How Much Voltage and Current Do I Need for 8 LEDs in Series and Parallel?

[ChrisToffer]

If i have a LED which drops 3V, and 30mA through each LED and i need 8 LED in series.. does it mean that i need a total of 24V and 30mA from power supply or less?
And if i use the same circuit in parallel, does it mean i will use 3V but much more current like 0.24A?? or least current?

 

[psparky]

For your series circuit, I believe you would need at least 24 volts at 30 ma.

That's 720 mwatts from your source.

For parallel, 3 volts will work with 240 ma coming from source, yet each branch will get it's 30ma.

That's also 720 mwatts from your source.

 

[mfb]

If i have a LED which drops 3V, and 30mA through each LED and i need 8 LED in series.. does it mean that i need a total of 24V and 30mA from power supply or less?

Why less? You'll need 24V and 30mA. It is safer to use a voltage a bit above 24V together with a suitable resistor in series.

And if i use the same circuit in parallel, does it mean i will use 3V but much more current like 0.24A?? or least current?

8*30mA of course. The same advice: use a resistor in series.

 

[Baluncore]

LED current is highly sensitive to applied voltage and temperature.
Each parallel path will require a resistor to stabilise the current in that path.

Consider a 12.00 volt DC supply for 8 LEDs.
4 LEDs in series would give no control of current at varying temperatures.

3 LEDs and a 100 ohm resistor in each series chain would stabilise the current at 30 mA. One of the three chains will need an extra 100 ohm resistor or a hidden LED to make 8 visible LEDs total.

2 LEDs with 200 ohms in series will be less efficient than 3 LEDs with 100 ohms.