Small relay switching 120V - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: April 11, 2020, 02:21:21 am
content: Head-scratcher I am hoping someone here can shed some light on.Take this relay, EC2-3NU.https://content.kemet.com/datasheets/KEM_R7002_EC2_EE2.pdfIt can handle 125VAC.From what little I know, there is a creepage minimum for 125VAC - a few mm?. With the pins so close together, then some PCB pads, and finally some blobs of solder, how is this actually possible? Would a potting compound be mandatory or something?I'm using mine for low voltage applications, but I was surprised to see such a small form factor be able to handle high voltage.

author: amyk
author_rank: Super Contributor
timestamp: April 11, 2020, 02:55:22 am
content: Dielectric strength of air is 3kV/mm.

author: tkamiya
author_rank: Super Contributor
timestamp: April 11, 2020, 04:32:08 am
content: Spec sheet calls for maximum 125V 0.5A resistive load, or 110V 0.3A resistive load. That's the maximum. It also doesn't say if it can switch LIVE. Some relays and switches are rated for connecting and disrupting connections with while voltage is applied. Some are designed to operate DRY where when switching takes place, no voltage should be applied. Use it over the specification, arc will weld contact shut.Also, wise design calls for safety factor, and I usually use relays for less than half its rating. Long story short, unless I'm switching very small current at 120V, I wouldn't use relay designed like this.

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2020, 12:26:18 pm
content: It only has basic insulation between the coil and contacts, which means the control circuit's 0V or +V will need to be connect to mains earth/ground.

author: win22
author_rank: Newbie
timestamp: April 11, 2020, 06:05:41 pm
content: u can use it in ur PCB if 125 VAC, 0.5 A resistive load. And u can to protect pins by capacitor and to increase the max current.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 11, 2020, 06:15:41 pm
content: Quote from: win22 on April 11, 2020, 06:05:41 pmCan you explain that?

author: Gyro
author_rank: Super Contributor
timestamp: April 11, 2020, 06:18:36 pm
content: Quote from: win22 on April 11, 2020, 06:05:41 pmAn RC snubber will help to protect any contacts against arcing with reactive loads but you will never reach or exceed the resistive load rating. Remember that many loads have startup surge currents too.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2020, 08:27:49 pm
content: Can rout the PCB between pins. Not much, but it's something.Would recommend transient limiting if you must use it on AC mains.Tim

author: InsertUsernameHere
author_rank: Contributor
timestamp: April 12, 2020, 12:08:07 pm
content: I too am interested in this, what are the basic design fundamentals of running AC on a PCB that has a lot of other low voltage dc control circuitry?I've seen a lot of boards with slots cut under opto-isolators etc, I presume this is for protection and separation?I need to switch a single phase AC pump motor 240VAC/50Hz with a maximum running current of 5 amps and I'm unsure of the start current.Ive found a few 25A relays that look like they would do the job, but the pins do look very small to handle 25A.Something like this:https://au.mouser.com/datasheet/2/164/ftr-k3wg-23364.pdfis what I am thinking.

author: wizard69
author_rank: Super Contributor
timestamp: April 12, 2020, 05:23:46 pm
content: Quote from: InsertUsernameHere on April 12, 2020, 12:08:07 pmThe minute you start talking about "motors" you should be using relays rated for the motor. More so you most likely be required to implement running overload protection. In some locations the regulations are governed by the NEC (National Electrical Code). So at the very least you need a relay rated for motor duty. That can be an onboard relay but at some point you have to consider external starter relays. In between you can find octal based relays that can do the job on a socketed PCB but you still need to consider running over load. There are lots of other considerations, for example motors can be purchased with built in thermal overloads in this size. This is a bit of controls engineering or the domain of a good electrician.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: April 12, 2020, 05:50:01 pm
content: Thanks everyone for the info!It sounds like with air having a di-electric strength of 3kV/mm, even if a PCB is an order of magnitude less (300V/mm), half a millimeter should be enough celerance for 120V. (A quick google says FR4 has about 800-900V dielectric, but 300V should be used for aging purposes).

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 12, 2020, 06:06:04 pm
content: While it’s off topic, the issue of running a pump came up so here’s my two cents.Single phase motors usually handle their own thermal protection, that is to say it is internal to the motor. Look for nameplate clues like “thermally protected” or on smaller motors “impedance protected”. This is most often an internal thermal circuit breaker (you’ve seen them - push button to reset) or a simple, internal klixon thermal switch buried in the stator/run coil somewhere. For such a motor, a proper HP rated relay will need to be used if the design is being submitted for UL approval. You will not find the contactor on a pc board in those designs. For a retrofit, the bigger the better with respect to the relay and if it’s 240 vac, many times a 24 vac coil HVAC definite purpose HP rated contactor is usually the easiest/cheapest solution. It just comes down to the motor HP.Three phase motors, 99.99% of the time are protected externally with the thermal overload being integral to the motor contactor. When the contactor and overload are physically assembled into one piece, it’s referred to as a motor starter.

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2020, 10:06:40 pm
content: Quote from: joeyjoejoe on April 12, 2020, 05:50:01 pmYou seem to have missed the point. The relay only has basic insulation between the coil and contacts, which is insufficient for protection against electric shock. If it's being used to switch mains, the circuit driving the coil should be treated with the same respect as mains and should have at least basic insulation from the user.To switch mains with a control circuit which can be touched, reinforced or double insulation between the coil and contacts is required.Half a millimetre of creepage, not clearance, is insufficient for separation between the mains and the safety extra low voltage circuit driving the coil. Several times that distance is required to meet the safety requirements.The insulation between the mains and control circuits needs to be able to withstand a much higher voltage than the normal mains voltage. If a mains cable is struck by lightning, the voltage to rise to several kV, which will cause an arc to form between the control circuit and the mains, if the insulation can't withstand the voltage. Even if the voltage spike isn't a high enough energy to cause a severe shock, the conductive path formed by the arc, between the low impedance mains and control circuit will be.Here are some links to more information.https://www.smps.us/pcbtracespacing.htmlhttp://blog.optimumdesign.com/clearance-and-creepage-rules-for-pcb-assemblyIn short:that relay should not be used to switch mains voltages, unless the control circuit is completely insulated from the user.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: April 13, 2020, 09:00:14 pm
content: Thanks Zero999!So in this case, I don't think I need to switch mains. It will be switching 24VAC HVAC equipment. Since the device is smart, in this case I could leave switching mains devices to a dedicated IOT device running tasmota or something so my design only deals with 24VAC

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2020, 01:19:37 pm
content: Quote from: joeyjoejoe on April 13, 2020, 09:00:14 pmThat's good to hear. You had me worried for a bit there.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: April 14, 2020, 01:22:54 pm
content: I do hope to play with mains one day when needed, but the R&D time will be much more for that.In the meantime, I've sent off v1 of the PCB to the fab! I stuck with an opto, and I can play with various resistor values based on the discussions above. I should probably just start a post in the Projects forum, but it's pretty quiet in there...