Short Takes (EE Shanty): What shall we do with a zero-ohm resistor?
In circuit board design you often need flexibility. It can cost hundreds or thousands of dollars to respin a circuit board, so I need flexibility for two main reasons:
- sometimes it's important to be able to use one circuit board design to serve more than one purpose
- risk reduction: I want to give myself the option to add in or leave out certain things when I'm not 100% sure I'll need them.
And so we have jumpers and DIP switches and zero-ohm resistors:
Jumpers and DIP switches are quick, but if I want something more permanent and reliable, I use a zero-ohm resistor — sometimes called zero-ohm jumpers. Plenty of companies sell them.
Here's the deal: you have to read the fine print. How close to zero do you need in your circuit? If you're working with milliamp signals it probably doesn't matter. If you have hundreds of milliamps or a few amps, well, be on your guard. Here's what a few common manufacturers promise:
- KOA Speer RK73Z: 50 mΩ maximum
- Panasonic ERJ series: No spec. Wah wah wah. You lose!
- Vishay RCWP series: 50 mΩ max for 0201 jumpers, 20-35 mΩ max for other sizes.
- Vishay CRCW series: 20 mΩ max
- Bourns CR0603/CR0805/CR1206: 10 mΩ max
- TE CRG series: 50 mΩ max
- Rohm MCR series: 50 mΩ max
- Stackpole RMCF/RMCP series: No spec. Wah wah wah. You lose!
- Harwin SMT jumpers: No spec. Wah wah wah. You lose!
- Yageo RC series: 50 mΩ for 0402, 0603, 0805, 1206, and 1210.
And then there are the specialty high-current zero-ohm jumpers:
- Susumu YJP series: 0.3 mΩ max. That is not a misprint! These are high current (10A rated) jumpers with ultralow resistance.
- TT IRC LRZ series: 2 mΩ max
- Isotek SMK/SMS/SMT-R000: 0.2 mΩ max. (rated at 50A for 1206 package, 100A for a 2512 package!)
- Rohm PMR series: 0.5 mΩ max.
- Stackpole HCJ series: 3 mΩ max for 0402, 0.5 mΩ for other sizes
- Vishay CRCW-HP series: 10 mΩ max for 0402, 8 mΩ for 0603, 5 mΩ for 0805, 1206, 2010, and 2512, and 4 mΩ for 1210 and 1218.
The problem with these is that they're pricey. Digikey carries the Rohm high-current jumpers, but even at 100 quantity they're still in the 30 cent range. The Susumu YJP jumpers are 20 cents each at 100 quantity but are only available in the 0603 size. Vishay's CRCW-HP series seems to be the least expensive at 14c each in 100 quantity from Mouser.
I recently needed a bunch of 1206 high-current jumpers (16 per board) and ran into this dilemma. And then I had an epiphany. "You know what, this is stupid," I thought. "There is no such thing as a zero ohm resistor. What I need to look for is 1206 resistors below 5 milliohms." (At the time, I hadn't found the CRCW-HP series yet, and they're 5 milliohms max, which is just on the hairy edge of what I needed for my application.)
So I found the TT Welwyn LRMA series 1 milliohm resistors, 20c each in qty 100. Problem solved!
I still think there ought to be a more inexpensive and reliable way to add or remove an electrical connection without having to incur a significant voltage drop — but I haven't found any.
The same type of issue comes into play when you're considering fuses, or connectors, or wires, or even circuit board traces, for that matter! When you are working with more than about 100 mA, read the fine print, and make sure the current-carrying capacity and series resistance are adequate for your application.
© 2013 Jason M. Sachs, all rights reserved.
You could use non-standard footprints, cutting the pad on each side in half, so that the current wouldn't run through the resistor, but through each of the contacts on the ends. In other words, you would make a footprint with a very small clearance between the pads, and place the component sideways. I'm not saying it's elegant, or even that it's a good idea, but it might work if you're in a pinch.
The actual resistance of the resistor would be irrelevant, so maybe you could use the same resistor for both variants; regular for normal resistance, sideways for nearly 0 ohms.
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