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# Circuit Design Questions - Voltage Drop Answered

Hi there, I'm working on a circuit which will include a 3v, .7Amp motor and three 20mA LED chips, with a maximum forward voltage of 3.8v. The whole circuit is intended to run off a 6v battery.In calculating resistance for voltage drop, I've figured out that resistance for the motor is 2.1 Ohms and then (by Ohm's Law Resistance = Voltage / Current) the resistance of each LED chip is 190 Ohms. Is this correct? It seems astronomically high.?

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## 6 Replies

LEDs don't have a DC resistance and don't obey Ohm's Law. That's because they are semiconductor devices.

Say you want 20mA to flow throught an LED who's forward voltage drop is 3.8V (and that seems a bit high), then you need a resistor in series with the diode to drop the remaining 2.2V. i.e. : R = (6V - 3.8V)/20mA , therefore R=110Ohms

Zahv (author)2013-02-07

Thanks for commenting.
So, just to be sure I understand what you're telling me, once the LED has taken up 3.8v to glow, the remaining 2.2v needs to be prevented from reaching the power source as 2.2v? Otherwise the LED will burn out from excess voltage, correct?

iceng (author)2013-02-08

Akellyirl is very correct..

With no current limit the LED will gobble more current while raising it's
voltage only a little and really heating up.

This causes a visible change to the color output and eventually burns
the phosphors that change the blue to red and to green.

First blue should start and then the dark red glow or if there is enough power to a bang-crack dark !

It's worth pushing a fwe LEDs to see how they suffer over power
but not in a  NO SMOKING Zone  ;-)

akellyirl (author)2013-02-08

More or less... Really, the voltages from the source and loads need to balance (Thevenin's Law). So if the source supplies 6V and the LED drops 3.8V the resistor drops the remaining 2.2V . As the resistor obeys ohms law, you can work out the current from its resistance or vice versa.

frollard (author)2013-02-08

+1

the semiconductor once saturated (at 3.8v) acts as a short circuit, allowing almost as much current wants to flow as the circuit will allow. Adding the resistor knocks down the excess voltage and forces a more predictable ohmic current response. Over-Current kills semiconductors.

The Skinnerz (author)2013-02-07

You need to connect the LEDs (with current limiting resistors) in parallel with the motor. LEDs don't have a fixed resistance, that's why you need extra resistors to limit the current through them. You will need a voltage regulator to drop the voltage to the motor down to 3V, unless it will run on 6V.