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Multiple negative ground radios on a single power supply

Discussion in 'General Technical Questions and Answers' started by NQ8J, Sep 10, 2020.

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  1. NQ8J

    NQ8J Ham Member QRZ Page

    As Derrick, @KF5LJW has drilled into our heads the problems created by having a negative-ground bond in our DC power supplies, it got me to thinking that multiple negative ground radios sharing the same supply could have the same problems.

    Radio 1 and radio 2 share the same DC supply, both have there respective antenna coax shields bonded to AC service ground per NEC. The neutral-ground bond in the power supply has been removed, but each radio has an alternate path for current to flow to the other radio, from its negative wire -> printed circuit board ground plane -> to chassis through mounting screw and/or SO-239 -> up the coax to the bonding point -> down the coax to the other radio drawing high current on transmit.

    For example, I measured 17.9A on the negative lead of radio 1 in transmit, and 2.6A DC current on the coax, which came through radio 2, for a total of 20.5A.

    I suspect if it was a major issue, we would hear of more radios being damaged because of this, but it still bothers me that some of the supply current is flowing though the other radio.

    Changing the length, wire sizes and arrangement can change the ratio between current supplied by the power lead vs the coax, but I can't conceive of a perfect setup besides one power supply for each rig.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I run seven "radios" (transceivers, for HF through UHF) from a single Astron RS-70M power supply here with all DC power lines in parallel and have never, ever experienced what you're talking about.

    Automobiles have dozens of electrical systems from ignition to the radio to the lights...to everything...all running from a common, parallel-connected DC source.
     
    CX3CP, WA9SVD, WA8FOZ and 5 others like this.
  3. N8YX

    N8YX Ham Member QRZ Page

    Triple that count here (two RigRunners connected to the shack power supply and feeding a large bank of receivers, five transceivers and a transmitter) and have also not experienced any weirdness.
     
  4. NQ8J

    NQ8J Ham Member QRZ Page

    The problem is not being parallel connected across both battery terminals, what I am trying to point out is that with negative ground radios, electrons not only flow up the negative power lead, but also up the coax because there is a path from the negative battery terminal through the body of the vehicle to the antenna end of the coax and through the coax to the radio. Two paths for the negative, which converge at the radio and return to the positive.

    In a base station install, there is no vehicle body, but the required ground bound means that all coax coming from said ground bond are now connected and provide a secondary path for current flow, which is through another transceiver.

    Each additional radio creates an additional parallel negative circuit.

    Take a clamp on ammeter, and place it around any one of the coaxes in your system. Assuming that they all have a common bond to ground, you will measure some current, obviously more noticeable on transmit. My example used two radios, but your system has seven radios, which create six other paths for negative current to flow into a given radio, with current divided between each path according to its resistance.

    I'm not saying you would necessarily notice a problem in operation, but you do have parallel circuits on the negative side and being relatively low resistance, current will be flowing in all of them.
     
    Last edited: Sep 10, 2020
  5. KB7WG

    KB7WG Ham Member QRZ Page

    Can you verify your suspicions? Can you see a difference in #1 neg power terminal current, when disconnecting #2 antenna?

    Can you get a similar result when transmitting on #2? Vice versa.
     
  6. NQ8J

    NQ8J Ham Member QRZ Page

    Yes. These are not suspicions. Try it yourself. Pull a negative fuse (if you have one) on one of the radios in a multi radio setup. Does the radio remain powered? If so, there is another path for the negative. What is it?
     
    KB0MNM likes this.
  7. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    Every lead going to a power supply terminal is in essence, a very low value resistor. Current will of course flow in each line in proportion to its resistance. To have such a high value of current running in a non-designated power lead, I can only imagine that your power leads are a bit too high of a gauge and/or too long. That would be my main concern as the power leads should be large enough that current doesn't want to flow by other routes. Not that it causes a huge problem, but that if you ever have to rely on the power leads alone, you would want them big enough to avoid a large voltage drop. Another term for what you are describing is "ground loops".

    73, Mark
     
  8. KB7WG

    KB7WG Ham Member QRZ Page

    Pardon me. The suspicion was mine.
     
  9. KF5LJW

    KF5LJW Ham Member QRZ Page

    You finally see it now, good. It is real and you know it now. It can easily be measured. So now the question you ought to ask is why it is happening and how to resolve it.

    Start with your house electrical system and compare it to a battery system. Your utility provides you 3 wires from a transformer. L!, L2, and Neutral the center tap of the transformer. We will only look at L1 and Neutral for a 120 volt circuit. L1 is equivalent to Battery Positive or the un-grounded circuit conductor, and Neutral equivalent to Negative battery the Grounded Circuit Conductor. When the utility enters your main disconnect device, you are required to solidly bond Earth to the Service Neutral where it enters. It is at this point where Equipment Ground originates. From that point Neutral and Ground are isolated from each other. Code forbids them to make contact again. NEC requires all electrical systems in homes offices and buildings to be, here it is again, Single Point Ground. Every branch circuit has a dedicated Equipment Ground Conductor originating from a Single Point where Neutral and Earth meet.

    Now look at your gizmos in the house with 3-wire plugs with Ground. Take an Ohm Meter and read the resistance between Neutral and Ground and you will see an Open Circuit ever time. Equipment manufactures know you cannot bond the grounded circuit conductor to Ground more than once. If the ground and neutral were bonded down stream, then normal load current would flow on Ground because it is now in parallel with the Neutral circuit. You completely defeat the purpose of the equipment ground by forcing current onto it. Not onlyu is that a safety issue but also an operational issue adding unwanted noise in your ground circuits. Ground now has voltage and current to shock you if you touch it, or noise if you are listening to it. That dang annoying hum you hear in audio.

    Turn the page on the DC power supply. Open the hood and take a look and we find a bonding jumper on the secondary winding being used as Battery Negative. That may or may not be a problem down stream. Depends on how the utilization equipment treats Negative and Ground. All but one industry would use the same path for both a Circuit Conductor and Ground, and that industry is Automotive Electronics. No other technology would ever do that because of the noise and common mode issues. Touch voltage on a 12 volt system cannot exceed, you guessed it 12 volts. If ground opened up, 12 volts will not hurt you. Couple that with all the weight and space savings in a car, along with the large surface area of a plane, allows the automotive industry to get away with it. But can and will give you fits if you bring that car radio into your home.

    If you look at commercial LMR radios designed to work in an vehicle or inside as a control station or base station with 12 volts, the negative is not bonded to chassis. That allows it to work in either environment without any modifications or worries.

    Back to the DC PS issue. First not all DC PS have negative have negative bonded to chassis. Higher end models do not and leave that as an external option to bond either Positive or Negative. There is not technical or safety issue that requires a DC PS output to be grounded. No electrical or equipment codes require it. NEC and equipment codes do not require the outputs of electrical device to be Grounded if less than 50 volts nominal or greater than 1000 VA output. No 120 volt 12-volt DC power supply exceeds those limits. Even if you are lucky enough to have bought a ham radio without negative being bonded to chassis (there are a few out there), you still want to remove the chassis bond to the secondary for two very good reasons.

    1, It breaks the galvanic bond between AC and DC systems, thus providing electrical isolation between your house 120 VAC system from your 12 volt DC system. Transformers can be configured as Separately Derived System (SDS) or non SDS. Put another way Isolated or not isolated. To configure a transformer non SDS or Not Isolated is bond on of the secondary conductor to the chassis of the transformer using the EGC of the supply conductors to complete the circuit to earth. To wire it isolated or SDS either leave the secondary float, or bond one of the secondary taps with a dedicated Ground Conductor bonded to the nearest Ground Electrode System access point.

    2. By breaking the galvanic bond and isolating AC and DC system you break the current path for DC to flow on your Ground Electrode System and come back in on your coax sheilds.

    So the solution is stupid simple once you understand what is happening. Make sure your DC Power Supply bonding jumper on the secondary has been removed form that Astron boat anchor, or replace it with a good Power Supply. You will have DC current flowing on Ground Conductors between pieces of 12 volt utilization equipment inside the shack, but non exiting or entering on AC Equipment Grounds, Ground Electrode System, and reentering on coax shields. Just on that rats nest some of you call station ground. :)
     
  10. AG6QR

    AG6QR Premium Subscriber QRZ Page

    What's being described is a ground loop. When there are multiple separate routes back to the negative side of the power supply, they form one or more loops. I wouldn't worry much about the DC effects of splitting current between various paths, but with significant RF radiation in the air, it's worth remembering that a loop can make an effective antenna. Another way of thinking of things is that the reason power leads don't normally pick up RF is that the parallel wires form a transmission line, with any electromagnetic fields producing equal and opposite effects on the two parallel conductors. When part of the current is returning via a different path, the effects of RF are no longer balanced between the positive and negative leads.

    Will it cause a problem? Maybe. If it does, there are various ways to break the loop, including connecting only one radio at a time, loading up the transmission lines with ferrites to choke RF, etc.
     

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