Need help with electrical circuit knowledge

[vivioo7]

Hello, as the title says I have a few gaps in my understanding. I'll list my questions as best I can but if anyone knows of a good place to read about all of these I'd be grateful for that too.

I'm trying to make my own LED lighting so most of the questions stem from that.

1. When constructing a grounded circuit will the voltage drop across the resistor(s) be equal to that of the power supply? My circuits will be only contain one LED, the forward voltage drop is 3.5 V, and the power supply is 12 V. Does this mean there will be too much energy going through the LED? I assume the way to fix it is a resistor in series but does it matter the order?

2. I'm using the tc420 controller and my DMM won't give me a reading for V or mA, I'm not sure why. Does anyone know what is adjusted by the controller? When entering a schedule for the controller, you have the option for 0-100% for 5 circuits. Does this alter the voltage or amperage? Does 100% mean 1/5 of the total power available or does increasing one ouput decrease the others?

3. Probably could have found this one but I just thought of it while writing. What are the limits of LEDs? Just generally how far above and below can you go on voltage and amperage?

Thanks for any help

 

[rwb500]

led's are designed to be run by supplying a given current. the voltage drop is then dependent on the current they are run at. that is why it is best to run LED's on a constant current driver, which supplies a constant current, regardless of voltage (within limitations). You can't supply a given voltage and amperage. You can pick one, and what you get for the other is dependent on the characteristics of the LED. As I said, you are supposed to pick the amperage, and let the LED settle into its Vf.

I don't know what a tc420 is, and I don't know why your multimeter isnt working, sorry.

 

[oreo57]

Hello, as the title says I have a few gaps in my understanding. I'll list my questions as best I can but if anyone knows of a good place to read about all of these I'd be grateful for that too.

I'm trying to make my own LED lighting so most of the questions stem from that.

1. When constructing a grounded circuit will the voltage drop across the resistor(s) be equal to that of the power supply? My circuits will be only contain one LED, the forward voltage drop is 3.5 V, and the power supply is 12 V. Does this mean there will be too much energy going through the LED? I assume the way to fix it is a resistor in series but does it matter the order?

2. I'm using the tc420 controller and my DMM won't give me a reading for V or mA, I'm not sure why. Does anyone know what is adjusted by the controller? When entering a schedule for the controller, you have the option for 0-100% for 5 circuits. Does this alter the voltage or amperage? Does 100% mean 1/5 of the total power available or does increasing one ouput decrease the others?

3. Probably could have found this one but I just thought of it while writing. What are the limits of LEDs? Just generally how far above and below can you go on voltage and amperage?

Thanks for any help

http://www.dx.com/p/tc420-1-4-led-p...gclid=CO3T973p5sQCFQmTaQodbpUAGA#.VSUufLGmXnc
TC 420 pulses the voltage.. It is for running constant voltage "strings"..
supply voltage = output voltage.
Only thing it does is chop the voltage timewise.. At dimming at 50% you get full voltage at 50% of the time..

LED's have a VERY NARROW window of voltage...
too little they don't go on, too much they overheat and burn out..
The difference between the 2 is usually < 1V..
ALL approximated for reference.
A 18Ohm 10w resistor would be needed in series to drop the 12V to 3.6 Current to the LED would be 500mA
39 Ohm 5W to run at 250mA

 

[sleepydoc]

To expand on what RWB said -
LEDs are non-linear devices, unlike resistors. With a resistor you can use the equation V=IR, so the current is directly proportional to the voltage and vice versa. With an LED, the current is related to the voltage, but not in a linear or consistent fashion. The relationship changes both with the voltage applied as well as with the temperature of the device. As they warm up, they tend to conduct more current and can rapidly burn out. (there are some specifically designed for a constant voltage supply, but they are typically not the LEDs we use)

This is why most LEDs need to have a constant current supply; unlike a 'normal' supply that we think of supplying xx volts, these supply yy milliamps and will automatically adjust the output voltage to achieve the desired current.

Like all devices, constant current supplies have limits. Your supply should give a minimum and maximum voltage that it will supply to give the rated current. Outside of these limits, all bets are off and you don't know what it will do. You need to look at the specs of your power supply and your LED to see if they are compatible. If the power supply says it runs at a minimum of 10 volts, and desired operating point of the LEDs is 3 volts at 1000 mA, then you need to have at least 4 LEDs in the circuit so the voltage is over the 10 volt minimum.