Double switch for 230V 13A heating elements - Page 1
Forum section: electronics / projects

author: lem0nhead
author_rank: Contributor
timestamp: January 19, 2021, 04:04:33 pm
content: Hi! This is my first post here. I was in doubt if I should post to the beginners or the projects area, because this is a project from a beginner, but I hope I made the right call.My apartment has a water tank with electric heating. It has 2 heating elements, one on the top and one on the bottom of the tank (the idea is that since hot water goes up, the top one can be used for heating a smaller amount of water, while the bottom one for the main overnight heating).I'm not happy with the way the timer works, so I'm trying to design my first high-voltage project.There are 3 block connectors on the top of the board, the one on the left is the input from the mains, the 2 others are the output. All Neutral and Ground are connected together. Live goes through the 2 relays (+ 1 ACS712 current sensor) and then output to the first OR second output (both can never be on at the same time).The low-voltage logic is pretty simple: just some LEDs for status, push buttons for debugging (they won't be exposed in the board), DS18x20 temperature sensors and an ESP32 for publishing some metrics via wifi.Some measures I'm taking:Live comes to a manual-reset thermal cut-off (60°C / 140°C) attached to the board using plastic boltsMinimum 2.5mm clearance for live/neutral/ground3mm track width when 13A is required (this is enough for just 5A with 1oz thickness, but see below)Exposed copper on those high-current tracks, so I can solder a 1.5mm² wire to reinforce those connections + tinning.Ground track connected to one of the mounting holes.Cut-out between high and low-voltage areasEnclosed in a IP65 plastic boxSince this is my first time dealing with a 230VA / 13A project, I'd like to know if someone more experienced can tell if this looks safe.Thanks

author: Gribo
author_rank: Frequent Contributor
timestamp: January 19, 2021, 05:26:02 pm
content: You can't have right angle internal corners, they need a certain radius (the router's drill radius).

author: Ian.M
author_rank: Super Contributor
timestamp: January 19, 2021, 05:59:09 pm
content: Get rid of the cutouts - they make the board ridiculously weak, and thus dangerous. Only use a cutout if you cant otherwise get 8mm creepage distance between tracks/pads on the live and low voltage sides of the design. If you do use one, give the slot rounded ends and no sharp corners so it can be milled with any milling bit less than or equal to the slot width.

author: Benta
author_rank: Super Contributor
timestamp: January 19, 2021, 07:03:09 pm
content: From what I see, there's easily 8 mm between the mains side and the low voltage side. No reason for the cutouts.Also, the tracks on the low voltage side are most anaemic I've ever seen, and for no reason.

author: EEauroro
author_rank: Contributor
timestamp: January 20, 2021, 01:47:26 am
content: Some thoughtsWhen dealing with AC, always recommended to add a fuse in case something smokes up. Example, your AC/.DC converter might have an internal short of L to N. Size the fuse appropriately for your application.Read the datasheet of your AC/DC converter, sometimes, they require external filters on the AC side for proper operation (ie EMC requirements, but in your case, maybe it does not matter ? ). Also, check that you have enough output capacitors on the AC/DC converter outputGet a UL plastic enclosure ( no fire hazard)

author: richard.cs
author_rank: Super Contributor
timestamp: January 20, 2021, 10:11:41 am
content: I quite like using this kind of relay with top spades and no high voltages or currents on the PCB:https://www.ebay.co.uk/itm/5pc-Galanz-Galanz-microwave-oven-JD2-1A-computer-board-relay-12V-16A-/174319304345

author: lem0nhead
author_rank: Contributor
timestamp: June 23, 2021, 01:11:01 am
content: Hey, I'm back after almost 6 months with the project "done".Even though I like the idea, I couldn't find a 13A+ relay with spades connector for 5V input, so I went with a soldered one.I reinforced the tracks with 1.5mm² wires, but the areas marked as "switch1" and "switch2" are overheating (I stopped when it reached 70°C / 160°F).Now, I feel a bit dumb asking this, but perhaps this is because I soldered only the ends of the wires on the terminals instead of stripping the wire and soldering it on top of the copper track?Thanks!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 23, 2021, 06:03:34 pm
content: Look for a trace width calulator eghttps://www.4pcb.com/trace-width-calculator.htmlcheck it against another calculator for sanity checkinghttps://www.digikey.co.uk/en/resources/conversion-calculators/conversion-calculator-pcb-trace-widthOr get some crimp terminals and a Panasonic ALE14B05. Very easy to get micowave oven relay.

author: lem0nhead
author_rank: Contributor
timestamp: June 24, 2021, 12:03:58 am
content: The copper track is definitely thinner than it needs to be to carry 13A (IIRC it would need to be about 30mm wide); but that's why I soldered the wire on top of it.Does it make sense for the track to still overheat?

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2021, 02:35:38 am
content: It would be a good idea to put an RC snubber across each relay contact to make them last a long time.

author: ajb
author_rank: Super Contributor
timestamp: June 24, 2021, 05:43:58 am
content: What relays are you using, and what is their specified contact resistance?Looking at the datasheet for the ALE14B05 part mentioned by Terry Bites, the initial contact resistance is given as 100mR max, and if your relays are similar then they could be dissipating up to 17W at 13A (I2*R). That's a fair bit of power, so would make sense that the tracks attached to the contacts are heating up, since they form a more thermally conductive path than the plastic casing of the relay.

author: james_s
author_rank: Super Contributor
timestamp: June 24, 2021, 06:24:17 am
content: That board is a little scary, I don't think I'd trust it in that kind of heavy duty service in my house. The 240V inputs in particular, those connections are way too small, the pads should be large with a nice beefy solder joint completely surrounding the wire that comes in. The tracks should be wider on both the line voltage and low voltage side, the low voltage traces are ridiculously scrawny. Those slots milled in the board are also much larger than necessary and as someone else noted it results in the board being very weak.Definitely do look at the characteristics of the relays, 13A is a significant load, and your pads that the relays attach to are too small. Don't use little round pads, use large oval pads, oval to increase the gap between pads, but large enough to provide more surface area. Add more copper where you can too, in some places the traces have to narrow down but you have lots of space where you could put copper pours to reduce the resistance. Another technique you can do is use traces without solder mask and coat them with additional solder to make them thicker, or solder bare wires over them, soldered along the whole length rather than just at the ends.

author: fourfathom
author_rank: Super Contributor
timestamp: June 24, 2021, 06:30:49 am
content: Quote from: Circlotron on June 24, 2021, 02:35:38 amDoes this really make a difference when switching a resistive load such as a heating element? Is there enough inductance in a water heater element to generate an inductive kickback disconnect-spike?

author: Circlotron
author_rank: Super Contributor
timestamp: June 24, 2021, 07:48:14 am
content: Quote from: fourfathom on June 24, 2021, 06:30:49 amThere is not much at all in the way of inductance, but if the relay contacts happen to open near the peak of the voltage (and therefore current) there will be something of an arc until the voltage and current pass through zero. Likely many installations don't use a snubber, but it's the difference between running nearly forever without failure and "rough enough is good enough". Some might call it gilding the lily but it's quite simple to do. Just make sure you use an across-the-mains x-rated cap and preferably a wirewound resistor for it's instantaneous surge rating.

author: lem0nhead
author_rank: Contributor
timestamp: July 01, 2021, 03:08:22 am
content: Ok, I did many many tests and I thinkajbis rightThe circuit is reaching around 45°C (113°F) on the yellow area, but around 80°C (176°F) on the red circles:They're all relay pins. I don't know why the relay on the right is handling it better. I could try to replace the relays, but I think the main issue is with the board tracks - I tried just soldering the relay on a prototyping PCB and it barely heats.I guess I need to get back to the drawing board, but I'm wondering if it's better to create a bigger unmasked track to deposit solder on the PCB or just avoid tracks and rely on the wires.I searched more, but I really couldn't find a double switch relay with spade-connectors (and being double switch adds one more layer of protection, since I can't have both on).

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 05:19:13 am
content: To OP: Bravo for the effort but its dangerous.A 230V 5 kVA CONTACTOR is needed, not a PCB mt relay.The relay and PCB are a fire hazard due to arcing, and creep/strike, besides the heat dissipation in the PCB Cu and solder joints.Industrial controls at 5 kVA is screw terminals and crimp splices.If this runs a heater 24/7 and is unattended, you have a FIRE HAZARD.IF a fire were to happen the use of such DIY electrical devices without regulatory or safety compliance will cause you liability.Please don't use this, consult a licensed electrician in you location.Bon Chance,Jon

author: lem0nhead
author_rank: Contributor
timestamp: July 01, 2021, 10:43:57 am
content: Quote from: jonpaul on July 01, 2021, 05:19:13 amThanks for the input.I don't mean to dismiss it and the risk of this design has, but I see there are some commercial products that do something similar using PCB relays; e.g. Shelly (230V 15A) and SONOFF Pow R2 (240V 15A).Do you think the danger comes from doing this on a PCB (and so those products have the same level of risk) or something specific to my design?Thanks!

author: themadhippy
author_rank: Super Contributor
timestamp: July 01, 2021, 11:24:10 am
content: QuoteAnd the difference between a contactor and relay is? and why 5Kva when the circuit dont even reach 3Kva?There are numerous commercial products out there rated for switching 13A using similar relays ,as long as the everything is suitable rated there shouldn't be a problem.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2021, 12:37:03 pm
content: Quote from: themadhippy on July 01, 2021, 11:24:10 amThere's a big difference between a contactor and a relay. The main one is safety. In a contactor, each pole consists of a pair of contacts, which are brdged by a sping-loaded bar, when it closes. A relay has just a single contact. In a single phase application, a contactor will disconnect both the live and neutral, for extra safety. A contactor also has an arc extinguishing device, but a relay doesn't.https://www.google.com/search?client=firefox-b-e&q=difference+between+a+contactor+and+relayQuoteDepends on the requirements for reliability and safety.The heater should obviously be protected by a thermal fuse, to prevent fire, in case the control circuitary fails. In this case, the heater is small enough to enable the thermal fuse to be wired in series with it. A much larger heater would require a contactor, with its coil in series with the thermal fuse.

author: themadhippy
author_rank: Super Contributor
timestamp: July 01, 2021, 01:11:49 pm
content: Quote99% of the time id agree,however there are exceptions,like some ancient allan bradley contactors, there defiantly marked as contactors but operate like the above description of a relay, but they do use some rather nice oil dash pots on the current overloads,Quoteso will a double pole relayQuoteso do some relays and some contactors dontThe difference between the 2 has been argued about for years,and i don think there ever will be a definite definition of the 2

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2021, 01:46:34 pm
content: Quote from: themadhippy on July 01, 2021, 01:11:49 pmAncient technology aside, the difference betwee a relay and contactor is clear: a contactor is designed to dissconnect large loads, safely, without welding. It can have auxiliary contacts for monitoring.There are safety relays, but they're designed to swtich smaller loads, such as contactor's coil.It's importat to know the difference. For example, if you were to replace a contactor on an industrial machine, with a relay, it would severely compromise the safety. If it failed to stop, when the emergency stop button is pressed and someone was injured, you would be liable.

author: jonpaul
author_rank: Super Contributor
timestamp: July 01, 2021, 04:17:29 pm
content: Sorry for confusion....rather than Contactor, I meant an industrial relay.The 13 A PCB mt are not going to last in a such an application.Derating 50% I would use a heavy duty relay with double break contacts rated 260VAC, 25A.Reason....in permanent install on water heater, continuous use.We used thesePOTTER & BRUMFIELD - PRD-11AYO-240 - DPDT 240VAC 25Ahttps://www.digikey.com/en/products/detail/te-connectivity-potter-brumfield-relays/PRD-11AY0-240/365939Jon

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2021, 07:46:33 pm
content: Quote from: jonpaul on July 01, 2021, 04:17:29 pmI don't see why you need such a big relay.15A should be enough, so long as it's not turning on and off too frequently. Breaking both poles helps a lot to reduce arcing.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2021, 02:31:36 am
content: HVAC type contactors are so cheap there's little reason to not use them. They're readily available with 120V or 240V coils, which work great with small SSRs. Or just control the contactors using off the shelf Sonoff smart switches.If you want some more advanced control over it, add a temperature sensor on the cold water line right at the heater in addition to one sensing the tank temperature. Have it sense the cold line getting cold (easy way to determine if hot water is in use) and switch on the top element ahead of time if the tank temperature is below the safety limit.You'll get the most savings building a heat pump for hot water but that's a fairly involved project.