Two power sources switched via SPDT switch - Page 1
Forum section: electronics / beginners

author: Doody
author_rank: Newbie
timestamp: December 28, 2020, 10:35:33 pm
content: Hi all, needing some expert advice. Crude drawing attached :-)I have a 200 Amp relay where the coil is energised via a 240V AC to 12V DC transformer.Back story, I have installed a Combi inverter/charger in my van (yes this is in a vehicle), when there is shore power the 240VAC to 12VDC transformer energisers the coil in the relay, which allows the combi charger/inverter to charge the vehicle batteries, if shore power is lost the DC link to the inverter is lost (this is what I want) as I don't want any AC load to failover to the inverter. (e.g. I have a 500W heater running in the winter and in the event of AC power loss I don't want the inverter to take over the load for obvious reasons).But I want to install a toggle switch that allows the possibility of supplying 12V DC to the relay coil via the batteries so that the inverter will work when away from shore power, e.g. when wild camping.The attached drawing would work to switch the positives from both power supplies, but I am not sure if the negative connections are correct. e.g. should I just take a lead from the relay negative to the vehicle chasis and leave the battery negative and 240V AC to 12V DC negative disconnected all together?

author: msat
author_rank: Regular Contributor
timestamp: December 29, 2020, 12:41:51 am
content: What do you mean by " should I just take a lead from the relay negative to the vehicle chasis and leave the battery negative and 240V AC to 12V DC negative disconnected all together?"?Presumably, the battery ground and perhaps even the negative on the 12V side of of the inverter are connected to the chassis, which, as far as a circuit is concerned, is just a wire.As drawn, your circuit would work enabling you to toggle between either your battery or transformer energizing the relay. Just make sure the switch you select is rated to handle the voltage and current required by the relay's coil. Both parts should list this information, generally on the device itself.If I'm understanding you correctly, it seems a bit odd to be using the inverter to provide AC when you're attached to shore power. Of course, it makes sense to use the charging capability when external power is available.

author: Doody
author_rank: Newbie
timestamp: December 29, 2020, 09:39:32 am
content: I was meaning as the batteries negative and the inverter ground are already attached to the chasis then could I just take a negative lead off of the relay to the chasis e.g. not bother taking another negative from the battery and from the transformer to the relay (as shown in the drawing).It is a charger/inverter combi unit, when shore power is attached the unit works in bypass mode and the inverter is in standby mode. I have the unit set to AC priority modeNB. the transformer is connected to the shore power AC and not the ouput AC of the Combi unit, so with no shore power the relay is not energised and therefore the inverter is disabled, having the ability to energise the relay from the battery will allow the inverter to take over the AC load if shore power is lost or not available. I don't want the inverter to take over the AC load when the van is in storage (e.g. a power cut could end up draining the batteries) but I want the ability to use the inverter when I am using the vehicle.

author: Ian.M
author_rank: Super Contributor
timestamp: December 29, 2020, 09:57:06 am
content: Its best to avoid chassis return in automotive wiring as it doesn't take much current to cause accelerated electrolytic corrosion in damp conditions. However there's no reason not to tap the battery feed at the inverter for power and ground for the relay and switch, as long as you put an inline fuseholder in the positive right next to the tap-off point , fused appropriately for the added wiring's current rating. I'd probably use four core cable and take ground to the switch so I could use an illuminated center off rocker switch to allow the inverter/charger to be disabled if the supply was poor quality or very low current rating (i.e. repeated tripping), with its light showing me if the relay was on (i.e. normal with AC supply, or override).

author: msat
author_rank: Regular Contributor
timestamp: December 29, 2020, 11:57:22 pm
content: Galvanic corrosion can happen pretty much anywhere. The fact if billions of vehicles use the chassis for grounding, so I wouldn't be overly concerned. So yes, you don't have to run an additional ground wire.Ian.M does make a good point about adding a fusible link as close to your power source as possible, though.

author: Ian.M
author_rank: Super Contributor
timestamp: December 30, 2020, 01:02:00 am
content: Its often more work to get a good ground that wont corrode than to run a ground wire. Check your vehicle's wiring diagram - if most of the loom has ground wires, you should do likewise. If OTOH as Msat comments, it uses chassis grounding everywhere, and there are convenient point to pick up reliable grounds at the switch, mains powered 12v PSU and relay, there's not much point in running a ground wire, you might as well use chassis grounding though the chassis surface must be cleaned to bright metal and the joint assembled with a light smear of Noalox or similar conductive anti-corrosion electrical jointing compound on the mating surfaces. More is *NOT* better as it contains conductive zinc particles which you don't want on the surface of nearby insulation.If there's any chance of moisture at the chassis ground, after wiping off all excess Noalox, give it a quick wipe with a rag lightly moistened with thinners and over-spray it with zinc 'cold-galvanizing' paint to protect any exposed bare steel, and if there's any chance of road spray, mud etc. when dry overcoat with a generous dab of underbody sealent.