110 AC DPDT latch relay - Page 1
Forum section: electronics / beginners

author: tubejim101
author_rank: Newbie
timestamp: October 08, 2011, 11:43:42 pm
content: Have a solar panel running to a batter. Have an inverter for the battery.So I would like to tape into one of my house ceiling lights.The idea would be to have a microcontroller check the battery.If their is enough "juice" in the battery, run the light off the battery.If not, then run the light off the city/house power.Can anyone recommend some kind of latching AC relay that I can control with my microcontroller?Hopefully, one that does not cost to much.

author: IanB
author_rank: Super Contributor
timestamp: October 08, 2011, 11:53:42 pm
content: You shouldn't mess with mains wiring unless you are an electrician, or unless you really know what you are doing.Unfortunately, asking this question means you don't really know what you are doing. Best to stay way from the mains. Use a separate lamp, like a table lamp or standard lamp, and power this solely from the inverter. Don't make any connection to the mains wiring.

author: tubejim101
author_rank: Newbie
timestamp: October 09, 2011, 12:21:00 am
content: Quote from: IanB on October 08, 2011, 11:53:42 pmHere in the states we are encouraged to do our own work. We have many warehouse like stores that you can buy everything you need to do the house wiring. They will assist you with any questions and even have classes you can take in the stores. Only thing, is it is "old school" for the most part. Most house electricians don't know what a microcontroller is.

author: IanB
author_rank: Super Contributor
timestamp: October 09, 2011, 01:11:36 am
content: Do you have a Fry's near you? Last time I was in there they had a whole range of solar panels, charge controllers, batteries, mains switching interfaces and everything you need for a home solar power system. All properly designed with the right safety certifications and installation instructions. Take a look there.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2011, 02:22:08 pm
content: Quote from: tubejim101 on October 09, 2011, 12:21:00 amBecause they don't need to know what a microcontroller does, the same as a digital designer doesn't need to know about wiring regulation.Would you be able to work out the fault circuit current of your mains supply or work out the derating factor for PVC cables buried under thermal insulation? Are you aware of the requirements for connecting an inverter to the grid in your country?

author: bilko
author_rank: Frequent Contributor
timestamp: October 09, 2011, 02:26:31 pm
content: Quote from: tubejim101 on October 08, 2011, 11:43:42 pmWhy can't you use the micro to latch the relay ?If you use relay logic you can very easily create a latching circuit, does the relay circuit have to survive a power cycle ?

author: tubejim101
author_rank: Newbie
timestamp: October 09, 2011, 03:17:17 pm
content: Quote from: yachtronics on October 09, 2011, 02:26:31 pmJust planning on running house ceiling lights. So the power cycle should not be an issue.

author: bilko
author_rank: Frequent Contributor
timestamp: October 09, 2011, 03:52:52 pm
content: Some properties have a standby generator in case of mains power failure. The safety requirements would be similar. There are issues in not back feeding the supply if you lose utility power. I would do as another poster suggested earlier, that is to run a totally separate light, standard lamp e.t.c.The problem is if you interface to your domestic lighting and do not have the correct protective measures, you could end up getting in trouble or getting sued.If you use a relay with DPDT contacts the commons would connect to the lamp, the nc contact to utility power and the no contacts to your solar powered inverter. The system would work OK, but in no way would I recommend that you connect the system up like I described. Should the relay fail or the contacts weld together there could be significant problems.To do the job properly and safely requires a lot more thought and as others have said, you need to follow local regulations including fitting safety equipment.

author: Rufus
author_rank: Super Contributor
timestamp: October 09, 2011, 03:56:13 pm
content: Quote from: tubejim101 on October 09, 2011, 03:17:17 pmUnless your mains power is really expensive or your batteries are really cheap the wear on batteries will cost you more than the electricity you save.

author: Jimmy
author_rank: Regular Contributor
timestamp: October 12, 2011, 04:04:50 am
content: You don't need a microcontroller to check batt voltages you can just use a LM3814 in bar mode to check voltage with some magic then drive a relay with that.Assuming that you are allowed to modify mains wiring where you are.With the Relay you would wire the common to the Light the NC to the House live wire and NO to the Inverter Live wire.Then when your circuit gives power to the relay it will change to inverter power. This way if your circuit breaks you will have mains power lights. This would act like an automatic change over switch. If wired correctly there is physically no way you can back feed power into the grid.Or you could just put some 12v led lights in your room and and have a 12v mains psu as backup power that automatically takes over once batteries drop below a set voltage. You can then use the inverter to run other stuff.

author: PetrosA
author_rank: Frequent Contributor
timestamp: October 13, 2011, 02:54:14 am
content: If you're in the US, I can't think of anything in the NEC that would allow you to do this for less than a few hundred dollars - legally, that is. When you power mains fed loads from a transferred battery powered source, it's what's called a Separately Derived System (the same as a permanently mounted generator). If you intend to do this, you better read up on the requirements for SDSs. It will mean installing separate grounding electrodes, panels, transfer switches, etc. Big money if you want to do it and have insurance cover a loss if something goes wrong.

author: bilko
author_rank: Frequent Contributor
timestamp: October 13, 2011, 11:16:56 am
content: Quote from: Jimmy on October 12, 2011, 04:04:50 amHave you ever seen relay contacts weld ?That is why I suggested the method in the previous post and then scored through. I would suggest that the OP speaks to his insurance company because if his house burns down or if a utility worker gets electrocuted then he will be in big trouble.Not wanting to pour water on the idea but there is a right way and a wrong way of doing things.

author: Jimmy
author_rank: Regular Contributor
timestamp: October 13, 2011, 11:58:35 pm
content: QuoteYes I have seen many relay contacts wield together. About 8 years ago I had the pleasure of replacing about 80 relays in a 240v emergency backup system that failed to operate correctly. This did not create a fire only incorrect operation.QuoteThat is why I suggested replacing it with a 12v system. In most places you can burn your house down if you get a permit.QuoteIt a utility worker gets electrocuted form this it is his own fault for failing to isolate all sources of supply and failing to test before you touch. If the contacts on the wield together and the power goes out the inverter will not start supplying power to the grid only your lights. It will only supply back to the grid if there is a short circuit which would quickly blow the fuse in the inverter.

author: Zero999
author_rank: Super Contributor
timestamp: October 14, 2011, 05:27:27 pm
content: Quote from: Jimmy on October 13, 2011, 11:58:35 pmWhy on earth would a 12V system be less of a fire risk?That makes no sense. In reality a 12V system will probably pose a greater fire risk than the mains. Batteries can catch fire and hydrogen gas can cause explosions. You may also need to replace the wring and switches. The current in a 12V system will be 20 times greater than the equivalent 240V system, causing the power dissipation in the cables to increase 400 fold which may cause a fire if the cables aren't suitably rated. Fortunately the cable in most modern installations is suitable overrated but you should check it, just to be safe. You could also increase the voltage to 48V to reduce the current four fold.As far as regulations are concerned, in the UK the fact that it's ELV will not absolve you from abiding by the wiring regulations which also cover wind power, battery and PV installations, regardless of the voltage. You still need to use suitable fuses or circuit breakers and the correct cable. The only really difference is you don't need to use an RCD if it's 120VDC or 50VAC.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 17, 2011, 01:32:11 am
content: Quote from: PetrosA on October 13, 2011, 02:54:14 amIf a plug in solution would work, a UPS nearly does just that. You'll need to somehow hack it to use the mains as a backup.