how much voltage and current is required to turn on 10 led's with voltage each (3.8v) and current (20ma)?

Answer 1

Although the spec may say 3.8V forward voltage. The actual value will vary considerably. You *must* use a series resistor with the LEDs. Best if you have a resistor in series with each LED and wire them all up in parallel.

So, say you have a 12V power source, then each resistor will be:
(12-3.8)/0.02 = 410 ohms (next common value is 470 ohms)

You will need 10 resistors, one for each LED

Answer 2

It depends on the wiring configuration. If you put them all in series you will need 38 volts at 20 mA. If you put them all in parallel you will need 3.8 volts at 200 mA.

If you wire the up with 2 in series to get 5 sets, then wire all the sets in paralles you will need 7.6 volts at 40 mA.

If you use only 9 LED's, you can wire 3 in series for 3 sets, then wire the sets in parallel for a voltage of 11.4 volts and 60 mA. Put a 50 ohm 1/2 watt resistor in series with each of the sets of 3 and you can use a common 12 VDC power supply. Of course you could also use 12 LED's and add a forth set.