# Simple LED circuit question

#### 6FiRE9

Joined Oct 5, 2009
10
So I'm really rusty, but I am trying to build a simple circuit and want to verify it would work. I can't seem to find a decent free simulator that I can confirm this on, but I know most of you with a simple look can tell me if I'm off base here.

So I have two 12v LED's for car use rated for 9w (750mA), and I want to connect them to a small 12v battery and use a switch to turn one on or both on. I don't want to complicate this with much circuitry or relays as it's just a simple battery box for camping.

I was thinking a SPDT switch with a diode running between the LED's should work. When the diode biases both LED's would power up, when the switch is in he other position, the diode reverse biases so only one LED would turn on.

Let me know if I'm missing something here, and if you have any recommendations.

Thanks!

#### AlbertHall

Joined Jun 4, 2014
10,558
Yep that will work. The diode must be able to handle 750mA so one from the 1N4000 series would be a good choice.

#### 6FiRE9

Joined Oct 5, 2009
10
Thanks Albert, I thought I was right but wanted to be sure & thanks for the diode recommendation!

#### Delta prime

Joined Nov 15, 2019
576
Hello there
to be on the safe side I put 16 ohm resistor in there.
you have to excuse my drawing I think I'm having a stroke.

#### AlbertHall

Joined Jun 4, 2014
10,558
I believe these LEDs are for direct connection to a vehicle 12V so the resistors are not needed and will severely dim the LEDs.

#### SamR

Joined Mar 19, 2019
2,791
16Ω? Odd number... If dropping resistors were needed, 12V/20mA = 600Ω minimum.

#### AlbertHall

Joined Jun 4, 2014
10,558
16Ω? Odd number... If dropping resistors were needed, 12V/20mA = 600Ω minimum.
Post #1, 12V LEDs, rated at 9W, 750mA.

#### SamR

Joined Mar 19, 2019
2,791
Yup forgot about that. At 750mA x 12V = 9W that is a pretty serious resistor.

#### upand_at_them

Joined May 15, 2010
547
That's a pretty serious LED!

#### Bernard

Joined Aug 7, 2008
5,686
Rated for automotive use, so might have about 3 LEDs and small R around 4 ohms?
How about a SPDT with center off ?

#### 6FiRE9

Joined Oct 5, 2009
10
Yes will definitely have the center off 3 position switch, and they are some fairly serious LED's!

I'm on the fence on the resistor, these are made to use without however probably not in a circuit like this.

My electronics skills are way too rusty these days to be confident on what value to use, and I don't want to dim them much if I can keep from it. If there is a consensus here I would be willing to throw one in however.

#### AlbertHall

Joined Jun 4, 2014
10,558
Yes will definitely have the center off 3 position switch, and they are some fairly serious LED's!

I'm on the fence on the resistor, these are made to use without however probably not in a circuit like this.

My electronics skills are way too rusty these days to be confident on what value to use, and I don't want to dim them much if I can keep from it. If there is a consensus here I would be willing to throw one in however.
If they are intended for automotive use, without specifying a series resistor, then they need no resistor.

#### dl324

Joined Mar 30, 2015
11,897
I'm on the fence on the resistor, these are made to use without however probably not in a circuit like this.
The 12V rating means they operate from the car's power; which will be closer to 13.8V when driving.

#### 6FiRE9

Joined Oct 5, 2009
10
Yes dl324, that's correct. My battery at full charge is between 13.2-13.5V, but calculations were done at rated 12V value given, as I don't have a proper spec sheet. Since my battery will be getting used a fair amount, I would guess normal operating voltage, since no alternator involved would most likely be under 13V anyway.

Thanks again Albert, I am leaning towards no resistor unless someone sees a valid reason for having one.

Thanks for the good feedback everyone!

#### dl324

Joined Mar 30, 2015
11,897
Yes dl324, that's correct. My battery at full charge is between 13.2-13.5V, but calculations were done at rated 12V value given, as I don't have a proper spec sheet. Since my battery will be getting used a fair amount, I would guess normal operating voltage, since no alternator involved would most likely be under 13V anyway.
My point was that the LEDs were designed to operate over a range of voltages and you don't need a current limiting resistor.

At 750mA, a 16 ohm resistor would drop a significant amount of the available voltage and the LED would likely be dim.