I have been looking at the guides for interfacing a CT sensor to an Arduino and I see that I need a handful of parts to use with my SCT-013-000.
My Arduino is a 5V version, not a 3.3V version
A Burden resistor @ 33 Ohms.
2x 10K Ohm resisters
10uF capacitor
My question is, since the guides seem to mostly be referring to 220V, and I have 110V, do any of the resistors or capacitor change in values to take into account the voltage difference? I am only interested in determining load (amps), not voltage for my setup.
I know itâs a âsilly questionâ for engineers, but Iâm not oneâŠ
As youâre dealing with current, not voltage, the effect is only a second-order one, i.e. you might need larger c.tâs than our standard 100 A one (which generally wonât fit on your service entrance wires), and consequently your burden resistor might be wrong for two reasons. Have you checked âLearnâ and the article about using the emonTx in N.America? Pretty much all of that is applicable to the Arduino too, but as well as changing the value of the burden resistor because of the different c.t.'s different output current, you might want to increase it also to suit the higher 5 V input of your Arduino. (The aim is to get 1.6 V rms developed across the burden resistor at the maximum current you expect.)
The sums are dead simple: Take the rated c.t secondary current, scale it proportionately in the ratio of your max. current to the c.tâs rated current, then calculate the resistor to give 1.6 V at that current, and choose the nearest available value below.
The rest of the input circuit wonât change.
If youâre looking at monitoring the main incoming supply, then just because you have a (say) 200 A service, donât assume you will take that. Go round and add up all the appliances you can reasonably expect to have on at the same time, then add (say) 20% for safety. Youâll find it will still be significantly less. See here for the details.
OK, I am not going to be monitoring my mains, but just one 15A circuit. My application that I will be using the CT for is very specific (one circuit) and every time I do a web search on how to use this type of CT, pretty much everything comes back to the Open Energy Monitor web site. Thatâs why I asked my question here.
My specific application is for a load sensor for a couple of my shop tools. One in particular has an induction motor that pulls a lot of current when it starts. I intend on crafting a controller that will sense when the motor starts, wait for the current to level out from itâs peak, then turn on a shop vac.
I am use to working with mains, so I already have a healthy respect for intermingling mains and low voltage. I will be using an IoT mechanical relay (not a solid state one) for switching on the shop vac.
The demo sketch that uses emonLib as it stands wonât be a great deal of help for you there, because it pre-supposes that you will measure a short sample of mains every few seconds. You donât actually need the rms average value, or to remove the bias offset even (which both happen inside emonLib). If you simply detect the peak currents positive and negative, and take the difference as the peak-peak value, when that falls to a steady value you can start the vacuum motor.
You might find you need quite heavy suppression on that to prevent any spikes getting into the Arduino.
âYour peak âinrushâ current is quite likely to be approaching the 100 A rating of the SCT-013-000â!! 100A inrush on a 15A circuit? That sounds scary.
âYou might find you need quite heavy suppression on that to prevent any spikes getting into the Arduino.â Can you elaborate a bit on this? What would that look like. The relay module is similar to this one (2-Channel 5V Relay Module | SainSmart â SainSmart.com).
Iâm a programmer by profession, so that part I can figure out (usually), but itâs the hardware I need some handholding when itâs something I have never done before.
I didnât say exactly 100 A inrush, but âapproachingâ. Without knowing all the details of the motor, itâs hard to say, but depending on where you catch it on the mains cycle, and the size of the motor, it could be 5 or 6 times normal full load current. And if thatâs 10 A, itâs half way to 100 A.
You might be able to find a value from the motor manufacturerâs catalogue.
Maybe time for a rethink. Take a look at the data sheet for the relay: http://old.ghielectronics.com/downloads/man/20084141716341001RelayX1.pdf
The contacts are rated at 3 A for inductive loads. You havenât mentioned the motor youâll be switching, but if itâs an extractor and another induction motor, and itâs rated at 3 A or more (~ 250 W ), you could be in trouble.
The inductive motor is in my old ShopSmith Mark V 510. Without removing the cowling (non-trivial task), I canât see the label on my specific motor, but a little internet search says it should be rated at 1-1/8 HP and 13.5A @ 115V. The other load I would be using this with (not at the same time!) is a DeWalt DWP611 compact router. It is rated at 1.25 HP peak and 7A @ 120V.
As far as the motor to be switched (thanks for checking on that), no, the shop vac is just a plain-old ShopVac brand vacuum (not a dust collector). It doesnât have an inductive motor on it. That said, your research did give me pause to consider getting a relay with more than 10A capacity on it though.
Itâs almost certainly got a universal motor. You should still check the rating and if possible measure the current, because I think it might be more than you suspect, especially at 120 V. Unless the load is a pure resistance, then you need to look at the contact rating with an inductive load, and choose the relay based largely on that. (The reason is, energy is stored in the magnetic field inside the iron of the motor, and when you break the current, itâs got nowhere to go, so it draws out an arc as the contacts separate, to try to keep the current circulating. And thatâs why the contacts burn up and eventually fail.)
If I understand Songleâs part numbers correctly, this one is the âForm Aâ (only normally open), which has a resistive load of 30A. As opposed to some models in the SLA line that are âForm Câ (both NO & NC components) and only have a resistive load of 10A on the NC side and 20A on the NO side (which is the side I would use anyway). Iâm not sure how they rate the relay at 30A in that case⊠http://www.songle.com/en/pdf/2008414165561000.pdf
Now, back to your other statement in regards to using a mechanical relay:
Any suggestions on what I would need to do for this? Or would that already be handled in the mini-boardâs packaging for the Arduino interface?
But that 30 A rating is still for a resistive load, whereas the âGeneral Motorâ (whatever that means?) rating is 1 HP. Itâs a bitter choice than the first one you mentioned, but I wouldnât guarantee that youâll get a decent life out of it even so.
As for suppression, itâs something of a black art, and youâll need to wait and see what happens. If you get resets or your sketch gets scrambled or there are odd effects that you canât explain, thatâs the most likely cause. The best way is to treat the problem at source. Keep the relay and the switched power wiring as far away as possible from the processor and its inputs, and keep the line and neutral as close to each other as possible (donât have big loops). My starting point would be to add this: http://uk.farnell.com/ampohm-wound-products/fe-sp-hdr23-47-100/contact-suppressor-0-047uf-100r/dp/1438454 in parallel with the relay contacts.
Well! This has truly been a learning experience! My primary intent was to ask about the CT Sensor and I am learning all about mechanical relays.
I originally said mechanical relay and not solid state because I didnât think that solid state relays would handle the amps, but it appears that some will handle larger amperageâs. This company has one that, on the small side, is rated at 25A (well OK, they have one at 10A, but I canât use that one), then another at 40A. http://www.fotek.com.hk/solid/SSR-1.htm
Maybe this will be a better route. Just requires a minimum of 3V to trigger (up to 32V), so an Arduino should be able to trigger it just fine.
I donât think âmaybeâ - more like âMaybe this will be a better route.â. The zero crossing bit means that itâs going to generate much less interference both at switch-on and switch-off. I wonât guarantee it, but youâre unlikely to run into problems with this. It may even be cheaper in the long run when you factor in messing-about time and effort.