Resistors knowledge: To know the resistor requirement.
Resistors work both with ac and dc, but don’t convert ac to dc or vice versa current.
===================== Home Mobile charging adapters, called AC-Dc/Smps power supplies or power adapters. They consist of:
Transformer (Transfers work with ac to ac only): Steps down the AC voltage.
(Bridge) Rectifier: Converts the AC to DC.
Optionally: Buck Converter (is a kind of switching reducer or regulator) to reduce dc voltage only, not current, nor ac. Small and efficient. Many smps power adapters use buck converters internally to step down the dc voltage before regulating the current.
Filter (Capacitor): (further) Smooths out the DC voltage.
(Linerar) Regulator (e.g. LM7805): Provides a stable (emphasis on ‘Stable’ not on ‘Regulated’) DC output voltage.
Most power adapters and laptop adapters are sort of these Smps AC-DC power supplies only. And they’re best for me.
Some power adapters, specifically used to power LEDs, they can handle current, and voltage both, are also called Led Drivers. They maintain a stable current through the LED, regardless of voltage fluctuations. Many LED drivers offer dimming capabilities.
Also, we can’t directly reduce current (i.e. amperes) the same way as we do with voltage. Rather, electric devices only need specific voltage (pressure of water), but they can handle higher amperes (=volume of water) easily. Device would easily use only that much current/amperes (volume of water) as it needs, till it gets AT LEAST that much current as it needs. Also current needs of any electric device keeps vaying as it operates.
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--------Terminology-------
v = voltage supply (= source voltage)
vf = voltage forward (=voltage load = v needed)
vi = current forward (=current load needed)
R = resistor
P= Power dissipated in watts (=When current flows through a resistor, it dissipates power as heat).
------ formula -----------
R= (5V (supply)−2V (load))/0.02A (load)
=150Ω.
A resistor cannot reduce ‘current’ without dropping ‘voltage’ (isn’t it written vice-versa?), e.g: R= 5v-5v/0.5A = 0 ohm
But we can create a ‘Voltage Divider’ by using multiple resistors (in series or parallel?) to reduce volts only:
Volt Out= volt In * (R2/ (R1+R2)
-------- Wattage reqirement/current dissipation
A 10Ω resistor with 0.5A current dissipates:
P= (0.5a)(0.5a) ×10Ω = 2.5W
P=(0.5A) 2 ×10Ω=2.5W
Use a resistor rated for at least 2.5W (or higher for safety).
========= Ex of 220v 5amp reduction for an led of 2v 0.002 Voltage Drop:
220V (source) - 2.5V (LED) = 217.5V
Resistance:
217.5V / 0.02A (20mA) = 10,875 ohms= 10.8Kohm (approximately)
Power Dissipation: 217.5V * 0.02A = 4.35 watts
========== Ex of 5-12v 1-2amps reduction to 2v 20 miliamps:
Assumptions: LED Forward Voltage (Vf): 2V (red LEDs typically range from 1.8V to 2.2V)
LED Forward Current (If): 20mA (0.02A) (common for small LEDs) Calculations:
Voltage Drop:
For 5V supply: 5V - 2V = 3V
For 12V supply: 12V - 2V = 10V
Resistance (R): (Ohm’s Law: R = V / I)
For 5V supply: 3V / 0.02A = 150 ohms
For 12V supply: 10V / 0.02A = 500 ohms
Power Dissipation (P): (P = V * I)
For 5V supply: 3V * 0.02A = 0.06W
For 12V supply: 10V * 0.02A = 0.2W
Resistor Selection:
5V Supply: Use a 150-ohm resistor (or the closest standard value). A 1/4 watt resistor is sufficient.
12V Supply: Use a 500-ohm resistor (or the closest standard value). Again, a 1/4 watt resistor is fine.
---------- Color Code:
Resistors use a color code to indicate their value. For example:
Brown-Black-Red-Gold: 1kΩ ±5%.
------------- Series & Parallel resistor calculations/behaviour:
Total resistance of a Series :
R(sub: total)= R1+R2+…
Total resistance in Parallel:
1/ R(sub: total) = 1/R1 + 1/R2 + …
That is 1+2 ohm in series would be 3ohm in total and 1/3ohm in parallel.