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Electrical Formulae
Useful equations
| Power (P) | = | Current (I) | x | Voltage (V) | P=IV |
| Voltage (V) | = | Current (I) | x | Resistance (R) | V=IR |
| Power (P) | = | Power (kVA) | x | Power factor (pf) |
Power is usually expressed in watts (W), current in amps (A), voltage in volts (V) & resistance in ohms (Ω).
Resistance
Series : RTOTAL = R1 + R2 + R3 + ...
Parallel : 1/RTOTAL = 1/R1 + 1/R2 + 1/R3 + ...
Capacitance
Series : 1/CTOTAL = 1/C1 + 1/C2 + 1/C3 + ...
Parallel : CTOTAL = C1 + C2 + C3 + ...
Voltage Divider
VOUT = VIN x { ROUT / RTOTAL}
Resistor Colour Code Chart
The colour code used for resistor identification is shown in the diagram below.
The resistor is coded with a series of coloured bands, each band having a different value and meaning. Resistors are generally marked with either 4, 5 or 6 bands. A 4-band resistor has two significant figures, whereas a 5 or 6-band resistor has three. The following band in each case is the multiplier. The final band on a 6-band resistor is the temperature coefficient.
| Significant Figures | |||||||||||
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||
| Multiplier | |||||||||||
| 1 | 10 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 10-1 | 10-2 |
| Tolerance (%) | |||||||||||
| 1% | 2% | 0.5% | 0.25% | 0.10% | 0.05% | 5% | 10% | ||||
| Temperature Coefficient (ppm/oC) | |||||||||||
| 100 | 50 | 15 | 25 | 10 | 5 | 1 | |||||
| Black | Brown | Red | Orange | Yellow | Green | Blue | Purple | Grey | White | Gold | Silver |
So, for example, a resistor having bands brown, black, orange, gold will be a 10kohm resistor with a tolerance of 5%.
See also:
- Go back
- Other Electrical reference information
- Main Technical Reference index
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