The power source components are the most significant part of electrical products as they feed power to all the other components. Moreover, they provide the principal separation between non-hazardous parts (the low-voltage secondary) and hazardous voltages (the mains). The following types of power source components exist:
I. Power supplies
Feeding power to an electrical product can happen with one of the following means of connection: power supply cord, simple plug or thermal blocks.
The power supply cord may be equipped with industrial or nonindustrial type plugs depending on their end-application. There are two varieties of power supply cords: detachable and non-detachable. The former represents a flexible cord that connects to the power supply through a suitable appliance coupler. The non-detachable power supply cord is either:
- An ordinary flexible cord that is easily replaceable and doesn’t require the use of any special tools
- A special cord that requires the use of a special tool for replacement or which replacement is impossible without damaging the power supply.
The simple plug is a power source component with no mains on/off switch. It serves as a disconnecting device because of which it is mandatory to ensure non-obstructed access to the mains plug and the associated mains socket/outlet.
Thermal blocks help establish permanently connected power supplies. Service personnel and licensed electricians are usually responsible for installing thermal blocks, and they must follow the local electrical codes and/or regulations when doing so.
II. Fuses as power source components
A fuse is a circuit-protection device that provides protection in the event of circuit overload by limiting the fault currents to low values. The metal wire or strip in the fuse will melt when the electrical current exceeds specific amounts, opening the circuit path and disconnecting the electrical equipment from the power source.
There are two main types of fuses:
- Fast-acting fuses, which will open an overload and short circuits very quickly. This type of fuse often consists of a single element.
- Time-delay fuses, which can withstand a heavy amount of electrical current overload for a limited time.
International regulatory compliance of fuses can be ensured by fulfilling the requirements of standards IEC 60127-1 and IEC 60127-2. In North America, standards UL 248-1 and CSA 22.2 no.248.1 also apply. (Identify applicable EN & UL standards in minutes by using cloud-based product compliance tools.) Each fuse standard specifies non-fusing and fusing time current limits. The latter is the minimum time that a fuse can carry for a specified electrical current without opening a circuit. The fusing time represents the allowable time range (min-max) that a fuse can hold for a specified electrical current before opening a circuit.
Check this list of design hints for using fuses in electrical products.
III. Power entry modules
Power entry modules are a type of power source components that reduce the risk of accidents. Their design makes it necessary to remove the power cord assembly from the unit before making any product changes (e.g. fuse replacement or voltage selection).
Power entry modules are available in a variety of component combinations. The main components of a power entry module are:
- an AC power inlet for Class I grounded connection or Class II (without ground) connection.
- a power on/off switch that keeps the power control near the power source
- a fuse holder with either 1/4 in x 1-1/4 in or 5 x 20 mm fuses
- voltage selector allowing the equipment to operate at a specific VAC
- EMI filter that can prevent conducted noise radiation and suppress conducted interferences on a signal or power line.
A switch is a means of disconnecting all electrical current-carrying conductors. It must be rated as equal to or must exceed the load it controls and must disconnect all ungrounded conductors. The switch should never be able to open the grounding conductor of a power-supply cord or cord set. The ON and OFF positions of the switch should have proper markings. “O” for ON and “I” for OFF.
When adding a switch to a product’s design, the designer should ensure that the switch:
- Is adequately rated for the intended purpose
- Is easily accessible
- Has a clear position ON and OFF
- Is an easy-mounting type for the application
- Has regulatory approval for the intended market.
V. Power Supply Units (PSU)
The PSU is the heart of any electrical product because it affects everything in the product. If it malfunctions, its malfunction may damage other components within the product.
There are two major types of power supply units: regulated and unregulated power supplies. The former is further divided into linearly and switch-mode power supplies. Nowadays, all power supplies are of the switch-mode type due to their small size and high efficiency.
When selecting a PSU, product designers should consider the following characteristics:
- Input and output voltage
- Input and output current
- Compliance with IEC 1000-3-2 for all load conditions (Relevant: IEC standards for product basic safety and EMC testing)
- Hold-up time
- Inrush and touch currents
- Dielectric strength voltage
- Efficiency at full load
- Operating and storage temperature
- Temperature derating.
Any power supply unit incorporated into the design of an electrical product must come as an already certified component. Thus, the following documentation must be available: approval status according to international regulations for electrical products, a declaration of conformity, a declaration regarding the production line tests and technical documentation.
A varistor is a power source component that can absorb higher transient energies and suppress positive and negative transients. When a transient occurs, the varistor resistance changes from a very high standby to a very low conducting value. Consequently, the transient is absorbed and secured to a safe level. In other words, varistors are surge voltage protection devices that limit electrical product hazards generated by transient surge voltages. The latter are voltages that occur for a very short time on signal inputs and low-voltage AC or DC power lines.
The most commonly used varistor is the metal-oxide varistor (MOV) which consists of arrays of zinc-oxide balls. These balls are fused into a ceramic semiconductor which produces a crystalline microstructure allowing the varistor device to dissipate very high levels of transient energy. After fusing, the surface is metallized, and leads are attached via soldering.
When selecting a varistor for a specific overvoltage-protection application, the product designers must consider the following characteristics:
- Circuit conditions
- The leakage current at maximum DC working voltage
- The acceptable let-through voltage for the protected circuit
- Operating temperature of the equipment
- The continuous operating voltage of the varistor
- The number of surges the varistor must endure.
Transformers are electrical devices that transfer energy from one circuit to another without changing the frequency. The most common transformers are of laminate steel or toroidal format and often consist of five parts: an input connection, output connection, windings, bobbin and core. (BONUS: A sample description of the construction details for a transformer)
There are two types of transformers often used in electrical products: power transformers and isolation transformers. The latter isolate the grounded conductor of a power line from the chassis or any portion of an electrical circuit load. On the other hand, the power transformers connect electrical energy from a power supply line to an electrical circuit system or one or several system components. Both types should comply with the following requirements:
- They have dielectric strength after the humidity conditioning treatment
- Overload on any output winding or overheating of insulation in the event of short circuits shouldn’t cause unacceptable risks.
- Their creepage distances and clearance have the values required by the type of insulation.
The compliance of this type of power source components is evaluated by properly measuring the transformer fuse values with respect to the ampacity of the transformer.