Equipment ground question

[Nashua Area Radio Society]

Has anyone who uses a laptop with their radios found a good way to connect their laptop to their common radio equipment ground bar?

I always assumed my HP laptop was grounded because it has a 3-prong power cord attached to its power supply. However, in checking with an ohm meter, I discovered the AC power ground is not passed on through with the DC voltage to the laptop itself. So, the laptop is floating with respect to ground even when plugged-in.

However… the laptop does get connected to my radio ground through the USB cable I leave connected to the radio. But the USB cable is not a very hefty ground and would not be able to carry much energy.

I probably don’t need to be concerned about RF on my coax getting to my computer, since the radio cases are connected to a common ground connected to a ground rod right outside my window. But who knows what static or a close lightning strike might do.

And then there’s also the issue of a possible ground loop.

I’ve had this laptop apart when I replaced the cooling fan. It’s not a job for the faint of heart. But I suppose I could find a ground plane on the main board somewhere to solder on a heavy ground wire.

It would be easy if I could use my desktop computer, but that’s not possible at present.

[Hamilton]

Hello Marvin,

Grounding laptops can be difficult. I occasionally see radios with USB issues likely due to someone with a good static charge touching the laptop. The charge is terminated at the USB circuit in the radio.

Here are a few steps that can help minimize the risk:

1) Check the resistance between the shells on your USB cable. Even with probe and contact resistance a good 6′ cable will have 2 ohms or less of resistance. When I first became aware of this I went through my collection of cables and found R over 20 ohms to no connection at all in 1/3 of my cables. A good cable will provide an adequate connection.

2) Make sure the charger (or a PC) is plugged into the same surge protector as the rest of the equipment. If there are monitors make sure they are also plugged into the same surge protector. Monitors can provide a significant current path from the laptop, on its own a laptop has a small charge “mass”.

3) The first two will likely prevent damage from a large ESD spike. The next step is a USB surge protector. It would be inserted in the cable near the radio with a short cable from it to the radio. The shell of the protector would have a wire to the radio’s case. These are available from L-Com and others and can be expensive. They have TVS diodes on all 4 lines to ground. Do not use a galvanic isolator, they do not support USB 2.0 speeds and the DC isolation does nothing for high voltage AC spikes.

Did you see the related post from last week?

How do I add lightning protection for me feed lines

Hamilton K1HMS

[Nashua Area Radio Society]

Hi Hamilton

Thanks for your response. I did see that post last week. That’s what prompted me to review my ground connections. We’re also starting to get low humidity static in the house now that we’re doing more heating. It’s time to get the humidifier running.

Last night I took this laptop apart far enough to discover an unused threaded stud on the ground plane. It must be there for service purposes. I made a slight modification to the case so that I could install a 1/2″ x 4″ copper strip that now comes out the back where I attached a stranded 12 gauge ground wire. It is connected to my common radio ground.

In the past I have metered my USB cables, and like you, found all sorts of low cost issues. Of course, some were only intended for charging, not data. But some are shielded, some are not. I marked all of the good data ones.

I had also looked into USB isolators (mainly low cost ones), but quite a few reviews reported issues with them. I did see devices that use the external 15-pin display housing on a laptop for a ground connection. But that doesn’t seem any better than relying solely on the USB ground.

I’m a believer in surge protectors, especially if they are replaced every few years. Most have no indication they have sacrificed themselves to a surge and no longer offer any protection. When I upgraded my main panel to 200 amps this past spring, I did install a whole house surge protector. But it won’t protect forever, either.

Marv – KC1LML

 

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