The discussion revolves around the use of optocouplers to manage potential ground differences in circuits with multiple 12V inputs from various appliances. Participants explore how these devices help isolate signals and prevent issues related to differing ground potentials, particularly in applications involving multiple power sources.
Participants generally agree on the utility of optocouplers for managing ground differences, but there are varying interpretations of how they function and the implications of voltage measurements across different grounds. The discussion remains unresolved regarding the specifics of voltage readings and the best practices for circuit design.
Participants mention the importance of understanding the relationship between voltage measurements and ground potentials, as well as the potential dangers of connecting grounds at different levels. There is also a note about the limitations of optocouplers in terms of bandwidth and circuit design considerations.
.Scott said:Most appliance that use or generate a 12VDC power or signal will already have that signal isolated. In other words, if you measured the impedance from the 12V signal or its local ground to Earth ground, you would discover that it was very high - well over 1MOhms.
However, there are conditions where these 12VDC signals may not share the same ground and may be indirectly tied together though shared circuits or through grounding issues.
So, for example, if you look at the signal from source "A" by comparing the source "A" signal (Sa) with the source "A" ground (Ga) you will get the signal you expect. And the same for signals "B" (Sb,Gb) and "C" (Sc,Gc). But is you try to tie those grounds together, you could get some very high current conditions. For example, you might discover that there is a 120VAC difference between Ag and Bg.
So, by using opto-isolators, you can encode each signal into an IR level and read that IR level using circuitry with your local ground. Since there are no electrical connection between your signal monitoring circuit and circuits A, B, and C, there is no problem with possible currents among those circuits.
Let's take a specific example:
Let's say you have 16 12V batteries connected in series. What you might normally do is 16:1 MUX those sixteen signals using a common ground and then pass the result to an ADC (Analog to Digital Converter). But tying all the battery negatives together would create dead shorts across 15 of the batteries - and associated fireworks. So instead, you opto-isolate each one, and then use the MUX and ADC.
If those are same kind of sources then they will all send you 12V, but only relative to their own ground.grasscut said:I was told that because each source might have a different ground potential, so maybe one source which supposed to input 12VDC, didnt actually input 12 VDC, say maybe input around 10VDC instead.
You might have the right idea - but, no, you're not saying it right.grasscut said:Am i right to say that, in order not to build up the current when all the sources are tied up, a opto-isolator is better as it reads the IR level rather than the amount of voltage coming from each sources?