1-Wire and ground voltage offset - Page 1
Forum section: electronics / beginners

author: Tantalum
author_rank: Regular Contributor
timestamp: August 28, 2021, 02:58:12 pm
content: HelloI have a 3 wire cable over 5m (H05RN-F, 0.5mm^2, ~170mOhm per wire) with:- GND- 12V- 1-Wire bus (3.3V)On both sides of the cable there are micro-controllers with their 3.3V voltage regulators.Now I need to know if the 1-wire bus will still work if a 2A current or more, flows through the cable?There will be a ground voltage offset of at least 0.3V (probably more because of the connectors resistance).Thank you

author: Manul
author_rank: Super Contributor
timestamp: August 28, 2021, 06:18:47 pm
content: If we talk CMOS levels, then rule of thumb is 0.3 x VDD and 0.7 x VDD for low and high level threshold respectively. But you need to add some margin to that if you want something robust. So I would say voltage drop of 0.5V should be absolute maximum. You might choose a thicker wire.Also, suggested maximum drop of 0.5V is because you don't want ESD diodes to start conducting. Levels will shift, so one side will see high levels above 3.3V and another side low levels below 0V (negative). 0.5V above VDD or 0.5V below ground is usually the limit after which ESD diodes might turn on.That is not full story though, because piece of wire also has inductance and voltage drop might be a lot higher for short periods of time if current is changing. So you need some electrolytic capacitors at both ends for 12V. Avoid ceramic because they have low ESR and might cause ringing and voltage overshoots.There is still possibility of some voltage transients, so consider adding protection for IO pins used for 1-Wire if you want real robust solution. Series resistors together with clamping schottky diodes, zenner diodes or TVS diodes are some options.And this is why you generally should avoid using same ground return path for both signals and high currents.

author: Tantalum
author_rank: Regular Contributor
timestamp: September 14, 2021, 08:45:50 am
content: Ok thanksYet another question.How about the radiated EMI from several 1-Wire buses using a NYY-J 2,5mm² over 5-20m cables (= not shielded, no twisted wires, and wires SIG/GND spaced by 3-4mm causing a little ground loop)?Could it be harmful or exceed any regulation?

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 14, 2021, 10:22:09 am
content: just don't do ground loop, make a star configuration and start powering from the star centeras for emi, after telling us you have 2amps and no spec about those 2 amps and some real bus architecture, you can't be serious about putting this question

author: Manul
author_rank: Super Contributor
timestamp: September 14, 2021, 06:48:24 pm
content: Quote from: Tantalum on September 14, 2021, 08:45:50 amAnd you are planning to use 1-Wire at 3.3V logic levels? Not so sure about it. It is not only about radiated EMI (which is hard to predict, depends on rise/fall times and many other factors), but also about absorbed EMI. How sensitive is your bus to external interference. I don't know about bit rate of your bus, but in some industrial environment it will likely fail. However, you are free to try it.For long distances or harsh evironments I would go for differential signaling and a twisted pair (see RS485). Again, you might know better what you need.