# MiscEl: Leds

 Back to MiscEl main page Write a message to author Calculate/analyses component values for different LED bias circuits. The color bar at the top of the form shows the different wavelengths and their color, the marked areas are the typical diode wavelengths.

### Supported circuits

 R (DC) Only a resistor, used for DC circuits T (DC) Discrete constant current source, used for DC circuits IC (DC) IC constant current source, used for DC circuits R+D (AC) Resistor+Diode, the most simple AC circuits, low efficiency, very high power loss at high voltage R+C+D (AC) Resistor+Capacitor+Diode, a better AC circuit, can be used over a wide range of voltages D+C+R (AC) Diode+Capacitor+Resistor, best for low voltages, rectifies the voltage, this eliminates the mains frequence flashing of the LED DD+CC+R (AC) Dual diode+Dual capactir+Resistor, best for very low voltage, rectifies the voltage with a voltage doubler, this eliminates the mains frequence flashing of the LED R+Bridge (AC) Resistor+Bridge, double efficiency of a single diode R+C+Bridge (AC) Resistor+Capacitor+Bridge, the best AC circuit, can be used over a wide range of voltages with high efficiency

### Input values

 Input voltage Ac or DC voltage used to supply the circuit Freq Frequence of the AC voltage, when field is empty 50Hz is assumed Led voltage drop Volts accross LED while on, depends on LED type, check datasheet for actual LED, 2 volt can usual be used (for green/red/yellow leds) when the voltage drop over the resistor/capacitor is more than 3 volts R Voltage drop resistor, for series capacitor solutions this resistor is used to limit the peek current in the LED C Capacitor, this can be either series or parallel capacitor depending on circuit, be carefull with the voltage rating! LED current Design for specified current, most LEDS can handle 10mA, some LEDS can handle up to 30mA or more Peek current Worst case one time peek current (Vpp/R), usual in the range of 0.1 to 1A, a high peek current gives a good efficiency (Because most voltage is dropped accross the capacitor) LED Ron Dynamic on resistance of LED* Diode Ron Dynamic on resistance of diode* Drop Diode on voltage drop* Bridge Ron Dynamic on resistance of bridge* Drop Bridge voltage drop* Transistor Only visible when circuit T is shown, select transistor used in circuit IC Only visible when circuit IC is shown, select IC used in circuit
* These values are used to make simple components models, when drawing the voltage waveforms, and calculate the current/voltage/power of the components.

### Output values

 Resistor Designed resistor, remember to check the power rating Discharge resistor Optional capacitor discharge resistor. This resistor will discharge the capacitor to 50 volts in 1 second. This resistor is not included in the calculations. Capacitor Designed capacitor, remember to check the voltage. For mains connected circuits also be sure to use capacitors with the correct safety marking. Diode/Bridge A standard diode/bridge, check peek current rating and voltage rating Current (rms) RMS current in component Current (peak) Peak current in component, component must be able to withstand this current, for a short time, without fusing. Voltage (rms) RMS voltage over component Voltage (peak) Peak voltage over component, component must be able to withstand this voltage, for a short time, without damage Power (rms) Power dissipation in component Power (peak) Peak power dissipation in component, component must be able to withstand this power, for a short time, without damage Peek (each cycle) Peek current due to the AC frequence Peek (one time) Maximum peek current under worst case conditions: Capacitor charged to Vp, circuit switched in while supply is -Vp, giving Vpp accross the resistor Peek falloff How fast the one time peek disappears Power input (rms) Power used from supply Power LED (rms) Actuel power delivered to the LED Efficiency A low number means your have designed a heater

### Hints

• Mains connected circuits: Capacitor must be safety marked
• Mains connected circuits: Remember a discharge resistor for capacitor
• AC circuits with capacitor: The resistor must be able to withstand high inrush currents, using of wirewound types is recommended
• Want to use another transistor/IC: Select "Config" page
• Want to use another transistor/IC, do a design: Copy the component values from design to analyze and select transistor/IC
• T (DC), due to tolerances on diodes and transistors large tolerances may occur in the final circuit