# You can't think about it, can you? A small resistor can be great too

2023-03-18Archive

The resistor is a normal component, but it has unusual paths. There are many options for resistors. Usually, we only need to pay attention to value, accuracy and power rating. These three indicators are suitable.

It's true that in digital circuits we don't have to pay attention to too many details. After all, in numbers consisting of only 1s and 0s, we don't care about minor influences.

But in analog circuits, when we use a precise voltage source, do A/D conversion, or amplify a small signal, a small change in resistance will make a big difference.

When we care about resistance, it is, of course, when working with analog signals. Later we will analyze effect of resistance parameters according to analog circuit application.

01

Rated resistor resistance

The choice of resistor value is often determined by application, such as limiting current of an LED lamp or sampling a specific current signal, there is basically no other choice for resistor value. .

But in some cases, resistors can be used in different ways, for example, to amplify a voltage signal. As shown in Figure 1, increase is related to ratio of R2 and R3 and has nothing to do with R2 and R3 values.

Currently, there is still a basis for choosing a resistor value:

• The larger resistor value, more thermal noise and worse performance of amplifier;

• The lower resistance value, greater current during operation, greater current noise and worse performance of amplifier;

• This is why resistance of many amplifying circuits is tens of kΩ. Where a high resistance value is required, either a voltage follower or a T-network is used to avoid this.

• 02

Resistance accuracy

The accuracy of resistors is easy to understand, so I won't be verbose here. Resistors are typically 1% and 5% accurate, and 0.1% accurate. The price of 0.1% is about ten times price of 1%, and price of 1% is about 1.3 times price of 5%.

Typically, accuracy code A=0.05%, B=0.1%, C=0.25%, D=0.5%, F=1%, G=2%, J=5% , K=10%, M=20%.

03

Resistor power rating

The power of a resistor is very simple, but often misused. For example, rated power of resistor of 2512 chip is 1 watt. According to specification of resistor, when temperature is above 70 degrees Celsius, resistor must be lowered.

How much power can a 2512 chip resistor consume?

At room temperature, if PCB pad does not have a special treatment for heat dissipation, when power of 2512 chip resistor reaches 0.3W, temperature may exceed 100 or even 120 degrees Celsius.

At 125 degrees Celsius, derating curve requires 2512 to be derated by up to 30%.

In this case, you need to pay attention to any packed resistors. Don't be superstitious about power rating. It's best to double-check location of keys to avoid hidden dangers.

04

Resistor Withstand Voltage

The voltage withstand voltage of resistors is rarely mentioned at all, especially for beginners, they often have no idea, thinking that capacitors have voltage withstand values.

The voltage that can be applied to both ends of resistor is determined by rated power. It must be ensured that power does not exceed rated power. The other is voltage withstand value of resistor.

Although power of resistor body does not exceed rated power, too high voltage will lead to resistance instability, current leakage between resistor terminals and other malfunctions. When using it, you must select a reasonable resistor according to voltage used.

The withstand voltage of some kits include: 0603 = 50V, 0805 = 100V, 1206-2512 = 200V, plug-in 1/4W = 250V.

Moreover, when durationIn a proper application, voltage across resistor should be more than 20% lower than rated withstand voltage, otherwise problems will easily occur after a long time.

05

Temperature Coefficient of Resistance

The temperature coefficient of resistance is a parameter that describes how resistance changes with temperature. It is mainly determined by material of resistor. Generally, thick film chip resistor packages above 0603 can reach 100ppm/℃.

This means that if ambient temperature of resistor changes by 25 degrees Celsius, resistance value can change by 0.25%. If it is a 12-bit ADC, a 0.25% change is 10 LSBs.

Therefore, op amps such as AD620 use only one resistor to adjust gain. Many older engineers will not use it for convenience. They will use conventional circuitry to adjust gain due to ratio of two resistors. When resistance is same. When using same type of resistor, temperature-induced change in resistance will not cause a proportional change, and circuit will be more stable.

In more precise instruments, metal-film resistors will be used. It's easy to achieve a temperature drift of 10 to 20 ppm, but of course it's more expensive.

In short, in precise application of instruments, temperature coefficient is certainly a very important parameter. If resistance is not accurate, parameter can be adjusted during calibration, and change in resistance with external temperature cannot be controlled. .

06

Resistor structure

There are many resistor structures. Here are apps that come to mind. The machine's starting resistor is generally used to pre-charge large capacity aluminum electrolysis with a resistor. After aluminum electrolysis is completed, close relay and turn on power to work.

This type of resistor must be shock resistant. It is best to use a large wire resistor. The power rating of resistor is not very important, but instantaneous power is very large. resistors that meet requirements.

For high voltage devices such as capacitor discharge resistors, actual operating voltage is over 500V. It is better to use glaze high voltage resistors instead of conventional cement resistors.

For example, to apply peak absorption, both ends of thyristor module must be connected in parallel with RC for absorption and protection against voltage drop and drop.