Benefits For Zero Ohm Resistor in Circuit Design
I was recently engrossed in a popular strategy game, Rise Of Kingdoms. I can’t believe how addictive it is, as I spent hours building a castle and sending troops to war. Besides the heated battles, I managed to pick up some in-game jargon, one of which is ‘zeroed.’
In the game, getting ‘zeroed’ basically means having the wall protecting the castle totally destroyed. With smoke and flames billowing from the castle, getting zeroed in the game is pretty bad news. Thankfully, you don’t get zeroed in PCB design, but the existence of a zero ohm resistor can be quite confusing.
What Is A Zero Ohm Resistor?
A zero ohm resistor is exactly what the term described. It is a single resistor that is manufactured to have zero ohm of resistance when measured. A through-hole resistor with zero ohm is often marked with a single black band, which indicates the value of the resistance.
Meanwhile, a surface mount ceramic resistor will have a single or a multiple number of zeros to represent this zero ohm resistance. You can also imagine zero ohm resistors as jumper wires that are shaped as typical resistors. Measuring a zero ohm resistor with a multimeter will return in a reading that approaches zero ohm.
Why Do You Need Zero Ohm Resistors?
PCB designers who have never encountered zero ohm resistors are often fuelled with questions on the applications and practicality of the component. After all, why the hassle of manufacturing resistors with zero ohm when one could easily use jumper wires on the design?
It’s common that circuits are usually built with jumpers that allow certain subcircuits to be connected flexibly. Often, a circuit is replicated in a few product variations where only selected subcircuits are used. Zero ohm resistors are commonly used to replace jumpers in enabling certain connections on the PCB
If you’re building a PCB with manual placement and soldering, using a jumper wire is probably the best way to establish connections between pads. However, mass production involves a pick-and-place machine, which doesn’t have the capability of unrolling and cutting jumper wires. In such cases, a separate machine is needed for jumper wires.
Using zero ohm resistors reduce the cost of jumper wires insertion.
For mass production, it’s important to keep the cost down. Using an additional machine for jumper wires results in a separate setup and process cost. Therefore, a zero-ohm resistor is the perfect alternative that saves cost and time in assembly.
Single Layer Design
PCB designers are often pressured to keep minimize cost. There’s nothing cheaper than confining a design to a single layer PCB. However, getting the traces to their destination without overlapping is tough when you have quite a number of components on board.
Zero ohm resistors are the perfect remedy to ‘magically’ route a single layer board. Rather than making holes and turning the PCB into a double-layer structure, you can route a trace between the pads of a zero ohm resistor. These resistors are then assembled along with other components during the manufacturing process.
Preventing Reverse Engineering
It’s a bitter truth but there are copycats in the market that will not hesitate to reverse engineer a PCB that’s proven to be a hit. Short of coating the entire PCB with epoxy, zero ohm resistors are a great option to confuse the unethical copycats.
Intentionally placing zero ohm resistors on certain tracks is a good strategy to mislead the perpetrators copying the design. If possible, order zero ohm resistors with no markings to further frustrate their efforts.
Placing zero ohm resistors may prevent illegal reverse engineering.
Design Considerations When Using Zero Ohm Resistors
Using a zero ohm resistor isn’t a complicated process but you’ll need to remember that zero ohm resistors aren’t ideal components. In other words, it has a certain amount of resistance as with other conductive wires and traces.
As there is a tiny amount of resistance in a zero ohm resistor, it can dissipate heat when current passes through. The maximum power rating for the zero ohm resistor is defined by the thermal characteristic of the material and size. You’ll want to ensure that you’ve chosen a zero ohm resistor that can handle the current passing through without burning out.
Whether it’s to protect your design or to fully automate the assembly process, it’s quite easy to add zero ohm resistors if you have a good PCB design software like OrCAD PCB Designer. You’ll be surprised how these tiny components that confused many can result in substantial cost saving. Even more, the whole suite of PCB design and analysis tools from Cadence can augment your design process into excelling at any scenario.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
About the Author
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A zero-ohm link or zero-ohm resistor is a wire link packaged in the same physical package format as a resistor. It is used to connect traces on a printed circuit board (PCB). This format allows it to be placed on the circuit board using the same automated equipment used to place other resistors, instead of requiring a separate machine to install a jumper or other wire. Zero-ohm resistors may be packaged like cylindrical resistors, or like surface-mount resistors.
One use is to allow traces on the same side of a PCB to cross: one trace has a zero-ohm resistor while the second trace runs in between the leads of the resistor, avoiding contact with the first trace.
The resistance is only approximately zero; only a maximum (typically 10–50 mΩ) is specified. A percentage tolerance would not make sense, as it would be specified as a percentage of the ideal value of zero ohms (which would always be zero), so it is not specified.
An axial-leadthrough-hole zero-ohm resistor is generally marked with a single black band, the symbol for "0" in the resistor color code. Surface-mount resistors are generally marked with a single "0" or "000".
In practice, zero-ohm resistors can be useful as configuration jumpers, but caution is exercised for PCB designs which may use zero-ohm resistors to select between options which require larger trace currents in the design. For these situations, it is better design practice to specify a low-ohm resistance, such as a 0.001-ohm to 0.003-ohm resistor, rather than the generic "zero-ohm" resistor in which the actual resistance may be higher, and tolerance is not given. The low-ohm resistors are easily obtained with 5% or 1% tolerances on a maximum specified resistance and can be safely utilized to pass much higher currents.
For example, a surface-mounted 0805 size resistor of 0.003 ohms, rated at 0.5 watts can, in theory, safely pass up to √P/R = 12.9 amperes of current. In practice, when approaching the power limit for a given package, it is good practice to either use an even lower-ohm product (more power-efficient but more expensive) or go up one package size larger (same power efficiency but cost may be kept lower). The "cost" associated with taking up more board space for the larger package may also be a consideration. In this example, for 12 amperes to pass through the jumper, a lower resistance or a bigger surface-mount package (such as a 1206) are usually specified. In contrast, a "worst-case zero-ohm" real-world jumper with 0.05-ohm impedance in a similar 0805 package could only pass 3.1 amperes maximum. The use of specific tolerance resistances is a much safer design practice for higher currents than the "zero-ohm" option, although the bill-of-materials cost can be higher for low-ohm devices.
Why is a Zero Ohm Resistor used instead of a Jumper Wire?
What is Zero Ohm Resistor?
A Zero Ohm Resistor (also known as Zero Ohm Link) is a passive device which has ideally “0 Ω” resistance like any other short wire and pure conducting materials.
0-Ω Resistor is a jumper or any other wire packaged the same like the shape of a resistor. It has almost 0-Ω resistance (although it has some resistance in real life as like other conducting materials and ± tolerance). Zero ohms resistors are available in a range such as 1/8W having 4mΩ and 1/4W having 3mΩ.
Many of us have seen these “Zero-Ohm” resistors in the modern PCB design and circuit. So one must ask about “What is the purpose of a zero ohm resistor and why 0-Ω Resistors are used instead of jumpers or wires?” Well, we will discuss the different reasons behind the story.
Types of Zero Ohm Resistors
Zero ohms resistors are available in the following two shapes.
- Wire Wound Zero Ohm Resistor
- Surface Mount (SMD) Zero Ohm Resistor
Wire Wound Zero Ohm Resistor
It is the general shape of an ordinary resistor. In the wire wound zero Ω ohm resistor, there is only and only one black line ( single black band) printed on the resistor. The single black strip o-ohm resistor is shown in the following fig.
Surface Mount (SMD) Zero Ohm Resistor
In the recent technology, surface mount resistors are used instead of wire wound or any other types of resistors due to the cost, simplicity and covered area of PCB board etc. In a zero ohm surface mount resistor, the manufacturer prints a “single zero “0” or “three zeros “000” on the surface. The single and three zeros labeled zero ohms resistors can be seen in the following fig.
Good to know: SMD stands for “Surface Mount Device”. We have discussed in the previous post about how to to find the value of SMD Resistors with color codes.
Why a Zero Ohm Resistor used instead of Wire?
There are multiple reasons to do so. Some of the zero ohm resistors application are as follow:
Automatic Placement by Insertion Machines
In PCB mass production, an automatic insertion machine is used to pick and place the components and devices such as diodes, capacitors, inductors, resistors etc. For the ease and reduce the production cost, it is an easy approach to use zero ohm resistors instead of wires and jumpers because you will have to manage another automatic wire placer machine or you will have to manually place it. In other words, you will have to use two machines, one for components and other one for wires and jumper which leads to a costly production and more time needed to fully assemble the PCB.
Single Layer PCB Design
A single layer PCB is less costly as compared to the double layer PCB design and assembly. To use wires in the single PCB assembly, you will have to make holes which turns it in double layer PCB. Instead of doing so, a 0-Ω resistor is the perfect match as compared to the wires and jumpers used in the single layer PCB structure. Hence, it reduces the overall cost again.
Preventing Copycats using Reverse Engineering
Yes, they exist. Copycats simply copy the well known PCB designs using reverse engineering tactics. In this case, zero ohm resistors are the best alternative to wires which confuse and prevent the copycats. To do so, designers and manufacturers put 0-Ω resistors with no marking or use different resistor color codes. This way, the chances of copying a PCB design are lesser with more efforts.
Should I Use a 0-Ω Resistor or Jumper Wire?
Well, if you are a hobbyist and just love making PCB designs and circuits for your own purposes, It is better to take the soldering iron and place a wire instead of picking those tiny so called 0-ohm resistors (they are having little bit resistance).
If you are doing mass production and working on a brand, you must use 0-Ω resistors instead of jumpers as all of your competitors are doing the same due to the above mentioned reason and advantages.
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