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# What is the Current Limiting Resistor and Its Function？

Author: Apogeeweb
Date: 3 Apr 2019
12586

## Introduction

In the circuit, a resistor is connected in series with other components, and there is no output signal in the series. Therefore, when the series components are short-circuited, the voltage applied to the resistor will not burn the resistor. Such a resistor is a current limiter resistor. Otherwise, it is not called a current-limiting resistor, but a protection resistor or load resistor.

## Catalog

 Introduction I What is Current Limiting Resistor? II How Does Current Limiting Resistor Work? III The Role of Current Limiting Resistor IV Specific Examples about the Function of Current Limiting Resistor V Cause of Burning Current Limiting Resistor VI Calculation of Current Limiting Resistor VII How to Choose A Current Limiting Resistor 7.1 How to Choose the LED Current Limiting Resistor? 7.2 How to Choose the Zener Diode Current Limiting Resistor? Ⅷ FAQ

## I What is Current Limiting Resistor?

The current limiting resistor is a protective resistor connected in series to avoid excessive current burning of the appliance. The principle is to reduce the current by increasing the total resistance of the load. Generally, it can also play a role of partial pressure. Usually, in a local circuit, a resistor that has no other function in series with the consumer can be regarded as a current limiting resistor for limiting the magnitude of the current.

Many components have a limit on the maximum input current. If the input current is too large, the components will not work properly or even burn out. To control the current, add a resistor at the input to reduce the current strength and avoid unnecessary risks.

LEDs & Current Limit Resistors Simple Explanation

## II How Does Current Limiting Resistor Work?

The resistor RL is a load resistor, R is a voltage regulator resistor (also called a current limiting resistor), and D is a Zener diode. According to the design principle of the voltage regulator circuit, when the input voltage is substantially constant, the RL becomes smaller, the current flowing through the RL increases, but the current flowing through D decreases.

The current limiting resistor is used to reduce the current at the load end. For example, adding a current limiting resistor at one end of the LED can reduce the current flowing through the LED and prevent damage to the LED lamp.

## III The Role of Current Limiting Resistor

In terms of the basic process of rectification and filtering, the low voltage and the high voltage are the same. Draw a rectifying and filtering circuit, as shown in Figure 1, and then say: "The key to the problem is that there is no charge on the capacitor before the power is turned on. The voltage is 0V, and the voltage across the capacitor cannot be mutated. That is to say, at the moment of closing, the ends of the rectifier bridge (between P and N) correspond to a short circuit. Therefore, when the power is turned on, two problems arise:

The first problem is that there is a large inrush current, as shown by curve 1 in the figure, which may damage the rectifier. The second problem is that the voltage at the incoming line will drop to 0V instantaneously, as shown by curve 2 in the figure. These two features, the high and low voltage rectifier circuits are exactly the same. "

Figure 2. Next: "The low-voltage rectifier circuit is to be stepped down by a transformer. The winding of the transformer is a large inductor. It acts as a barrier and can limit the inrush current during the closing, as shown in curve 1 in Figure (a). In the rectifier circuit of the inverter, there is no such barrier, and the inrush current is much more serious, as shown by curve 1 in Figure (b).

As for the voltage waveform on the incoming side, in fact, in the low-voltage rectifier circuit, the secondary voltage of the transformer also drops to 0V instantaneously, as shown in curve 2 in Figure (a). But reflecting on the original side of the transformer, such an instantaneous step-down, is buffered, as shown in curve 3 in (a), does not interfere with other devices in the same network.

There is no such buffer in the inverter rectifier circuit, and its incoming voltage is the grid voltage. Therefore, at the moment of closing, the grid voltage should drop to 0V, which will affect the normal operation of other equipment in the same network, usually called interference. Therefore, between the rectifier bridge and the filter capacitor, a current limiting resistor RL needs to be connected.

When a current-limiting resistor is connected, the inrush current during power-on will be reduced. At the same time, the instantaneous voltage drop is reduced to the current-limiting resistor, which solves the voltage waveform on the power supply side. Wait for the voltage on the capacitor to rise to a certain level, and then short-circuit the current-limiting resistor.

The size of the short-circuiting device (thyristor or contactor) varies with the capacity of the inverter, but the resistance and capacity of the current-limiting resistor are not much different. What is going on?

## IV Specific Examples about the Function of Current Limiting Resistor

Let's talk about it separately. Look at the current limiting resistor RL first. Strictly speaking, in a large-capacity inverter, the allowable current of the rectifier is also very large. The capacity of the filter capacitor should also be large, the resistance of the current limiting resistor should be small, and the capacity (wattage) should be large. But let's look at an example. Assuming that the resistance value of the selected current limiting resistor is RL=50Ω, what is the maximum inrush current even if the power supply voltage is equal to the amplitude value ULM=1.41&TImes;380=537V? ”

Only a little more than 10A.

And, assuming that the capacitance of the filter capacitor is 5000μF, how long is the charging time?

T=RLC=50&TImes;5000=250000μs=250ms=0.25s

That is the charging time constant, and the charging time should be 3 to 5 times. That is to say, the charging time is about 0.75 s to 1.25 s. The uniform point of the cage is about 1s.

Such charging current, and such charging time, are acceptable for most sizes of inverters? Therefore, in order to reduce the types of other components, the manufacturer has adopted the practice of selecting the same specification of current limiting resistors for inverters of various specifications.

As for the capacity (wattage) of the resistor, since the time for passing the current in the RL is very short, only 1s, the time to actually reach 10A is shorter. Therefore, in general, the capacity is not less than 20W. Look at the bypass contactor KM. Still use specific examples to illustrate it.

Assume that the motor capacity is 7.5 kW, 15.4A. The capacity of the inverter is 13kVA, 18A.

Generally speaking, the capacity of the DC link and the input capacity of the inverter should be equal. When the power supply voltage is 380V, the average value of the DC voltage is 513V. So, how big should the DC current be? ”

The three contacts of the contactor can be used in parallel, as long as the 10A contactor is sufficient.

However, if you use a thyristor, you still need to use 30A.

Then, if the motor capacity is 75kW, 139.7A. The capacity of the inverter is 114kVA, 150A. What is the size of the contractor?

Contactors with a rated current of 80A should be selected.

## V Cause of Burning Current Limiting Resistor

Why does the current limiting resistor smoke and blow? There are three possible reasons for blowing the current limiting resistor.

The first possibility is that the capacity of the current limiting resistor is chosen to be small. Because the current flowing in the current limiting resistor is exponentially attenuated and the duration is very short, as shown in Figure 4. Therefore, its capacity can be chosen to be smaller. In order to reduce component costs, some inverter manufacturers often take smaller values when determining the capacity of the current limiting resistor. In practice, however, the current IR flowing through the current limiting resistor is related to the resistance RL of the current limiting resistor and the capacitance CF of the smoothing capacitor. Comparing the graphs (a) and (b), the RL is large: the initial value of the current is small, but the duration of the current is long.

Comparing Figure (b) with Figure (c), it is known that CF is large and the duration of the current will be extended. Therefore, strictly speaking, the capacity of the RL should also be adjusted as appropriate. However, as mentioned before, there is no strict requirement for the charging process of the filter capacitor. Therefore, there is no clear regulation on the resistance and capacity of the RL. In general, if RL ≥ 50 Ω, PR ≥ 50 W is not a problem.

(a) RL = 80 Ω, CF = 1000 μF (b) RL = 40 Ω, CF = 1000 μF (c) RL = 40 Ω, CF = 2000 μF

The second possibility is that the filter capacitor has deteriorated. Every device with electrolytes has a characteristic: you always use it, it is not easy to break. If you don't use it often, it will break. If the inverter is stored in the warehouse for more than a year, you should first open the cover and observe the filter capacitor to see if it is a 'drum pack'? Is there even electrolyte leakage? The characteristic of deterioration of electrolytic capacitors is firstly an increase in leakage current.

A long-time unused inverter suddenly adds a high voltage, and the leakage current of the electrolytic capacitor may be quite large. When the power is turned on for the first time, the inside of the inverter emits smoke. It is likely that the electrolytic capacitor is seriously leaking or even short-circuited. The DC voltage is difficult to charge above 450V, the short-circuiting device does not operate, and the current-limiting resistor is connected to the circuit for a long time. Of course, it must smoke and blow.

When the electrolytic capacitor is not used at the time, it should be added with about 50% of the rated voltage first, and the pressing time should be more than half an hour, as shown in Figure 5. Its leakage current will drop and it will be used normally.

First, use a multimeter to measure whether the capacitor is shorted. If there is no short circuit, there is no abnormality in appearance. As shown in the figure, after half an hour of power-on, the capacitor can be recovered.

A third possibility is that the bypass contactor KM or thyristor has no action. As a result, the current limiting resistor is connected to the circuit for a long time.

The bypass device should operate when the filter capacitor has been charged to a certain degree (eg, the voltage has exceeded 450V). Therefore, when confirming that the filter capacitor is intact when the power is turned on, observe whether the bypass device operates when the DC voltage UD is sufficiently increased.

One of the specific methods is to connect a voltmeter PV1 in parallel across the current limiting resistor, and also connect a voltmeter PV2 at both ends of the filter capacitor, and then connect the two series-connected bulbs to both ends of the filter capacitor as a load. As shown in Figure 6. After power-on, if the PV2 shows that the UD is large enough, but the PV1 reading is not 0V, the bypass device is not operating.

Connect a load to the DC circuit. If there is no load, there will be no current in the current limiting resistor, even if the short-circuiting device does not operate, the current-limiting resistor will not be able to measure the voltage.

Because the electrolytic capacitor has a certain inductive property, it can not absorb the interference voltage in a short time, which easily leads to the malfunction of "overvoltage tripping". Capacitor C0 is used to absorb the interference voltage.

## VI Calculation of Current Limiting Resistor

Current limiting resistor (RS):

(1) Provide the operating current of VZ.

(2) Protect the VZ from overcurrent damage.

Two extreme cases:

1. (Input voltage VS)

VS = VS(Min), IL = IL(Max) (IL is the operating current of the load) When VS = VS(Max), IL = IL(Min)

## VII How to Choose A Current Limiting Resistor

How to choose the current limiting resistor?

First, you must know the working current and working voltage of the LED you choose. Generally, the LED operating current of 0805 is about 5mA, and the voltage is related to the LED color; the red, green, blue, and white led operating voltages are not consistent. For details, please see this link: SMD 0805 LED supply current, current limiting resistor and brightness

Taking the red LED as an example, the operating voltage is 2V and the operating current is set to 5mA.

R = U / I = (4.2-2) / 5 = 440 Ω. Consider that you are powered by a 4.2V battery, the current limiting resistor can be slightly smaller, and you can choose 330Ω.

Note that the operating current should not be too large, otherwise it will affect the LED life.

### 7.1 How to Choose the LED Current Limiting Resistor?

The calculation is relatively simple, but it is recommended to master a calculation method: the method is as follows:

1, according to the formula: U / I = R

2, according to the typical voltage in the specification of led general white light, blue light is 3.2V@20mA yellow, red is 2.0V@20mA

3. According to the electrical driving current of the LED. Ordinary 20 mAh piranha can reach 50 mA, high power can reach 350 mA or higher

4. Calculation start: A common white light-emitting tube is taken as an example: R=U (voltage drop across the resistor) /I (current through the resistor) sets the drive voltage to 12V; then R=(12-3.2V)/0.02A= 8.8V/0.02A=440

Ohm experience: In order to protect the life of the product, the general drive current is smaller than the typical drive current value. Such as ordinary diodes around 15ma.

### 7.2 How to Choose the Zener Diode Current Limiting Resistor?

Zener diodes can be connected in series for use at higher voltages, and more stable voltages can be obtained by series connection.

The Zener diode acts as a voltage regulator. When the load current decreases, the voltage drop across the current limiting resistor decreases, and the output voltage rises, that is, the reverse voltage of the Zener diode is relatively increased, and the Zener current IZ rises, making the IRS also ascending, the tube voltage drop of the current limiting resistor RS rises, the output voltage drops, and the output voltage remains unchanged. The disadvantage is that a large output current cannot be obtained.

Voltage regulation percentage ratio: %V.R

The stability of the voltage, the lower the ratio, the better. When the DC voltage input VS or the load current IL changes, the output Vo can be kept within a certain range.

VNL: Voltage output at no load VFL: Voltage output at full load

Example: The regulator shown above has an output voltage of 7.5V when there is no load, and 7.4V when the rated current is output, and the voltage stability of the regulator is obtained.

## Ⅷ FAQ

1. What is a current limiting resistor?

A current limiting resistor regulates and reduces the current in a circuit. This equation and calculator helps determine the value of the resistor to add to a light-emitting diode (LED) so that it can limit the current moving through the LED. The calculation also determines how much power the LED consumes.

2. How do you find the current limiting resistor?

Single LEDs

When computing the value of a current limiting resistor for a single LED, the basic form of Ohm's Law — V = IR — becomes: where: Vbatt is the voltage across the resistor and the LED. Vled is the forward voltage of the LED.

3. What is the formula of the current of resistor?

The resistor's current I in amps (A) is equal to the resistor's voltage V in volts (V) divided by the resistance R in ohms (Ω): V is the voltage drop of the resistor, measured in Volts (V).

4. Why do LEDs need current limiting resistors?

In the case of LED strips or commercial lighting, current limiting resistors are installed to minimize the effects of variation in the voltage source. These LED lights often state the voltage that they operate at and that they require constant voltage LED drivers. Get the right power supply for your LED configuration.

5. How do you find the current limiting resistor for an LED?

You need to be sure the wattage (power) rating for your resistor is sufficient for the power being used. The equation for power is: Let's say you are using the LED above with a supply voltage of 12V, an LED forward voltage of 3.9V, and a total forward current of 1400mA.

6. Does a resistor reduce current or voltage?

In short: Resistors limit the flow of electrons, reducing current. Voltage comes about by the potential energy difference across the resistor.

7. Does a resistor affect voltage?

The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor. ... Also, Kirchhoff's circuit laws state that in any DC circuit, the sum of the voltage drops across each component of the circuit is equal to the supply voltage.

8. What resistor is used as a current limiting device?

A current limiting resistor is a resistor that is used to reduce the current in a circuit. A simple example is a resistor in series with an LED. You would usually want to have a current limiting resistor in series with your LED so that you can control the amount of current through the LED.

9. What is the difference between current voltage and resistance?

Voltage is the difference in charge between two points. Current is the rate at which the charge is flowing. Resistance is a material's tendency to resist the flow of charge (current).

10. What is the current limit on a power supply?

The current limiting is the protection of the sensitive devices from large currents that can occur during either normal operation or due to faults. The simplest form of a current limiting device is a fuse.

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