# Wiring a DPDT switch for two LEDs as current output indicator

I need some advice. I’ve got some experience in DIY circuits and stuff, but still learning so I apologize if my terminology isn't correct and for any mistakes in my rough diagram.

Anyways, I’m making another portable metal etching/marking station from an old AC/DC linear power supply. At the basics it’s a 120 V AC input to a step down transformer that puts out 12-15 V DC, but I’ve been able to tap into the AC after it steps down but before it hits the rectifier to get 12 V AC as well and choose which one I want to choose output with a DPDT on/off/on switch since DC will etch metal and AC will mark it.

I’ve attached a wiring diagram that I’m 98% sure is correct from the last time I made this. On the diagram the "X" on the DPDT switch isn't part of the wiring for the project, it was just part of the schematic from the manufacturer to show which terminals are +/-.

I’m using all the same parts for my second one I’m building, except this time I want to add in 3 12 V 1/4W LEDs to signal power on, and then have one LED light up for AC out and another LED for DC out, but I’m not sure where to put the LEDs for the output since the DPDT switch only has one set of output terminals.

Diagram: https://postimg.cc/rK2gxL4c

• I've broken up your wall of text to let some air in and make it a bit more pleasant a read. Use 2 x Enter for paragraph breaks. Welcome to EE.SE. Commented Nov 23, 2023 at 15:28
• you shorted the AC and DC together at the switch Commented Nov 23, 2023 at 15:42

Let's redraw the schematic first.

simulate this circuit – Schematic created using CircuitLab

• The schematic should read from left to right, top to bottom. That means that power should flow from left to right and top to bottom.
• Add a fuse and draw the mains switch to the left of the transformer mains wiring. Now it's easy to see where mains starts and stops.
• We'll use a standard DPDT symbol for the switch.
• You are correct that there is no easy way to match your exact requirements with the DPDT switch but we can give clear visual indication as shown. D1 will light when DC is selected but D2 will not as it will be reverse biased. When AC is selected both will illuminate on alternate half-cycles.

Note that the cross-connections on the DPDT switch shown in your wiring diagram are shorting out the AC and DC supplies. Maybe that's what you're pointing out in the reference to 'X' - which I can't see.

Just out of curiousity, would it make any difference if I wired the LEDs in series vs parallel?

Yes. One would always be reverse biased so neither would light. Have a look at the non-return valve analogy in what I wrote here.

• Thank you so much for this. I’ll definitely give this a go. When you say D2 is reverse biased, does that mean I’d just wire that LED backwards? These LEDs are prewired with a .25w resistor, so would I need to change anything with that? And the crossed black wires on my dpdt switch going to opposite corners is the X I was referring to. I should have omitted that from my drawing, they’re not actually connected, it was just from the manufacturer of the switch to show which terminals were +/-. As for the fuse at the mains, what kind and rating fuse would you recommend? Commented Nov 24, 2023 at 16:33
• Yes, wire D2 backwards. If they're rated for 12 V the LED/resistor combination should be fine. Check the transformer datasheet or rating plate for the fuse rating. Commented Nov 24, 2023 at 18:06
• Thank you. Would I have to move the resistor to the opposite leg of the LED for D2 since I’m wiring it backwards? I don’t have a data sheet for the transformer since I’m using a Westell Model GPU481201000WD00-L power supply and cracking it open and using the internals Commented Nov 24, 2023 at 19:06
• Series connected components can be wired in any order and the circuit won't know or care. The LED's resistor could be in the anode (+) lead or the cathode (-) lead and it won't make any difference. Nominal current for your transformer will be P/V (watts / volts). The next standard size fuse above the calculated current should be adequate. Commented Nov 24, 2023 at 19:12
• Thank you so much for all your help. Just out of curiousity, would it make any difference if I wired the LEDs in series vs parallel? Commented Nov 24, 2023 at 20:10

One simplified option would be to use a single Bipolar LED. The LED actually contains a Red and a Green LED in opposing polarity. With DC voltage across the LED only the Red (or Green) section would light up. With AC voltage across the LED both the Red and Green would light up producing a combined Yellow output. The circuit would be close to this:
.

simulate this circuit – Schematic created using CircuitLab

If desired R1 could be adjusted to change LED brightness.
.

• I appreciate the idea and the diagram. Would you recommend and 2 pin bipolar led or 3 pin? If 3, then would I want common anode or common cathode? Commented Nov 24, 2023 at 17:02
• @JFalcon - Either type will work. I assume the 3 pin type is if you wanted to be able to drive the two colors separately. With a 2 pin (or 3 pin w/2 pins tied together) you would just need to decide which color you wanted to represent the DC condition, and wire that as forward bias to the DC +/-. Do note that with these types of 3 color LEDs it is usually better to find a defused lens as it lets the colors mix better.
– Nedd
Commented Nov 24, 2023 at 22:35
• @JFalcon - For some extra info, if you wire the LEDs separately (not wired directly opposing each other), the reverse 12 V will likely be too high a reverse value, check the LED's reverse voltage spec. If needed you could add an additional standard diode in series with an LED to prevent damage from reverse current.
– Nedd
Commented Nov 24, 2023 at 22:44