www.elperfecto.com I was connecting an LED to a microcontroller and needed to choose the values for the resistor. But first, I needed to calculate the forward voltage (Vf) of the LED. Later, I plan to drive some LED’s with an Arduino and a MSP430 microcontroller. This video shows how to calculate the forward voltage of an LED. From there, you can choose a suitable resistor to limit the current and protect the LED’s from being damaged.
How to use the 4 wire (Kelvin) resistance measurement technique to measure tiny resistances (in the milliohm range) using only a cheap multimeter!
The heart of the LED chaser is the PIC 16F628A microcontroller, IC2. The program that runs on this chip controls the LEDs attached to the output port pins. Resistors R1 thru R8 limit the current through LED1 – LED8 to a safe level that won’t damage the PICs I/O ports or LEDs. Resistor R9 provides a pull-up for the input connected to switch S1. R10 holds the PICs MCLR reset signal high. Capacitor C1 is used to decouple the 5 volt power supply to the PIC. The voltage regulator used is a LM2931-5.0, low-drop-out regulator and will maintain regulation with an input voltage down to 6 volts. Input voltage for the LED chaser should be between 6 volts and 14 volts to ensure power dissipation remains within limits. The LM2931-5.0 regulator is designed for battery powered and automotive applications and includes internal current limiting, thermal shutdown, as well as reverse battery connection without damage to itself or the circuit behind it. Capacitor C3 is important and must be fitted to prevent instability of the regulator output. i got this from: picprojects.biz
A Voltage Divider Discussion
If you are in an introductory level electrical engineering course you have probably heard of a voltage divider. Other people that may be interested in learning about voltage dividers include electricians, computer engineers, communication engineers, software engineers, and the technical crowd in general.
I will discuss this concept here without graphs or equations so bare with me. Consider a box that can contain a single source or any other combination of circuit elements. It will be hooked up to a few resistors that are all lined up in parallel and we need to calculate the voltage drop across each of the resistors. In order to properly understand voltage dividers you should understand the basic concepts behind Kirchoff’s Current Law and Kirchoff’s Voltage Law (KVL). Here we will be applying KVL to the resistors that I just spoke about. The law says that the cumulative drop in potential (voltage) across all of the series resistors will sum to the value coming out of our source (box). The voltage potential will start at the value of the source, and drop a certain percentage as each of the resistors elements is encountered.
These voltage drops are calculated using a voltage divider. Consider two resistors in parallel. In order to calculate the voltage dropped across the 1st resistor we need to multiply our voltage source by the value of that resistor and then divide the that value by the total resistance (add both resistors together because they are in series!). The resulting value is the voltage drop across that first resistor. Consequently, that leaves Vsource – Vresistor1 as the value that is left to drop across the second resistor!
Now you can apply this concept to calculate the differences in voltage potentials for just about any circuit and it is on of the most important tools you can learn.
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