i'm now using two 9V batteries and using a 7812 for +12V from +18V in the circuit. This +12V is also used in 7806 for +6V supply. Previously I used an adapter for this purpose. How do I resolve this dimming of LED??
i'm now using two 9V batteries and using a 7812 for +12V from +18V in the circuit. This +12V is also used in 7806 for +6V supply. Previously I used an adapter for this purpose. How do I resolve this dimming of LED??
Little 9V batteries have a low output current and don';t last long.
When you power 2V LEDs from an 18V battery then 89% of the power from the battery is wasted.
You can connect 6 LEDs in series and in series with a 240 ohm resistor for a current in the LEDs of 25mA when the batteries are new and a very low current when the batteries are each 6V.
hi,
If the LEDs are say 2V and the battery is a 9V PP3, allow a voltage from the battery to be 8V say.
Connect three, 2V LED's in series = 6V, leaving 2Volts to be dropped across the resistor.
If the current is 20mA, the Resistor= 2V/0.02 =100R
[Most efficient]
If you connected only two, 2 Volt LED's in series then the resistor would hav e to drop 4Volts, 4V/0.02= 200R [inefficient]
Six 2V red LEDs in series need a voltage at least 12V. If they are 3.5V white or blue LEDs then they need a voltage at least 21V.
A 9V beattery drops to 6V when it is considered to be dead so six LEDs in series with a 240 ohm resistor gives a current of 25mA when the batteries are new and almost no current when the batteries are dead. The batteries last much longer with 25mA than if the LEDs were in parallel like you had them before but each has a current of 25ma because then their total current is 150mA.
When the battery is new and the resistor is 120 ohms then the current is 33mA which is higher than the max allowed current for many LEDs (30mA max).
Only one LED was measured. Maybe the others have a forward voltage of only 1.6V and maybe the 120 ohm resistor is 5% low at 114 ohms. Then the current will be 37mA.
Use 150 ohm resistors.
It's awesome! LEDs are very bright!!! I've used 100ohms as resistance to connect the 3 LEDs in series. I have a dumb question to ask: Can I connect the LEDs to +12V instead of +9V? Will it burn out the LEDs?
To audioguru: I have 100 ohms and after that I have 220 ohm resistors. Don't have any 150 ohm.