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KOA Corporation > Product > How to Use > Applications of resistors

Applications of resistors

Precharge Resistors

A typical power supply circuit is shown below. Rush current flows to charge the capacitor having a large value, when the power is turned on. A resistor is used to eliminate the rush current. This resistor should withstand the rush current of several ten's of Amperes and neither emit smoke nor catch fire even when the switching transistor breaks down to be subjected to a larger current. From these standpoints, wire-wound ceramic cement resistors with excellent pulse withstanding,  BGR BWR and  CW are recommended. Wire-wound with a built-in fuse ceramic resistors  WF are also recommended to protect from continuous over-current. Contact us when you select a resistor.

Power Type Resistors

Power type Resistors include wire-wound: CW, and metal-oxide film: MOS, MO. Either type has different electrical characteristics, which should be selected depending on your circuit. Wirewound type applies metal resistive wire to achieve excellent stability and pulse resistant characteristics. Because resistive wire is wounded in coil shape, inductance or capacitance are relatively large and is sometimes affected by the frequency characteristics of the circuit. The product of high resistance generally tends to have long wounded resistive wire, and is more likely to be large with low resonant characteristics. Pay attention to the influence on frequency characteristics when you use wirewound type for load resistors of voltage amplifier requiring flat frequency characteristics. Non-inductive type wirewound resistor RW_N, with improved frequency characteristic, is recommended. Metal-oxide type resistors are physically small sized, excellent in frequency characteristics, and the resistance value can be high while they have inferior pulse resistance characteristic when compared to wire-wound type resistors. To compare the possible resistance range, wire-wound types are rather low between several ten's of Ω m and several kΩ while that of metal-oxide types covers from several ten's of ohm to several hundred of kohm. Our ceramic cement type resistors include both wire-wound resistors BGR BWR metal-oxide resistors BSR which are used for different applications depending on resistance range.

Application example of video output circuit

Discharge resistors(for safety standard)

In Western countries, secondary circuit is insulated by trans from primary circuit connected to commercial power supply. In electronic devices like audio or video, with outer antenna or various input/output terminals that people can easily touch, people can get electric shock if the secondary circuit retains electricity. By connecting ground and secondary circuit, the secondary circuit is required not to be charged. Discharge resistor is used for this connecting area. Typical inserting point of discharge resistors is the point between the ground of primary circuit and secondary circuit as below. The ground of secondary circuit is connected to e.x. terminal that people can easily touch normally. Commercial power supply is grounded galvanically so discharge resistors prevent secondary circuit from being electrical-charged, which avoids electrical shock hazard. Discharge resistors are also used for the area that crosses over the contact gap of switch of the main power supply. This is to avoid arc when it is OPEN. It sometimes connected between the different pols of power supply at primary side. The resistors used with this purpose are required to keep stable resistance under overload and required to be tested according to IEC60065 14.1.RCR50EN and RCR60 are discharge resistors comformed to the standard.

Snubber circuit

Snubber circuit is the circuit connected to both terminal of the element which produces surge voltage in order to absorb surge energy generated by switching. Snubber circuits include serial circuit(RC snubber) with resistor and capacitor, the circuit connected with RC and diodes to clip the top of the surge voltage. See inside of the circles in the image below. For example, rectifying diodes in the secondary side of switching power supply can be the source origin of surge by quick change of voltage and current caused by switching ON/OFF. RC snubber circuit is used for this surge voltage absorption. A snubber circuit is required to absorb noise but with minimum energy loss and resistors used in such circuit should have both optimized constant and considerations for safety. Flame retardant type resistors are required to control the smoking or firing of resistors when a capacitor or semiconductor is short-circuited depending on switching voltage and the constant of snubber circuit. Metal oxide film resistors MO, MOS have excellent heat resistance so therefore the smaller type can be used if the allowable power is the same. Those are suitable for snubber circuit because the products apply flame retardant coating. Resistors used in RC snubber circuit are required to have surge resistance characteristics, which requires attention.

Gain determination of amplifier circuit

See the images of inverting amplifier circuit and non inverting amplifier circuit that apply OP amp as below. Amplifying gain is determined by the ratio of R1 and R2. To achieve the necessary gain without any adjustment, each resistor in R1 or R2 should have high precision and is unlikely to be affected by the temperature. Metal film type resistors RN73, RN73H are suitable because of the high precision and excellent temperature characteristics. There is also compound type resistors which means one chip resistor include two resistors in pair. This type of resistor achieve excellent relative precision and TCR tracking because two resistors are included. CNN2A has two embedded resistors with TCR of ±25x10-6/K, which becomes 5x10-6/K or smaller in relative. KPC series has more resistors in one package, which enables improved relative precision and high density mounting.

Current noise reduction

Noise of resistors is divided into thermal noise and current noise. Thermal noise is logically determined by resistance, absolute temperature and frequency range, and has no relation with resistor material. The effective value in unit interval of frequency is extremely small and distributed uniformly in frequency domain, which leads to main factor of deterioration of S/N in broad band amplifier of wireless frequency. Normally, no problem occurs in low frequency range under audio frequency. The current noise, on the contrary, depends much on material, the lower the frequency, the larger the noise becomes. In the frequency range under kHz, the effective value of unit interval of frequency is larger than thermal noise; especially under a few Hz, it increases incomparably. Optical detecting circuit as below which amplifies micro detecting signal with large gain, or applications like elctrocardiographic monitors which handle micro voltage near direct current, current noise rather than thermal noise would be the main factor of deterioration of S/N. In these purposes, thin film chip resistors with excellent current noise characteristics like RN73, RN73H, MF, RNS, MRS are suitable.

Example of circuit easily influenced from current noise(optical detecting circuit)

Digital circuit

Pull-up Resistors / Pull-down Resistors / Damping Resistors

Buss in the digital circuit plays a role to transfer data in CPUs, memories and peripheral devices. Resistor of 1k to 100kohm is inserted between power supply, ground and signal wire(pull-up/pull-down resistor) to stabilize the signal electropotential of buss, or resistor of tens to hundreds is used between signal wire and ground to align the impedance of signal wire and LSI.(terminal resistor). Resistors of 10 to 33 ohm are used in parallel in signal wire to adjust waveform(damping resistor). Parallel buss consists of plural signal wires; each of signal wire requires pull-up/pull-down, terminal resistor, damping resistor, which needs much space. Chip network resistors are suitable for such areas. For 8-bit width pull-up resistor/terminal resistor, when the necessary area for 8 pieces chip resistors(size 2012) is 100%, two chip network resistors CN2A4 enables to reduce the area by around 20%.(* calculated when the interval of products is 0.5mm and the dominant area is 0.5mm from the product peripheral). This will also reduce the number of mounting components, which leads to the mounting cost reduction. For 8bit width pull-up/terminal resistors, suppose the necessary surface for 8 pcs of 2012 chip resistors is 100%, two chip network resistors CN2A4 enables to save about 20% surface(calculated the product interva for 0.5mm and product surroundings 0.5mm). Less number of mounting components will also lead to mounting cost reduction.

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