Understand Diode, LED and Diode Zener - Basic

Diode

Diode is semiconductor device that allow current flow single direction only (From Anode to Cathode). We can found difference type of diode in market, it design for difference application.

Diode type and Application
Diode - commonly used in rectifier circuit
Schottky diode - switching power supply, switching circuit, as switch in battery charger circuit.
LED (Light Emitting Diode) - indicator
photodiode - sensor
SCR (Silicon Controlled Rectifier) - power control circuit, use as switch
Pin Diode - RF circuit as RF switch
TVS (Transient Voltage Suppression) - circuit protection
Tunnel Diode - microwave circuit
Varicap - use it as voltage control variable capacitor, VCO (Voltage Control Oscillator)
Zener diode - Voltage regulator

Understand diode forward bias and reverse bias

Diode Forward Bias	Diode Reverse Bias

Understand diode rating
This post only focus on normal diode rating, table at below is specification for diode 1N400x series.

Table from datasheet

Important parameter for diode
Breakdown Voltage (Vr, Vrrm, Vrwm) - the diode can withstand maximum voltage in reverse mode. If reverse voltage is higher than rating value, the diode may damage.
RMS Reverse Voltage (Vr(rms)) - the diode maximum DC reverse voltage. (continues reverse voltage)
Forward Voltage (Vf, Vfm) - the voltage drop across diode in forward mode. The voltage can measure by multimeter.
Non repetitive peak forward surge current (Ifsm) - maximum forward pulse current (Surge Current).
Forward Current (If, Ifm) - the diode maximum DC forward current. (continues forward current).
Peak Reverse Current (Irm) -the diode maximum leakage current in reverse mode. when diode in reverse mode, there may have some leakage current from cathode to anode.

Example:-

Base on circuit on top, the transformer step down the voltage to 52V, the 1N4001 will no applicable use in this circuit, because its Vrrm is 50V only. The 1N4002 until 1N4007 suitable for this circuit.
For datasheet, we know that, 1N400x series diode forward current is 1A. Formula at below use to find the lowest R that can support by circuit:-

Voltage divider Formula
Vin = Vfm + (Ifm x R)
52V = 1V + (1A x R)
R = (52V - 1V) / 1A
   = 51 ohm.
The R value cannot lower than 51ohm, if R lower than 51ohm, the diode may damage.

LED (Light Emitting Diode)

The LED characteristics not much difference in electronic circuit. normally, the LED have higher forward voltage compare to diode. The diode forward voltage we can measure using multimeter.

Example:-

Choose resistor for LED.

Base on the circuit on top, the capacitor and series of resistor are connect parallel with LED.
We can ignore the capacitor path and series of resistor path, because parallel connection will have same voltage level.
Just assume, the input voltage is 5V, diode forward voltage = 2.2V Find the suitable R value.

Rule of thumb The LED forward current is around 5mA to 10mA. The forward current is parameter that control LED brightness.

Voltage divider Formula
Vinput = Vled + (If x R)
5V = 2.2V + (5mA x R)
R = (5V - 2.2V) / 5mA
R = 560 ohm

Find R power:-
P = VI
   = I²R
   = (5mA)² x 560
   = 0.014W

can use any resistor more then 0.014W.

Diode Zener

The diode zener package it look like normal diode. It forward voltage is around 0.7V (it close to normal diode), but it have difference reverse voltage value compare to normal diode.
Diode zener reverse voltage is low compare to normal diode.

Example 1

Input voltage =10V
diode zener voltage = 5.1V
Vref current = 10mA (target)
Find the R value, R power, diode zener power
1. The diode zener can regulate the voltage only, the current is control by R.
    Just assume current flow throw diode zener is 1mA and 10mA through Vref path.
    Total 11mA will flow throw R.

     Voltage divider Formula
     Vinput = Vzener + ((Iref + Iz) x R)
    10V = 5.1V + ((1mA + 10mA) x R)
     R = (10V - 5.1V) / (11mA)
         = 445 ohm
         ≈ 430 ohm

2. R power
    Find current across R
     Voltage divider Formula
     Vinput = Vzener + ((Ir) x R) 
    10V = 5.1V + (Ir x 430)
    Ir = (10V - 5.1V) /430 ohm
        = 11.4mA

    P = Ir² x R
       = (11.4mA)² x 430 ohm
       = 0.056W

3. Just assume, Vref open circuit. all current will flow across the diode zener.
    The diode zener power will be:-
    P = VI
       = 5.1V x 11.4mA
       = 0.05814 W (min)

4. The Vref = diode zener votlage = 5.1V

Example 2

Find the Vref voltage. if input voltage is 11V.
1. The Vref = Vinput - Vzener
                   = 11V - 5.1V
                   = 5.9V

2. Power for diode zener
    Itotal = Iz = Ir + Iref
    Pz = Iz x Vz

Understand Transformer - Basic

photo take from wikipedia.

Transformer

Transformer is an electric device used to transfer electric energy from one circuit to another  through inductively coupled conductors. The transformer basic application is step up / step down AC Voltage. Normally, the power transformer is design at operation frequency 50Hz until 60Hz.

3 basic parameter you will found on transformer. There are primary voltage, secondary voltage and VA rating. The primary voltage and secondary voltage is indicate the transformer is step up / step down transformer. 

We may found some different size transformer but have same voltage rating. what difference between those transformer? Please take note its VA parameter is difference. What is VA parameter?

The VA is maximum power of transformer. because of  transformer is an inductive device. When AC signal meet with inductance and capacitance may result in periodic reversals of the direction of energy flow.

The power will transform into real power (measure in watt) and reactive power (measure in VAr [Volt-Amp-Reactive]). As what we know, the real power and reactive power may change depend transformer's load type. In inductive circuit, the capacitor may help on improve the real power.

Note The real power is energy will drop on load and reactive power is energy will returns to the source in each cycle (AC).

The VA is total "apparent power" supplied. This is to prevent over loading a transformer. Relationship between VA, Watts and VAr:-

VA² = W² + VAr²

Example:-

The transformer rating as below:-

Primary voltage = 230V

Secondary voltage = 15V

VA rating = 3A/50VA

1. Input voltage at primary is 240Vac, how much output voltage and max current? 

    a. Secondary voltage = (Vin/Vpri) x Vsec

                                     = (240V/230V) x 15V

                                     = 15.65V

    b. Secondary current = VA / Vout

        (max)                    = 50VA / 15.65V

                                     = 3.19A 

                                     ≈ 3A (limit by current rating)

2. How much input current, when maximum output current?

    a. Primary current = VA / Vin

                                = 50VA / 240V

                                ≈ 0.208A

3. If input voltage at secondary voltage is 12Vac, how much output voltage?

    a. Primary voltage = (Vin/Vsec) x Vpri

                                = (12V/15V) x 230V

                                = 184V