Reactively coupled 240 volt sensor - Page 1
Forum section: electronics / beginners

author: rhodges
author_rank: Frequent Contributor
timestamp: September 27, 2019, 08:10:37 pm
content: I would like to have a decent circuit for sensing line voltage.My first need was to monitor a water well pump. I used a 120v / 240v universal volt power supply to give 12 volts DC when the pump was on. This 12 volts went into a optocoupler, and then into the MCU. It seems safe enough.Now I would like to monitor a water heater that has two 240 volt elements.I would like to use reactive coupling for a few milliamps for the optocoupler and have a circuit that I can use for other line voltage sensing. This is my very first attempt at using reactance with line voltage.I chose 20 milliamps as a relatively high, but not too high feed for the opto forward current. As you see, this 20mA may be shared between the LED (15mA) and R2 (5mA). I will be using the "C" version of the PC817, and the datasheet says I get 10mA to 20mA Ic with 5mA If.The input goes into a 100mA fuse and a 510 ohm resistor to limit inrush. But if the reactance is limited to 20mA, do I need R1? Is it a good idea? If C1 fails closed, maybe R1 will give the fuse time to blow before it gets exciting.I chose 0.22 for C1 to get 20mA at 240V and 60Hz. Is 400 volts enough, or should I go for 600 volts for C1?A bridge rectifier feeds C2 and R2. I just picked 20 uF just on the 1000 uF per amp rule of thumb. Is R2 a good idea? If the opto LED failed open, I was thinking R2 could give fuse F1 some time to blow before overvoltage kills C2.Thanks for all comments! I am sure I made some mistake somewhere!

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2019, 08:57:52 pm
content: The circuit will work, but why not simply use a relay with a 240VAC coil?https://www.mouser.co.uk/ProductDetail/TE-Connectivity-Schrack/RT214730?qs=8wHch9UpSvZoktAeFuphHQ==&gclid=EAIaIQobChMIy_L67vHx5AIVRkPTCh2vJA2qEAAYAiAAEgL2uvD_BwEhttps://uk.rs-online.com/web/p/non-latching-relays/6996865/

author: rhodges
author_rank: Frequent Contributor
timestamp: September 28, 2019, 12:06:39 am
content: QuoteWell, I did not think of that... While I have maybe used a couple 120V coil relays, it did not occur to me that I could buy a 240V one. It would be simple and "known safe".I was thinking my two options were a "universal" power supply or a reactive circuit feeding an optoisolator.The first uses a custom adapter cable, basically an extension cord with the plug cut off and replaced with ring terminals. The power supply (small 500mA wall wart) plugs into the other end of the extension cord. It works, but it looks like a real kludge.I have been intrigued by reactive power supplies for a long time, and this seemed to be a good excuse to get some experience. But it is obvious that there are safety details to work out! I would like to learn more about designing a circuit that is useful and safe.

author: GerryR
author_rank: Frequent Contributor
timestamp: September 28, 2019, 12:20:05 am
content: The relays will work fine and tell you that the circuit is live, but for the water heater, you might want to use a current transformer for each element so you know the elements are still good. Just a thought.

author: KMoffett
author_rank: Regular Contributor
timestamp: September 28, 2019, 11:54:44 am
content: Simple. 100-240VAC to 5VDC switch mode wall wart. Input to the AC line you want to monitor and the 5V output to the MCU input.

author: Calvin
author_rank: Regular Contributor
timestamp: September 28, 2019, 02:07:11 pm
content: Hi,You could use two optically coupled LED´s, one as transmitter and the other as receiver.You can generate >100mV of receiver voltage which You can easily monitor with an comparator or level translating transistor or an MCU´s ADC.I used such a configuration as voltage sensors for 115V and 230V for our diffusion furnace controller which fortunately features 150mV linear inputs.regardsCalvin

author: soldar
author_rank: Super Contributor
timestamp: September 30, 2019, 07:54:52 am
content: To detect voltage I have used1- Mains voltage relay. Already mentioned.2- Wall wart. Already mentioned.3- Capacitive voltage dropper. Simple way to drive an optocoupler.To detect current a current transformer can be used but I have used a couple diodes in opposition and an isolating transformer as shown in the attached schematic.

author: Zero999
author_rank: Super Contributor
timestamp: September 30, 2019, 09:21:26 am
content: Quote from: soldar on September 30, 2019, 07:54:52 amThat will work but it will need to be a step-up transformer to increase the forward voltage dropped across the diodes. A mains transformer connected backwards would work.

author: Kalin
author_rank: Regular Contributor
timestamp: October 01, 2019, 03:55:48 pm
content: Depending on the budget a current sensing relay could be a good option. it clips over the line you want to measure and relay actuates at a set current. that way you know when it is on and that the element is still good.