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# 13 Commonly Used Basic Schema Formulas Beginners Should Read

2023-04-06【Archive】

**01 Ohm's law calculation**

Calculate relationship between current, voltage, resistance and power in a resistive circuit.

▶ Ohm's law explained:

Ohm's Law explains relationship between voltage, current and resistance, that is, current flowing between two points of a conductor is proportional to potential difference between two points.

The law that establishes relationship between voltage difference between two points, current flowing through these two points, and resistance of this current path. The mathematical expression of this law is: V = IR, where V is voltage difference, I is current in amperes, and R is resistance in ohms. If voltage is known, then greater resistance, less current.

**02 Calculate total resistance of several resistors connected in series or parallel**

**03 Calculate the total capacitance of several capacitors connected in series or parallel**

**04 Resistor voltage divider calculation**

Calculate output voltage of resistor divider circuit for a given combination of resistance and supply voltage.

What is a voltage divider?

A voltage divider is a passive linear circuit that produces an output voltage (Vout) that is a fraction of its input voltage (V1). Voltage dividers are used to adjust signal levels, bias active devices and amplifiers, and measure voltage.

Ohm's Law explains relationship between voltage, current and resistance, that is, current passing through a conductor between two points is proportional to potential difference between two points.

This is a law that describes relationship between voltage difference between two points, current flowing through those two points, and resistance of that current path. The mathematical expression of this law is: V = IR, where V is voltage difference, I is current in amperes, and R is resistance in ohms. If voltage is known, then greater resistance, less current.

**05 Current shunt - resistance calculation**

Calculate current flowing through up to 10 parallel resistors connected to a current source:

**06 ****Reactance calculation**

Calculate reactance or conductance of an inductor or capacitor at a given frequency.

(1) Inductive reactance/conductivity

(2) Capacitive reactance/admittance

**07 ****RC time constant calculation**

Calculate product of resistance and capacitance, also known as RC time constant. This value appears in equations describing charging or discharging of a capacitor through a resistor and represents time required for voltage across capacitor to reach approximately 63% of its final value after a change in voltage is applied to circuit. At same time, calculator will also calculate total energy stored in a capacitor charged to specified voltage.

How to calculate time constant:

The time constant (T) can be determined by value of capacitance (C) and load resistance (R). The energy (E) stored in a capacitor (E) is determined by two inputs: voltage (V) and capacitance (C).

**08 series resistor calculator**

Calculate resistance required to turn on one or more LEDs in series from a voltage source at a given current level. NOTE: When choosing a resistor for this purpose, to avoid overheating resistor, select a resistor with a power rating between 2 and 10 times the power calculated below.

**09 ****dbm to watt conversion**

**10 ****inductance conversion**

**11 ****Capacitor conversion table**

Convert capacitance measurement values between capacitance units of various quantities, including pF, nF, µF, and F.

**12 ****Battery life**

Battery life formula:

**Battery Life = Battery Capacity (mAh) / Load Current (mA)**

Battery runtime is estimated based on nominal capacity of battery and average current drawn by load. Battery capacity is usually measured in ampere hours (Ah) or milliamp hours (mAh), although watt hours (Wh) are sometimes used.

Divide watt-hours by nominal voltage (V) of battery to convert to amp-hours, formula is as follows:

**Ah=Wh/V**

An ampere-hour (also known as an ampere-hour) is a unit of electrical charge, equal to amount of current flowing over a specified period of time. One ampere hour is equal to one ampere of connected current for one hour. mAh or mAh is one thousandth of an amp-hour, so a 1000 mAh battery corresponds to a 1 Ah battery. The above results are approximate and actual results may vary depending on battery condition, age, temperature, discharge rate and other factors. This estimate is closest to actual results using new, high quality batteries that have been used at room temperature and have been used for 1 hour to 1 year.

**13 ****Calculating PCB Line Width**

Use formula provided by IPC-2221 standard to calculate width of a copper conductor or PCB "track" required to carry a given current while keeping temperature rise of track below specified limits. In addition, if trace length is known, total resistance, voltage drop, and power loss due to trace resistance are calculated. Results obtained are estimates and actual results will depend on conditions of use. Also note that traces on inner layers of board should be much wider than traces on outer surface of board, use result that suits your situation.

How to calculate width of a printed line?

First, calculate area:

**Area[mil^2]=(current[amps]/(k*(temperature rise[℃])^b))^(1/c)**

Then calculate width:

**Width[mil]=area[mil^2]/(thickness[oz]*1.378[mil/oz])**

For IPC-2221 inner layer: k=0.024, b=0.44, c=0.725

For outer layer of IPC-2221: k=0.048, b=0.44, c=0.725

where k, b and c are constants obtained by fitting curve to the IPC-2221 curve.

Typical values are thickness (1 oz), ambient temperature (25°C), temperature rise (10°C).

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