Dual DC voltage to LED schematic help - Page 1
Forum section: electronics / beginners

author: vladio
author_rank: Newbie
timestamp: December 05, 2023, 04:39:43 pm
content: Hello,I'm working on an old AC/DC frig from our boat (old boat). I pulled the unit out of the boat at the end of the season and brought it back to the workshop, it didn't run on 120v AC. I replaced a couple resistors and a mosfet, it runs great again on AC again... DC wasn't an issue - we ran it this summer on 12v with the charger running when on shore power. I'm currently giving the frig a make-over, they're prone to rusting on the frame where the door seal meets. One thing that I'd love to upgrade is adding a couple LED's to the interior. I have them placed and have an idea on getting the line inside the cabinet, I've done a mock-up running the LED's off a variable power supply. I'd like to tap into both the AC and DC to drive the LEDs - AC using the AC power supply output which is 42V DC. I've included a schematic and was hoping to get some feedback. There shouldn't be a time where both the AC and DC are both on to the frig... there's an AC & DC main in the power panel. However, I'd like to plan for someone (myself included) switching both on at the same time. I'd love some feedback on whether this will work. I've also considered running two sets of LEDs, one for when the DC is on and one for the AC. These LEDs would simply remain on when the door is closed.Also, my experience with cool white LEDs from ebay has not been very good. In other projects they tend to get weak or change color over time... is there a better quality LED than ebay?Thanks in advance! I know... I'm no engineer.

author: BeBuLamar
author_rank: Super Contributor
timestamp: December 05, 2023, 11:24:29 pm
content: It would be fine except that you will be wasting a lot of power on the current limit resistors. Since you do not connect both power source at the same time it would be fine for that part.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 05, 2023, 11:29:55 pm
content: It would be more efficient to put 2-3 LEDs in series and run them on the 12V system all the time. Or get a 12V led strip that does that all for you.

author: Konkedout
author_rank: Frequent Contributor
timestamp: December 06, 2023, 06:35:28 am
content: I have 4 LEDs that have lit our "mud room" for a few years now. They are 1W rated white LEDs, rated for 350 mA and I have them running 24/7d at about 5% of rated current.I bought these LEDs from MPJA in USA. I judge that MPJA is better than e-bay; I have no idea what garbage you might receive from e-bay. But MPJA will not be as good as a mainstream distributor such as Mouser or Digikey.So if these LEDs are going to stay ON then be sure to derate the current. Run them much less than full rated. And make sure they are not getting hot. I do not know what are your LEDs or your resistor values. But realize that they are putting some heat into your fridge if running all the time, so it would be better if you do not have "Close the door and the light.......stays ON!!"

author: vladio
author_rank: Newbie
timestamp: December 08, 2023, 03:34:04 pm
content: I think I'm going with a dc to dc converter. would there be a problem with running both positives together if I use a diode on each to keep from back feeding the 12v or the 120v? My concern is if someone accidentally switches both on at the control panel.