owe you a little bit of something, Thank you again!

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 06:42:56 pm
content: Quote from: AndyChriss123 on April 27, 2022, 06:29:00 pmI couldn't tell you at what point the optocoupler won't work, but it is clear from the datasheet that it works down to <0.5mA, albeit with a variable output. You will have to filter the result on the phototransistor side with a small capacitor and pulldown resistor and that setup will also matter as to where the cutoff is. If you really don't care about anything less than 200VAC, then your dropper capacitor can be 0.01uF, which lowers the current even more. Depending on exactly what you do on the other side I suspect it will have a pretty low cutoff, so you'll have to plan and experiment a bit so that noise or leakage doesn't give you a false indication.Nobody owes me anything, I've gotten much more out of EEVBlog than I've put in. Thank Dave Jones for putting it all together.

author: AndyChriss123
author_rank: Newbie
timestamp: April 27, 2022, 06:51:26 pm
content: Awesome! Thank you and good luck!I've been watching the youtube channel for a while now but somehow I never visited this forumSo much time wasted Haha.. In my case I guess this is the disadvantage of having so many modules for everything that you just plug in and configure without thinking too much about it... You forget the basic stuff, back to school I guess)

author: AndyChriss123
author_rank: Newbie
timestamp: April 27, 2022, 07:11:57 pm
content: Sorry, one more question, the datasheet got me confused.. the PLC operates on 24VDC and the first feature presented is this (in the datasheet of the optocoupler)1) Collector-emitter voltage: 80 V (min) - Sooo, minimum 80V? Isolation/Breakdown maybe, what do you think?

author: chris_leyson
author_rank: Super Contributor
timestamp: April 27, 2022, 08:47:27 pm
content: Attached an ltspice schematic of a 50/60Hz zero crossing detector. It's based on a design from EDN, link below.https://www.edn.com/mains-driven-zero-crossing-detector-uses-only-a-few-high-voltage-parts/Resistors R1, R2 set the optocoupler LED current, 1.5mA peak at 230VAC input. I used Everlight EL816B opto-coupler with typical 100% current transfer ratio and my spice model assumed 100% CTR at low LED currents 100uA. The spice model may not be accurate in this respect, however, the zero crossing detector works from less than 20VAC up to 230VAC with no problems. To turn the zero crossing detector into an AC detector just add Miller capacitor across Q2 base emitter.Transistors Q2 and Q3 add some additional gain to the zero crossing detector allowing it to work down to < 20VACCheersChris

author: chris_leyson
author_rank: Super Contributor
timestamp: April 27, 2022, 08:58:07 pm
content: Sorry forgot to add, the four diodes D1 to D4 allow the zero crossing detector to work on positive going AC transitions going front-ve to +ve and negative going transitions. You could simplify things and make layout easier by removing D3 and D4 so that the zero crossing detector works on one AC transition only.

author: chris_leyson
author_rank: Super Contributor
timestamp: April 27, 2022, 09:02:58 pm
content: Also, AC load current is 300uA peak, 100uA RMS at 230VAC

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 09:08:20 pm
content: Quote from: AndyChriss123 on April 27, 2022, 07:11:57 pmThe LED and phototransistor are isolated for thousands of volts. The 80V rating is the CE breakdown voltage of the phototransistor, the (min) designation just means they guarantee it is good for at least that much. You could use the 24V directly on it, with current limiting and so forth of course. That optocoupler should work fine for DC supply voltages from logic levels up to 48V.

author: pigrew
author_rank: Frequent Contributor
timestamp: April 27, 2022, 09:19:29 pm
content: I forgot to ask if this is a fixed frequency application (i.e., 50 or 60 Hz), or is there some range of frequencies it must operate within? If it's a wide range, the capacitive dropper might be be a bad idea.With "only" a 10:1 range of operating voltages (200 to 2000 Vpk is a good range), the photodiode with appropriate resistive or capacitive dividers are easy and good solutions. For a wider range, constant current regulators or zener diodes could be thrown at the design, but these seem unnecessary given the requirements.Remember to use physically large components (maybe even through hole) and large clearance distances for the high voltage tracks.

author: tooki
author_rank: Super Contributor
timestamp: April 27, 2022, 09:23:24 pm
content: There was a recent thread about this same topic, has some useful info:https://www.eevblog.com/forum/projects/non-contact-mains-voltage-presence-sensing-techniques/

author: bdunham7
author_rank: Super Contributor
timestamp: April 27, 2022, 09:29:08 pm
content: Quote from: pigrew on April 27, 2022, 09:19:29 pmI assumed he meant mains, but rereading I see he didn't explicitly say that, he just referenced a generator. Obviously if it isn't a 50/60Hz system, a power resistor dropper is in order.OP--what is the source of the AC voltage?

author: mariush
author_rank: Super Contributor
timestamp: April 27, 2022, 09:41:10 pm
content: Microchip makes a LR8 linear regulator which can do up to 450v DC input, and as little as 1.25v output if my memory is correct... and the input must be at least 12v above the output voltage.Not sure if the design would make it possible to put 2 or 3 of them in series so that each one would only "see" a few hundred volts difference ... but if you can get the input voltage to 1/3 or 1/4 of 1000v such IC would safely give you some voltage at a very low current, just enough to get it detected by a micro.