Question with common ground

Discussion in 'General Electronics Chat' started by c0de3, Jan 23, 2012.

  1. c0de3

    Thread Starter Active Member

    May 1, 2009
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    I have purchased a cheap digital panel meter (DPM). After I got the meter I found out what many people have found out. I can't power the meter from the same circuit that I'm trying to measure... At least not without isolation. I've been reading for a couple days now on this topic. I think I understand the *why* I need to isolate, because I can't measure something without having a independent baseline... However, whatever the reason I know I need one. So I've found online a few ideas but they are all a bit complicated for a newbie. Question, can anyone recommend some simple (newbie) reading on how to understand creating this separate (floating?) voltage and ground from the power supply in question? I've seen discussion around opto-couplers and 555 timers being used to make things work. But they are all completed circuits with no explanation of why they work or what the different parts are doing.

    Any help would be great. I have some 555 chips, no opto-couplers, so perhaps something down that line would be most helpful. A common question I have from the diagrams I've seen is that they do all seem to use the same "ground" at some point. So I'd like to see discussion around what makes the "grounds" different.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Look at post #5 on this thread:

    http://forum.allaboutcircuits.com/showthread.php?t=64927

    You will see three symbols for ground, Signal Ground (Common), Earth Ground and Chassis.

    Earth Ground is used for electrical safety.
    Chassis is the metal box of your equipment. This is wired to Earth Ground through the GREEN wire (in North America) on the 3-wire AC cord.
    Signal Ground or Common is the 0V reference point in your circuit. If there is no connection to Chassis Ground, then your circuit is "floating".

    It is common practice to connect Signal Ground to Chassis Ground.

    There should be no current flowing from Signal Ground to Chassis Ground to Earth Ground.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    Yes, you have bumped into a common problem with inexpensive digital panel meters. There are some that don't need an isolated supply (for example I bought one from Futurlec, that didn't need isolation, to monitor the battery on my motorcycle), but many do.

    One solution is to build a small DC-DC converter to power the meter, such as shown here. It uses a 555 as an astable oscillator to generate a 100kHz square-wave which is coupled through capacitors, for isolation, to a couple of diodes. These rectify the square-wave to generate 9V DC for powering the meter. The capacitors (C3 and C4) allow the 9V DC to "float" as needed above or below the input ground of the meter.

    What is the supply voltage you have to power the meter?
     
  4. c0de3

    Thread Starter Active Member

    May 1, 2009
    50
    1
    The power supply "pre-regulation" is a 16v supply. I'm using a LM317 to regulate. I tried to plug the meter in before the LM317 with some resistors in series (DPM needs 7 - 11V) but it read a "-1". When I used a separate 9v battery things worked. Also thanks to the link for that circuit, I had found that one also, I just don't understand what it is doing. Specifically what are the caps doing that isolate things?

    Actually that circuit is a perfect example of my question. Notice the 4 ground connections? The "isolated voltage" is connected to the same ground symbol (via a cap) as the other side of the circuit. Actually, thinking about this, wouldn't that cap only ground AC and not DC? I was thinking Caps were a sort of DC filter. Maybe I'm way off. Again, thanks for any help!





    I'm familiar with the different ground symbols, but thanks for the link.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You are close to the explanation. Caps do indeed block DC and conduct AC.

    Perhaps I should have clarified the word "isolation" in this application. The meter power needs DC isolation so its DC ground (common) voltage can be different from the meter input ground, but it doesn't require AC isolation. Thus the capacitors allow conduction of the AC square-wave (C3 conducts the output from the 555 and C4 provides the AC return path through ground) while still allowing the DC voltage generated to float.

    Does it make sense now?
     
  6. c0de3

    Thread Starter Active Member

    May 1, 2009
    50
    1
    I think I'm getting closer but I'm not there yet. Thanks for your help though. A few followup questions:

    1. The output of the 555 is actually generating AC but it doesn't go negative correct? It is going from 0 to 0+x and back to 0? This may be incorrect due to R1 connecting to ground, that may allow current to flow back in... please advise.

    2. Can someone give a brief description of the zener diodes role? I'm thinking it keeps the voltage at 9v instead of 12v by allowing voltage over 9V to pass through it right to ground? That is a guess, so correct me please.

    3. d1 and d2 are simply a half wave rectifier?

    4. So is the DC ground (floating ground) going to end up being negative voltage in this example, or just "0"?

    5. The output shows a1mA MAX? Is that correct? I'm thinking I can't power my panel with only 1mA? Thoughts?

    Sorry for all the questions. . . I know now that I need to order yet *more* parts for my project! :)
     
    Last edited: Jan 24, 2012
  7. Adjuster

    Well-Known Member

    Dec 26, 2010
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    It may help you to read up about voltage and current. This could make it easier to discuss things like Zener diodes.

    1. The output voltage from a 555 can be regarded as pulsating DC, or else as consisting of DC and AC components at the same time. In the circuit you are looking at C3 blocks the DC component, but couples the AC.
    2. A Zener diode has the property of drawing a sharply increasing reverse current if the reverse voltage is increased beyond a critical level. It is best not to talk about voltages passing through things. Voltages appear across components, currents pass through them
    3. D1 and D2 could be described as a voltage doubling rectifier. If the zener diode were not there, it could produce a DC output voltage just a bit less than the peak to peak input voltage. An ordinary half-wave rectifier only produces the peak voltage (half as much), and more importantly a simple half-wave rectifier will not work with a capacitor input, whereas this voltage doubler, or charge pump will.
    4. This example circuit is only suitable for light duty, with only 1mA available. You will need something bigger to power your panel, but this circuit illustrates some useful principles.
     
  8. c0de3

    Thread Starter Active Member

    May 1, 2009
    50
    1
    I've read on voltage and current. But unfortunately for me, I have to do more than just read a topic to have it make sense in my brain. Sorry about that... I'm just dense.

    1. thanks, I understand the purpose of the cap.
    2. I've never heard the statement: "Voltages appear across components, currents pass through them" Thanks, that makes sense. I'll read more about zener diodes.
    3. Thanks, I'll try to understand that.
    4. Augh. Back to the drawing board...
     
  9. c0de3

    Thread Starter Active Member

    May 1, 2009
    50
    1
    Of all the luck. I found a old 10Base2 network card in the basement. Turns out they have a sweet little isolater on them. Still don't understand the details around the isolation, but I guess that will have to wait. At least I've got a meter! :)

    Taking it off the card was a massive PITA, but I got it.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The floating ground will go to whatever voltage the meter requires. The meter likely has a high resistance value between its two commons and that, along with the internal meter circuit, establishes the voltage between the two. You'd have to analyze the meter circuit to determine what the actual voltage would be.
     
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