12 volt relay help

I need some advice from the electrical guys. We have some landscaping lights around the front and side of our house. We have two transformers to run them, 110 in and 12V output. One I have set to turn off 3 hours after they come on at dusk. The other one I leave on all night in order to keep a light on our flag. This means about 10 lights are on all night then when I would like to only have the flag on all night. So with my limited electrical knowledge I came up with an idea I need some help.

I am thinking that I will get a 12volt relay and wire it up so that when the one transformer shuts off after 3 hours it will trip the relay and shut off all the other transformer's lights except for the flag light. So the one transformer will be shut down, the other transformer will still be on but only one light on, and the relay will shut off the other lights. Think I would need a normally open relay to make this work.

Here is a rough sketch to help explain what I want to do. The transformer on the left will power it's landscaping lights and also be the trigger for the relay to turn off the other lights. The transformer on the right will power the flag from dusk to dawn and also go to the relay that will run that set of landscaping lights from dusk until transformer #1 shuts off 3 hours after dusk and opens the relay. Hopefully this makes a little sense!

Something like this.

Your power packs are likely AC so make sure your relay is capable of running on AC and that the contacts can handle the load of your other lights.

Sounds like a very simple circuit. Just be careful to get a relay rated to throw on 12VAC instead of 12VDC.

Makes total sense.......

If the output from the transformer is DC and I believe it is. You will need a DC relay that will accommodate the load and of all the lights and have two sets of contacts NO & NC.

A relay with two sets of contacts are marked: NO/NC Normally open and Normally closed.
1 you can hook up the coil of the relay to trans #1 output DC
2 wire the flag light to the normally closed contact set NC
3 wire the yard lights to the normally open contact set NO
4 wire the DC power output (lights) from trans#2 to the center contact located between the NC & NO contact set. When the relay energizes and de energizes it supplies power to each contact set.

The flag light will be off at the same time as the rest of the lights are on. With trans # 1 on relay energized, NC contact is now open and the flag light is off, when relay coil is energized. To correct this, you will need to place a jumper with a diode between the NO & NC contact sets.

Here is why, the diode will keep the power of the flag light that is on after trans # 1 was shut off. From feeding back to the other yard lights you wish to have off. A diode is the same a check valve, flows in one direction but not in the other.

So back to the relay
1 Wire for the yard lights, on NO contact
2 Wire for flag light, on NC contact
3 center contact/wiper, wire the DC power output (light power) from trans #2
4 Jumper the NO & NC contact with a diode


Reason, when trans #1 in on and the relay energized the power will feed all the lights and the flag through the diode. Then when the power is dropped at trans # 1 and the relay de energizes, closing the NC contact set. The power is now going to the flag light by itself, and will not power the other yard lights because the diode is preventing that.

To find the direction of the current flow use a meter with ohms set. Probe the diode with the leads, one direction will read infinity, open circuit. Swap the leads and it will read continuity. You will want the flow to be too the flag NC contact set.


FYI if you do not have a meter hang some alligator leads off of each end of the diode. Leave trans # 1 off: jump the NC & NO contact set on the relay, one way the yard lights will on and the other the will be out, you want them out when trans # 1 is off.

Size of diode: look at the flag light see if bulb gives an amp rate, if not 1 amp more then likely will do it.

Cheers
Greg

Pepi, All of the landscape lighting transformers I have seen put out AC. not DC. Light bulbs dont care as long as the voltage is right and its cheaper to run AC since they can leave out the rectifier that makes it DC.

One way to do it would be to grab the smallest (probably cheapest too) landscape lighting transformer and use it to power just the flag light. Set it up with a photocell to come on at dusk and go off at dawn.

Alternatively and probably the easiest way to do it is you can buy a solar powered LED light and aim it at the flag. On at dusk off at dawn and would not require digging up the existing wiring or running any new wires except for one if you want to locate the solar panel away from the light itself. This is how I would do it.

G_P said:

One way to do it would be to grab the smallest (probably cheapest too) landscape lighting transformer and use it to power just the flag light. Set it up with a photocell to come on at dusk and go off at dawn.

Click to expand...

Thought they were DC, AC certainly would nullify the diode function as describe.

Your idea of a cheap power supply with photo cell on off control. That would be the ticket and solve the problem. Another plus with the photo cell, no time adjustments to mess with, let mother nature have its way.


I like it!
Greg

Thanks for all the advice. The transformers are 12V A/C outputs. I personally haven't had good luck with solar lights, they last a season and then seems like they die. Another transformer would work I already have these two and don't really want to add another one if possible.

The relay should work and be fairly simple. The flag light won't need to be hooked up to the relay, it will run right off the transformer with the dusk to dawn sensor. The problem I have now is finding a 12V A/C relay, seems like all the A/C relays I can find are much higher voltages. Will a 120V A/C relay work with 12V A/C or does it need 120V to operate?

signcrafter said:

The relay should work and be fairly simple. The flag light won't need to be hooked up to the relay, it will run right off the transformer with the dusk to dawn sensor. The problem I have now is finding a 12V A/C relay, seems like all the A/C relays I can find are much higher voltages. Will a 120V A/C relay work with 12V A/C or does it need 120V to operate?

Click to expand...

Here:

http://www.automationdirect.com/adc...81_-z-_782_-z-_783_-z-_784_Series)/782-2C-12A

Not sure how long it would last, but should be ok. If you want something nicer go with an Allen-Bradley version.

signcrafter said:

Thanks for all the advice. The transformers are 12V A/C outputs. I personally haven't had good luck with solar lights, they last a season and then seems like they die. Another transformer would work I already have these two and don't really want to add another one if possible.

The relay should work and be fairly simple. The flag light won't need to be hooked up to the relay, it will run right off the transformer with the dusk to dawn sensor. The problem I have now is finding a 12V A/C relay, seems like all the A/C relays I can find are much higher voltages. Will a 120V A/C relay work with 12V A/C or does it need 120V to operate?

Click to expand...

The coil in a relay needs to be supplied with its rated voltage and will NOT operate on lower voltage.

One option would be to use a bridge rectifier (or make one out of 4 diodes) and a capacitor to change the 12VAC to 12VDC, then use a standard automotive relay.

However, a relay with a 12VAC coil is probably still your best best. Here's a link to one from Mouser Electronics:

http://www.mouser.com/ProductDetail...GAEpiMZZMtGt%2bn33CgIP/ucid3HAh0TFR7P1rHtiYI=

Relay is $6.40. Optional socket is $4.35.

Use a photo eye on the flag transfomer 120vac side.