Ohms, can you help me understand why I'm getting incorrect numbers?
So... I've taken apart a simple 3led touch button light. It has a 10ohm resistor (colours Brown, black, black, gold) 3 triple A batteries are used to power it.
What I don't understand is. If I have 3 white LEDs, gonna assume they have a voltage drop of 3.3 each. So power supply 4.5v - (3.3v x 3) / 20ma (assumed current of led) gives me -270ohms. What am I doing wrong?
What I want to do...is take these three LEDs and use a USB power bank output is 5.3v 1000ma and capacity is 2700ma.
I'm trying to work out what resistance I need but obviously doing it wrong.
So... I've taken apart a simple 3led touch button light. It has a 10ohm resistor (colours Brown, black, black, gold) 3 triple A batteries are used to power it.
What I don't understand is. If I have 3 white LEDs, gonna assume they have a voltage drop of 3.3 each. So power supply 4.5v - (3.3v x 3) / 20ma (assumed current of led) gives me -270ohms. What am I doing wrong?
What I want to do...is take these three LEDs and use a USB power bank output is 5.3v 1000ma and capacity is 2700ma.
I'm trying to work out what resistance I need but obviously doing it wrong.