Hello. I'm not formally trained in electronics, but I have done some simple projects like hook up a pid controlled system to a kiln, wire a vfd and controls to a lathe, wire battery powered leds. The last time I wired some leds in a trailer I used a dewalt battery for power. There was a switch on the socket you plug the battery into. 20 volts came from the battery to a buck converter when the switch was on. From the converter to the leds it was stepped down to 12 volts. I want to do another trailer in a similar fashion, but I want to put switches against each door that will turn on the lights when the door swings away from the switch. I want the lights to come on when either door is open, or when both doors are open. Even more amazing if you could also switch it on from a simple on/off switch. I'm thinking I can figure out what switches, lights, and buck converter to buy, but I don't know what else I need to make this work, or how to wire it. If this topic is covered, my apologies, I did try find it.

 

With the proper switches I don't think you will need anything else.
All you would need to do is install both door switches and the on off switch in parallel with each other. One side of the switches connects to the + output of the 20 volt battery the other side to the positive input of the buck converter.
The LEDS connect to the + output of the converter.
Negative side of LEDs to negative output of converter
Negative side of the battery to the negative input of the converter.

 

I can follow that. Is it ok if more than one of those switches is ON at the same time?

 

Yes it's OK.

 

Could that be done on the lower voltage side? ie the converter output goes to one side of the switches, and the other side of the switches goes to the leds.

 

Oh, and thank you for your help!

 

It could but then the battery is always being drained unnecessary from the standby current of the converter.

 

Ahh, ok then. I just need to hunt down the switches for the doors and I will be ready. I'm finding mostly stuff rated at .6 amps at 125 volt dc. Would those be up to the task when wired in before the buck converter? It looks like each of the four led's will draw 1.67 amps and I don't want to have my switches undersized. Any thoughts?

 

The switches need to rated above the total current draw of the LEDS. I suggest twice the total current demand.

 

Would a switch rated at .6 amps at 125vdc be sufficient for 15amps at 12vdc? How do I figure that? It seems like what I'm finding is not saying anything about amp ratings at 12 or 20volts.

 

The current rating of a switch is only slightly affected by the voltage so don't used the switch for above its 0.6A rating.

Likely would be better to use the switches to control a 12V, 20A relay (example).
That allows the use of those switches, and you can use small wire to go to the switches since it only has to carry the low relay coil current of a few tenths of an amp.

 

If I understand correctly, I would then need to somehow step my voltage down for the switches, then use the relay to give the buck converter full voltage. I think I would rather the more simple circuit if I can find more robust switches. Any suggestions as to a source of switches rated at 10 or 15 amps?

 

Any suggestions as to a source of switches rated at 10 or 15 amps?

10A would be a high current for LEDs. Are you sure you need that high of a current rating?

 

If I understand correctly, I would then need to somehow step my voltage down for the switches, then use the relay to give the buck converter full voltage.

No.
The relay and switches still operate from the 12Vdc but the switches just handle the low current needed to operate the relay coil.
The relay's contacts then switch the high-current 12V to the converter.

 

10A would be a high current for LEDs. Are you sure you need that high of a current rating?

He's switching the converter voltage, not the LED voltage.

 

sghioto
Joined Dec 31, 2017 5,464
Yesterday at 9:47 PM

• #2

With the proper switches I don't think you will need anything else.
All you would need to do is install both door switches and the on off switch in parallel with each other. One side of the switches connects to the + output of the 20 volt battery the other side to the positive input of the buck converter.
The LEDS connect to the + output of the converter.
Negative side of LEDs to negative output of converter
Negative side of the battery to the negative input of the converter.

 

The battery is 20v. the buck converter steps it down to 12v. There are four, 20 watt leds.

 

If the switches are in between the battery and the buck converter, do they draw the full load of the led's? I feel like I was so close and now I'm confused, lol.

 

My understanding was that the switches would experience the full load when the lights are switched on. They are in between the 20v and the converter that is drawing power for these lights. At 20v, 20 watts would be 1 amp each for a total of 4 amps. If I used switches rated twice that size, they would need to be rated for 8 amps at 20 volts. That is why I was hunting 10 amp rated switches. I just rounded up a bit.

 

If the switches are in between the battery and the buck converter, do they draw the full load of the led's? I feel like I was so close and now I'm confused, lol.

Use power calculation and assume equal amount of power consumed.
LED power = LED volts x LED current
Battery power = Battery volts x Battery current

Then take the battery current and multiply it by 1.2 to take into account 20% loss in power.