Why Do I Need A “0 Ohm Resistor”?

The 0 Ohm resistor is a special resistor that has to be used for a number of applications. So, we are actually in the process of circuit design or often used to a special resistor. 0 ohm resistors are also known as jumper resistors, is a special purpose resistors, 0 ohm resistors resistance value is not really zero (that is a superconductor dry things), because there is resistance value, but also and conventional chip resistors have the same error accuracy of this indicator. Resistor manufacturers have three accuracy levels for 0-ohm chip resistors, as shown in Figure 29.1, which are F-file (≤ 10mΩ), G-file (≤ 20mΩ), and J-file (≤ 50mΩ). In other words, the resistance value of a 0-ohm resistor is less than or equal to 50 mΩ. it is because of the special nature of the 0-ohm resistor that its resistance value and accuracy are marked in a special way. the device information of the 0-ohm resistor is marked with these parameters, as shown in the figure.

We often see 0 ohm resistors in circuits, and for novices, it is often confusing: if it is a 0 ohm resistor, it is a wire, so why put it on? And is such a resistor available on the market?

1. The function of 1.0 ohm resistors

In fact, a 0 ohm resistor is still useful. There are probably several functions as follows.

a. To be used as a jumper wire. This is both aesthetically pleasing and easy to install. That is, when we finalize a circuit in the final design, it may be disconnected or shorted, at which point the 0-ohm resistor is used as a jumper. By doing this, it is likely to avoid a PCB change. Or we a circuit board, may need to do compatible design, we use 0 ohm resistors to achieve the possibility of two circuit connection methods.

b. In mixed circuits such as digital and analogue, it is often required that the two grounds need to be separate and connected at a single point. Instead of connecting the two grounds directly together, we can use a 0 ohm resistor to connect the two grounds. The advantage of this is that the ground is split into two networks, which makes it much easier to handle when laying copper over large areas, etc. And we can choose whether to short the two ground planes or not. As a side note, such occasions are sometimes connected with inductors or magnetic beads etc.

c. For fuses. Due to the high fusing current of the PCB alignment, it is difficult to fuse in the event of a short-circuit overcurrent and other faults, which may lead to greater accidents. As the 0 ohm resistor current withstand capacity is relatively weak (in fact, 0 ohm resistor is also a certain resistance, just very small), overcurrent will first 0 ohm resistor fused, thus breaking the circuit, preventing a greater accident. Sometimes small resistors with a resistance of zero or a few ohms are also used as fuses. However, this is not recommended, but some manufacturers use this to save costs. This is not a safe use and is rarely used in this way.

d. A place reserved for commissioning. You can decide whether to install it or not, or other values, as required. Sometimes it is also marked with an * to indicate that it is up to the debugging.

e. Used as a configuration circuit. This acts similarly to a jumper or dipswitch, but is fixed on by soldering, thus avoiding random modification of the configuration by the ordinary user. By installing resistors in different positions, it is possible to change the function of the circuit or set the address. For example, the version number of some boards is obtained by means of high and low levels, and we can choose 0 ohms to implement the change of high and low levels of different versions.

2. Power of 0 Ohm Resistors

The specifications of 0 Ohm resistors are generally divided by power, such as 1/8W, 1/4W, etc. The table lists the through-current capability corresponding to the different packages of 0-ohm resistors.

0 Ohm Resistor Current Capacity by Package

3. Single point earth for analogue and digital ground

As long as they are grounds, they must eventually be connected together and then to earth. If not connected together is a “floating ground”, there is a pressure difference, easy to accumulate charge, resulting in static electricity. Ground is a reference 0 potential, all voltages are derived from the reference ground, the ground standard should be consistent, so all kinds of ground should be short connected together. It is believed that earth is capable of absorbing all charges, always remains stable and is the ultimate earth reference point. Although some boards are not connected to earth, the power plant is connected to earth and the power from the board eventually returns to the power plant into earth. Connecting analogue and digital grounds directly to each other over a large area would lead to mutual interference. Not short connection and not appropriate, the reason as above, we can use the following four methods to solve this problem.

a. Connected with magnetic beads: The equivalent circuit of magnetic beads is equivalent to a band resistance limiter, which only has a significant suppression effect on the noise at a certain frequency point, and requires a pre-estimate of the noise frequency when used in order to choose the appropriate model. For cases where the frequency is uncertain or unpredictable, magnetic beads do not fit.

b. Connected by capacitor: capacitor isolated through the AC, resulting in a floating ground, can not achieve the effect of equal potential.

c. Connection with inductors: inductors are large, have many stray parameters and are unstable.

d. 0 ohm resistor connection: impedance can be controlled range, impedance is low enough, there will be no resonance frequency point and other problems.

4. 0 Ohm resistor how to derating?

0 Ohm resistors are generally only marked with rated maximum current, and maximum resistance. The derating specification is generally for ordinary resistors, and rarely describes how to derate 0 ohm resistors separately. We can use Ohm’s Law to calculate the maximum resistance multiplied by the rated current of a 0 Ohm resistor, for example, if the rated current is 1A and the maximum resistance is 50mΩ, then we consider the maximum voltage allowed to be 50mV. However, it is very difficult to test the actual voltage of a 0 Ohm in practical use scenarios, because the voltage is very small, and because it is generally used for shorting, and the voltage difference between the two ends of the short is fluctuating.

So, generally we simplify this process by using a direct 50% derating of the rated current for use. For example, we use a resistor to connect two power planes, the power supply is 1A, then we approximate that the current of both the power supply and GND is 1A, in accordance with the simple derating method we have just described, choose a 2A 0 ohm resistor for shorting.