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on-off-on "light switch"

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Bob Faulkner:
Thank you for all the great info!


--- Quote from: Steve-White on February 25, 2023, 04:43:22 PM ---Pretty simple stuff Bob.  Just be sure it's all fused.  Lots of stories from the racing community on trailer fires with the ignition source being a battery.

With a backup battery that's not connected to the vehicle electrical system a SPDT will work fine.  Just connect the negative to the vehicle ground which will be the frame which is where the negative of the lights will end up being tied to somewhere in the circuit.

Inline fuse on the positive side of the battery is highly advised.

--- End quote ---
Thanks.  Yes, will be adding an inline fuse before calling it "done". 


--- Quote from: Jim Thorn on February 25, 2023, 11:08:30 PM ---Bob,

  The wiring diagrams you linked to may be a little confusing, because they show a commercial electrician how to wire two selectable loads to a single power source, and you're looking for two selectable power sources to a single load.  To an electrician, "L1" means incoming powerline #1, not "Load #1", so the labeling may be confusing, too.
 I've linked a diagram for using that DPDT switch for your situation.  Unless the cost of that switch is prohibitive, I would use DPDT for its sheer conceptual simplicity (and no splice required), but if you go with SPDT as others have mentioned, the link has a diagram for that, too.

Switch sketch

I hope this is helpful.

Jim Thorn


--- End quote ---
Exactly!!  I was looking for two power sources for a single load.  This is why I have found it confusing to understand how something (seemingly) simple could end up a confused mess (confusing for me).  All the info I've been able to find (prior to this original posting), appeared to point to using the switches in a particular manner, which was not how I needed a switch to be used.

Thank you for the explanation and image.


--- Quote from: Steve-White on February 25, 2023, 11:34:50 PM ---Try this one.  There are 3 negatives on one side of the switch and 3 positives on the other side.

--- End quote ---
Thank you for the image.  This is how I envisioned a DPDT switch to work with my setup, but couldn't prove it!

Patrick Tracy:
How about this? Just tie 2 to 4 and 6 to 8 on the output.

https://www.amazon.com/VictorsHome-Changeover-Positions-Terminals-Universal/dp/B07MZ739CS/

Craig Hauber:

--- Quote from: Bob Faulkner on February 26, 2023, 12:40:01 PM ---Thank you for all the great info!
Thanks.  Yes, will be adding an inline fuse before calling it "done". 
Exactly!!  I was looking for two power sources for a single load.  This is why I have found it confusing to understand how something (seemingly) simple could end up a confused mess (confusing for me).  All the info I've been able to find (prior to this original posting), appeared to point to using the switches in a particular manner, which was not how I needed a switch to be used.

Thank you for the explanation and image.
Thank you for the image.  This is how I envisioned a DPDT switch to work with my setup, but couldn't prove it!

--- End quote ---
I wouldn't be looking at 120VAC switches for building electrical.  I've been told they aren't the best for DC and contacts are designed assuming a zero-crossing every 1/60th a second.  I would look instead at automotive and RV type equipment.

I also think you may be overthinking this.  Honestly, how long are your load-outs?  I've run the LED work lights I installed in my trailer for over 12 hours straight on a basic riding mower battery -and still didn't see the end of it because I turned it off before the battery ran out!.  I can imagine I would be able to get an easy 4 hours off just a tiny UPS type gel-cell.  Before testing this I ran trailer lights long enough to think "Oh crap!" before dashing to the cab to start engine -and have never had any issues or even hesitancy in cranking.  (and it's an early-80's 7.3 Diesel so it definitely draws some juice)

Now that being said, I also have a big sine-wave inverter for power tools and possibly running PA gear (haven't had to use it for that yet but it's been used to fire-up and test a rig before main generator plant arrived) either way it runs my air compressor, DeWalt contractor saw or the compound miter just fine.  But it would drain even a big battery in short time.
I use a motorhome battery isolator under the hood to allow charging a second battery from the alternator but no draw-down of the main starter battery when engine isn't running  There's a second deep-cycle RV type "house" battery under the hood that runs the inverter and 12v feed to the trailer (pin-4 on the 7-round).  My interior lighting is just the typical plastic RV ceiling lights but with LED bayonet type (dome light) bulbs instead of the original stock incandescents.  Some of those have extra circuitry in them to not confuse modern vehicle (CANbus) lighting controllers so they draw significantly more current, but I got mine from a friend who is obsessed with off-grid solar and intended for minimal power draw.  there's a switch on the face of each one so I don't have to use all 8 lights if I don't need.  Master switch by back door that turns them all off if you forgot after loadeing and can't reach the front ones.  And they are all just the normal accessory toggle type switches you can buy at NAPA.

Bob Faulkner:

--- Quote from: Patrick Tracy on February 26, 2023, 02:20:04 PM ---How about this? Just tie 2 to 4 and 6 to 8 on the output.

https://www.amazon.com/VictorsHome-Changeover-Positions-Terminals-Universal/dp/B07MZ739CS/

--- End quote ---

This is a great design.  Though, I would be back to tying terminals together.


--- Quote from: Craig Hauber on February 26, 2023, 03:01:01 PM ---I wouldn't be looking at 120VAC switches for building electrical.  I've been told they aren't the best for DC and contacts are designed assuming a zero-crossing every 1/60th a second.  I would look instead at automotive and RV type equipment.

I also think you may be overthinking this.  Honestly, how long are your load-outs?  I've run the LED work lights I installed in my trailer for over 12 hours straight on a basic riding mower battery -and still didn't see the end of it because I turned it off before the battery ran out!.  I can imagine I would be able to get an easy 4 hours off just a tiny UPS type gel-cell.  Before testing this I ran trailer lights long enough to think "Oh crap!" before dashing to the cab to start engine -and have never had any issues or even hesitancy in cranking.  (and it's an early-80's 7.3 Diesel so it definitely draws some juice)

Now that being said, I also have a big sine-wave inverter for power tools and possibly running PA gear (haven't had to use it for that yet but it's been used to fire-up and test a rig before main generator plant arrived) either way it runs my air compressor, DeWalt contractor saw or the compound miter just fine.  But it would drain even a big battery in short time.
I use a motorhome battery isolator under the hood to allow charging a second battery from the alternator but no draw-down of the main starter battery when engine isn't running  There's a second deep-cycle RV type "house" battery under the hood that runs the inverter and 12v feed to the trailer (pin-4 on the 7-round).  My interior lighting is just the typical plastic RV ceiling lights but with LED bayonet type (dome light) bulbs instead of the original stock incandescents.  Some of those have extra circuitry in them to not confuse modern vehicle (CANbus) lighting controllers so they draw significantly more current, but I got mine from a friend who is obsessed with off-grid solar and intended for minimal power draw.  there's a switch on the face of each one so I don't have to use all 8 lights if I don't need.  Master switch by back door that turns them all off if you forgot after loadeing and can't reach the front ones.  And they are all just the normal accessory toggle type switches you can buy at NAPA.

--- End quote ---
I've used AC electrical things with DC for many years, no issues.  My trailer currently has a USA standard AC light switch in for the LED lights.  I installed this about 8 years ago to replace the factory 5-watt incandescent mood-light that came with it; have had no issues.  You are right, I am probably overthinking this whole project (I have a tendency to over-engineer things).  Over the years, I have found household electrical things to be a lot more "heavy duty" than automotive things.

Some of our load-outs can take 3 hours; and I do not want to have to worry about loss of battery power.  In my years of doing this, I've been stuck twice due to running the vehicle battery down too far to start the engine (plenty of power to light a few lights, but not enough to start).  Since then, I do what I can to ensure no issues.

Sounds like you have a decent setup! 

Steve-White:

--- Quote from: Patrick Tracy on February 26, 2023, 02:20:04 PM ---How about this? Just tie 2 to 4 and 6 to 8 on the output.

https://www.amazon.com/VictorsHome-Changeover-Positions-Terminals-Universal/dp/B07MZ739CS/

--- End quote ---

That's the switch style he outta be using.

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