Resistance in wiring - Page 1
Forum section: electronics / beginners

author: kalel
author_rank: Frequent Contributor
timestamp: July 25, 2017, 02:53:59 pm
content: When using protoboards, and connectors on breadboards, how important is the total resistance that you usually get?I suspect that in most simple projects this resistance is not tested for every wire and pin, so you can't know either the value or importance.One dupont connector short wire (those sold in breadboard kits) on those small multi-tester boards shows about 0.3 ohms.If that's true, then just 10 of those is already 3 ohms.What I want to ask, because every project is different, just how important this effect is, and whether you should always "worry about it" or just not spend much time thinking about it.I guess PCBs also have a resistance in traces, but it's probably much less since everything is tightly spaced together.So let's say a few to 10 or 20 ohms or so in total for small projects (could be wrong on that assumption), is it going to cause any effect that you really need to think about and factor in?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 25, 2017, 04:06:44 pm
content: All components have a tolerance so your 10k resistor won't actually be 10k. Those sort of variations probably outweigh your interconnects.....however...You are right the capacitance, resistance and inductance of the wires and the bread board itself can have an effect it depends on what you are doing. e.g. a switching power supply may behave differently on a breadboard and possibly not work at all, indeed it might not work with some PCB layouts but work with others.Don't worry about it until it doesn't work and then consider it.

author: rstofer
author_rank: Super Contributor
timestamp: July 25, 2017, 05:11:59 pm
content: The wires don't often add in series. There may be 1 Ohm per signal on a digital system but that is meaningless, especially with CMOS.Now, motor drivers might need some consideration on the power side. If the wire smokes, it is too small.Op amp circuits, of the variety that I play with for analog computing are already using 1 MOhm input resistors (100k for 10x) so an Ohm or two laying around isn't going to matter. What does matter is the high impedance makes it easy to inject noise. The wire may be the antenna. OTOH, integrators tend to deal with noise pretty well.I'm sure there are examples of where it matters but not on my bench.

author: Ian.M
author_rank: Super Contributor
timestamp: July 25, 2017, 07:56:32 pm
content: Solid 24 AWG tinned copper wire is 89.2 Ohms/Km. If you make your jumpers out of scrap Ethernet cable (not stranded patch leads) a 4" jumper will have a resistance of under 10 milliohms. For any practical jumper length, the wire resistance will be negligible compared to the breadboard contact resistance.Therefore throw out your crappy premade 0.3 Ohm jumpers (assuming you measured them correctly using a 4 wire Kelvin technique), and to minimise contact resistance, use a good quality breadboard that grips leads firmly, keep it clean and *NEVER* force oversize leads or pins into it as that stretches the contacts. Also don't let any components overheat or expect a breadboard to handle much over 100mA through any contact reliably.