A + B Power failover, will this work? Will it kill me? - Page 1
Forum section: electronics / beginners

author: pbgben
author_rank: Contributor
timestamp: October 01, 2015, 04:39:57 am
content: I was looking into making my own auto transfer switch for low power equipment, and have drafted up an idea of how I could create such a thing (Yes, I know you can buy them)Will this work, or will it kill me?Thanks.

author: John_ITIC
author_rank: Frequent Contributor
timestamp: October 01, 2015, 06:06:32 am
content: Unfortunately, nobody can possibly know what each wire does or what it is connected to. You would need to post a proper schematics of the PCB terminals and their connections...

author: Ian.M
author_rank: Super Contributor
timestamp: October 01, 2015, 06:23:25 am
content: It can kill you! Consider the possibility of a fault on one relay, either a wekded contact holding it closed or a coil fault holding it open. If the other relay switches correctly, a circuit exists via the bulb from the live pin of one plug to the neutral pin of the other. If you then pull out the wrong plug, you've now got mains live via the low resistance of a cold bulb filament on the exposed neutral pin of the plug in your hand.You'd need to use a single DPDT contactor rather than a relay and it would need to be rated for use in safety isolation applications, i.e. have enough contact clearance and a design that prevents operation of the other pole if one pole's contacts are welded.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 01, 2015, 06:29:06 am
content: It isn't to hard to see what he is trying to do. Even though he is probably not allowed to do it. (Australian plugs).Brown wires are active, Blue wires are neutral.But you're missing the Earth wire which should run in 3 core cable to every point in the circuit, especially the light. And even if it is not connected there. Someone may change the fitting later on.But instead of using 2 x SPDT you should use 1x DPDT.Because if the neutral relay fails you could have a live neutral wire.Otherwise it will only kill you if you dont wire it properly according to local standards.If safety is important to you then make sure you have RCD protection somewhere in the mains supply to this circuit.<edit> Beat me to it.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 01, 2015, 06:43:57 am
content: Yes, you can build this.No, you can't build this with the parts shown.The safest method is using a DPDT (Dual pole Dual Throw) relay. And not the relays shown in the picture, those are signal/indicator relays and they will burn.For example, use one of these:http://www.farnell.com/datasheets/1430603.pdfBut then buy the appropriate coil voltage and don't forget the mount.The url was very dutch, I changed that

author: djococaud
author_rank: Regular Contributor
timestamp: October 01, 2015, 09:39:35 am
content: You can use a mains powerred DPST relay like this :Not cheap but cool because rail mounted :http://fr.farnell.com/dold-soehne/ik-8701-12-ac-50hz-230v/relais-dpdt-230vac-16a/dp/9514856

author: Tomorokoshi
author_rank: Super Contributor
timestamp: October 01, 2015, 04:41:01 pm
content: It looks like your goal is redundant power.You could go to another level and instead of the relay switchers, replace those with battery chargers. Each would be able to charge a single battery in the system. Suppose it's a 12V AGM battery. From there connect an inverter, so you've basically synthesized a dual-input UPS.The chargers would have to be rated to supply full load while running with power available, as with the inverter.As long as the two chargers are happy working together it might work. It isn't very efficient but it would help with the safety problem.