The easiest way is to look it up in the datasheet, or the store where you bought it. Look for LED forward voltage and the Test Current. You could find the LED voltage by using a multimeter with a diode function. You could connect a battery to the LED and a potentiometer.
Typically, the forward voltage of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED. A red LED typically drops around 1.7 to 2.0 volts, but since both voltage drop and light frequency increase with band gap, a blue LED may drop around 3 to 3.3 volts.
LED lights require a certain voltage, such as 24 or 12V. When they run at higher voltages, they become extremely hot. Extreme heat damages the LED lights or the soldering around them.
The LED voltage VL is determined by the colour of the LED. Red LEDs have the lowest voltage, yellow and green are a little higher. Blue and white LEDs have the highest voltages. For most purposes the exact value is not critical and you can use 2V for red, yellow and green, or 4V for blue and white LEDs.
12v LED strips almost always have half the distance between cut points compared to a 24v strip. e.g., If on a 12v strip the cut points are 50mm apart, then the 24v type would have 100mm spacing. This is because the 12v LED strips are cuttable every 3 LEDs, whereas the 24v types are cuttable every 6 LEDs.
Most LED strips will have a mark showing 12V or 24V. Now, check if the voltage on the power supply specifications matches the LED strip. In this case, the power supply is also 12V, so we are good to go.
Determine the voltage and current needed for your LED. We'll use the following formula to determine the resistor value: Resistor = (Battery Voltage – LED voltage) / desired LED current. For a typical white LED that requires 10mA, powered by 12V the values are: (12-3.4)/. 010=860 ohms.
Most common LED's require a forward operating voltage of between approximately 1.2 to 3.6 volts with a forward current rating of about 10 to 30 mA, with 12 to 20 mA being the most common range.
An LED with a product label stating “120V-240V” will accept any one of these three voltages 120, 208 or 240 and will automatically sense the supplied voltage to ensure the same output to the LEDs, regardless of the input voltage.
A 5V LED must not be run on 12V.
Despite some of the most popular power supply voltage options being 12V or 24V DC, it's important to remember that (most) individual LEDs are actually 3V DC devices.
This is called Kirchhoff's Voltage Law. So if you have a 5V power supply and each of your LEDs have a forward voltage drop of 2.4V then you can't power more than two at a time.
Standard LED 3mm diameter. Forward Voltage (V) : 3 Volts for White, Warm White, Bright White, Green and Blue.
Most LEDS get damaged from long exposure to 5V; I (generally) just use a 330 ohm resistor to prevent this. You can check the ratings for the LEDs you're using too, and use a bit of math to figure out what resistor to use to meet these ratings.
Testing LED lights is simple with a digital multimeter, which will give you a clear reading of how strong each light is. The brightness of the LED while you test it will also indicate its quality.
Apply the multimeter contacts to the positive (+) and negative (-) copper pads on the LED strip, and test for the resistance value. If there is no short circuit, the multimeter should indicate infinite resistance. If it indicates any resistance value, that indicates that there is a short circuit.
Using a Digital Voltmeter
A digital voltmeter tests the voltage at your electrical sockets, transformer, and fixtures to find a drop in voltage along your lines. Voltmeters are effective in both low voltage and line voltage outdoor lighting. Typically, voltmeters will come as a device with two probes.
Adjusting the voltage lower or higher will change the brightness, but only because it also changes the current- higher voltage means the current will increase in an LED.
LED polarity
The voltage supply side of the diode is the positive (+) side, this is called the anode. The negative side is called the cathode.
Going the other way - 24V LED strips on a 12V power supply
If you're trying to go the other way and connect a 12V power supply to operate 24V LED strips, unfortunately, you are out of luck. You'll need to purchase a transformer or voltage booster, or, more straightforwardly, a 24V power supply.
The answer is “Yes”. A 12V strip can run on a power supply a little lower than its required electrical voltage, like a 9V alkaline battery. This setup is even entirely safe to operate.
If you have an LED strip with 12-volt DC operation that you wish to use in a 120-volt AC home wiring system, you'll need a power supply that includes 120 volts in its input range and has 12-volt DC output.