# Why would a circuit have a zero ohm resistor (3 black bands) in it?

1. Nov 16, 2005

### EIRE2003

Why would a circuit have a zero ohm resistor (3 black bands) in it????

2. Nov 16, 2005

### faust9

Zero ohn resisitors are used as jumpers.

3. Nov 16, 2005

### Staff: Mentor

And often they are placeholders for low value resistors whose value you haven't decided on yet. Also, you use zero Ohm resistors to let you stuff different configuration options.

4. Nov 17, 2005

### EIRE2003

What do ya mean there?

5. Nov 17, 2005

### Staff: Mentor

You wouldn't run into this use of zero Ohm jumpers on a production board, probably. But on test and prototype assemblies, especially mixed signal analog + digital emulation boards, you often are not sure what the best configuration will be in an analog area. So if there are two or three likely candidate configurations of the basic topology, you put all the hooks in to be able to switch between the configurations by stuffing different combinations of zero Ohm jumpers. Parasitics are often a concern in analog circuits, so you don't want to have to be reworking flying components too much onto an SMT assembly to try different options. I'll use the SMT zero Ohm jumpers to short around a component that I want to remove from the circuit (like a signal transformer) for particular characterization tests, for example. Or maybe I want to try a couple different topologies of an analog filter section, so I'll place zero Ohm jumper positions in places that I can use to change the way the parts are connected in the filter. Make sense?

6. Nov 17, 2005

### Ouabache

Hmmmmm... A zero ohm resistor sounds like a wierd item indeed... We know that even "wire" has some resistance per unit length. It would be logical to say that a "zero" ohm resistor then has some finite resistance. Suppose you put an voltmeter across this resistor. Lets say 5Vdc was the potential preceding the zero ohm resistor. How much current would you say is running through the resistor?? I = V/R = 5/small = 5/.0000001 = 250MAmps:surprised That would burn up!! Even a small potential above such a resistor would measure a large current through it.

So how do you avoid such a high current through a zero ohm resistor?

Last edited: Nov 17, 2005
7. Nov 17, 2005

### dlgoff

I don't think you would put them across your power supply. If you did, you would either smoke the wire or your supply(if not current protected).

8. Nov 17, 2005

### Ouabache

It doesn't have to be directly across the PS to have a voltage on it.

9. Nov 18, 2005

### Staff: Mentor

LOL. You use them for interconnect, just like wires and traces. Not as a circuit element. Funny guy :rofl:

10. Nov 18, 2005

### Staff: Mentor

Hey, I just remembered an application for zero Ohm jumpers where they are actually used in high volume production of low-cost assemblies. I'd make it a Quiz Question for y'all, but it's probably too obscure.

Zero Ohm jumpers are used to help a designer use the cheapest PCB possible in low-cost consumer goods like TVs and radios and calculators. Since jumpers are so cheap, adding a few to a single-sided PCB to keep from having to use a 2-sided PCB makes great economic sense. Same thing for a 2-sided PCB over a 4-layer PCB, although jumpers are less commonly used for this purpose on 2-sided PCBs. If you open up just about any low-cost radio and check out the PCB, it will be single sided with a few jumpers to help route the signals.

11. Nov 18, 2005

### Averagesupernova

One reason zero ohm resistors would be used on production level would be for hardware-programming. The end user cuts various resistors out to 'program' the way the machine is to behave. This can be done with jumper wires too, or even 10 or 100 ohm resistors depending on circuit design. But, wires are not used because it is probably difficult for auto-insert machines to insert them onto a circuit board.

12. Nov 18, 2005

### Ouabache

Well, I get the impression at least a couple of folks, have the idea why the zero ohm resistor doesn't burn up in the circuit, though it would be useful to see some analytical reasoning why that is so.

The reason the zero ohm resistor does not burn up (fuse open), is because in its application, it would be in series with a large impedance before reaching ground (0Vdc), so the current through it would be very small. (intuitively you may sense that)
However, lets take a closer look. Lets say the zero ohm resistor has 5V (= Vs) applied to it. If a large impedance R2 (e.g. 1Mohm) is in series with it, what would be the voltage drop across the "zero" ohm R1 resistor (real world: R1 = very small resistance = ~.00000001 ohms
We can solve this as a voltage divider. Vs(R1/(R1+R2)) = 5(1M/((1M+0.00000001)) = 5(1M/~1M) = 4.99999999...
So the voltage drop across the zero ohm resistor is 5-4.9999999... = very small voltage and the corresponding current through it, would also be very small (I = V/R = (very small)/small) = very small current.

13. Nov 21, 2005

### matejhowell

Another Question

I have another question on the same topic. Does anybody have any more analog (as opposed to digital -- like a switch -- or mechanical -- like PCB layer avoidance) ideas for the use of 0 ohm resistors? the reason why I ask is because they are sold with very high, very specified ratings (like a surface mount zero ohm resistor from Dale/Vishay might be rated at a maximum current of 4 Amps). Why not make it metal, just like a wire, that might not be rated so high in current? If they have specific deliberate attributes, it seems they might have a component purpose? ..... just a question.....
MH

14. Nov 21, 2005

### Staff: Mentor

I'm no help on that question, unfortunately, but we will sometimes use a wirewound resistor as an inductor in special situations. Good question, though.