To fuse or not to fuse…that is the question.
Ultimately, this web page will discuss the fusing of an amateur radio transceiver within a mobile installation. But before we can discuss that, we must mess around with a little mumbo-jumbo background. If you are in a hurry, you can skip ahead to that section.
The question of the day is to double-fuse or single-fuse. That is the question. You are going to install two power supply wires running directly from your amateur radio VHF transceiver to the vehicle’s battery. Do you fuse only the positive wire or do you fuse BOTH positive and return wires?
What are we Working with, Anyhow?
A few years ago, I bought a Yaesu DR-3200 Fusion transceiver to install in my car and work the 2-meter amateur radio FCC allocation. It came with a power supply cable designed for distributing the car’s 12 Volt power to the transceiver. The application for it was to bring the car’s 12 Volt power directly to the transceiver from the car’s 12 Volt battery terminals.
Why Have a Dedicated Power and Return Wire to the Battery?
Our amateur radio will probably work fine when powered out of a PTO socket inside the passenger cabin (assuming sufficient power to meet the transmitter demand). At Ford Motor, a PTO is always dedicated and will not be sharing loads with other vehicle modules. It may, however, be shared on other makes. Load sharing invites a ground loop which then appears as noise in your amateur radio audio. We don’t want noise and therefore, we don’t want a ground loop leading to the conclusion that dedicated supply wires are a good thing.
Easy as Pie or Punishing Myself
It would be easy to rig up a power supply cable for my DR-3200 Fusion VHF amateur transceiver by means of a convenient power point from inside the vehicle cabin. By comparison, it would be VERY hard to run the two wires Yaesu had given me through the vehicle’s firewall and connect safely to the battery. If I am going to punish myself, I want a good reason. In many cases, that good reason is simply getting the power to the transceiver it demands. Other than that, there must be a safety issue or something optimizing the transceiver’s performance if I am going to get my hands dirty or break my back.
I Saw Something Curious
But there was something very curious about this cable assembly that came with the radio from Yaesu. On one end of the cable assembly, there was a connector matching the one on the radio. But on the other end of the cable assembly (we are talking about a distance of maybe 10 feet) were two fuses–one for each wire. Yaesu was telling me that I should have a fuse next to the battery, not only for the positive feed but for the return wire as well.
I asked myself, “Why is this?” I came up blank. To me, it is an idea that won’t hurt anything but at the same time, is a dumb and senseless thing. I searched the internet and found conflicting stories on this dual-fusing business for amateur transceivers. However, there were two consistent things that I found. That was that nobody…
- …was claiming that the double-fuse approach in any way enhances the performance of the ham radio.
- and everybody was working from a cookbook. They did not explain why their conclusions were valid.
Above is page 9 from the Kenwood User Manual for the TR-751A.
I Hate Cookbooks
I hate cookbooks, and the present dilemma is a good example of why I hate cookbooks. What I consider to be a trustworthy authority on fusing for the DR-3200 (Yaesu) was at least suggesting a double-fuse configuration.
I also found a wealth of forum posts on the subject with all sorts of simple statements as if with authority. A general rule of thumb for recognizing posts is to ignore those where they open by telling you their authority. They might be an electrical engineer or rocket scientist. They will tell you about their many years of experience and then say, “…trust me.” Advice: ignore them.
Maybe it is time to throw away the cookbook and figure this one out for me.
Possible Reasons For a Dual-Fuse:
- Everybody else is doing it.
- Maybe there is a balancing act beneficial to optimal radio operation. (No)
- If the vehicle ground strap becomes disconnected, the entire vehicle return current will try to come down your transceiver return wire. But the transceiver return wire has to have continuity with the chassis somewhere for that to happen. The very purpose of the dedicated return wire for your transceiver is to absolutely not share return paths with other equipment returns. (Pay attention to this one, folks.)
- Protects against the return wire shorting against some positive source. (No)
Think about it. What is the return wire going to short against? This is extremely important for positive wires but not negative. There are many places where positive wires can find a path to ground if their insulation becomes compromised. But a compromised return wire will only find other zero potential paths to ground. We can make a case for its finding a path to 12 Volts, but this means a double failure which we do not protect against.
- The transceiver’s antenna may provide an alternate “dirty” path to the battery return parallel with the vehicle’s ground strap. This is not to be confused with the transceiver’s minus power supply input but is separate and distinct. What happens if the vehicle’s ground strap is compromised?
Time to Map This Thing Out
Please consult the figure above showing the power distribution architecture for a mobile amateur radio transceiver installed for a mobile application. In the illustration, we have essentially two operating systems shown.
- There is a vehicle manufacturer supplied electrical system powered by the vehicle PDB (power distribution module). The PDB distributes power to the various sub-systems within the vehicle through fuses designed to protect downstream wires. Those sub-systems return their current either to the PDB or to various points in the vehicle’s chassis. However, the return current is routed through the vehicle’s ground strap.
- The second electrical system is our transceiver. It does not return its current through the ground strap but it has its own dedicated return path directly to the battery negative post.
Our Yaesu DR-3200 is shown in the illustration as a second system within the vehicle. It has a dedicated supply and return path to the battery and therefore does not use the ground strap. But there is another “dirty” return path that the Yaesu can find for the battery return. That is through the transceiver’s antenna. If a mag-mount, the return is capacitively coupled to the chassis and returns through the ground strap. Any other type of antenna probably has a direct path to the vehicle chassis. But this system is isolated accounting for its value. There can be no shared loads or ground loops by definition.
Somebody Threw in a Wrench
Suppose that for whatever reason, the ground strap became null and void. The other vehicle sub-systems now have no path to ground and will look for an alternative. What they will find is a path through the transmission line coax, leading inside the transceiver, leading to the transceiver’s PCB, through various PCB traces possibly, but in any case, to ground.
Do you understand the significance of what has just been described? The vehicle’s entire compliment of sub-system return current (i.e. the starter, heated seats, steering EPAS motor, etc.) has a path (although dirty) to ground—possibly through the transceiver’s PCB.
What Happens Now?
Typically the power supply wires are AWG #10 and fused at 30 Amperes. Let us also suppose that the transceiver’s path to ground is dirty (not nearly as good as a ground strap) and providing some resistance such that current is limited. Let us suppose that the vehicle sub-systems would like to have less than 1 milli Ohm resistance but can only find 200 milli Ohms (0.2 Ohm) resistance through this dirty path to ground. This dirty path to ground now is conducting 70 Amperes through our transceiver’s PCB. I never learned how to spell very well. How do you spell the word…TOAST?
The reader may now say to him or herself, “Well, it’s okay because the 30 amp fuse on the negative wire will save it since 70 amps will trip the fuse right away. Consider the nature of the fuse. The typical 30 amp fuse will blow in 10 seconds given a current of 70 Amperes. While it is a foregone conclusion that the PCB is toast, consider that the #10 wire will now conduct 70 Amperes for 10 seconds. The wire may or may not survive without damage.
Suppose There Had Been No Negative Wire Fuse
But now suppose that there had been no fuse in the negative supply wire. Thermal damage will occur without question. But worse. How do you spell “fire?”
Consider an Alternative
It looks like we are damned if we do and damned if we don’t. There is an alternative.
In the discussion thus far it has been proposed to return the transceiver power to the negative battery post. Consider returning it to the south end of the ground strap. Doing so means that the alternate defacto ground strap was never created in the first place. If the ground strap becomes compromised, all current flow ceases and all of those problems never had a chance to occur.
Keep the negative fuse. But if not using a negative fused, consider attaching the transceiver negative power wire south of the vehicle ground strap rather than the negative pole of the battery. Problems with return power wire placement are relative to having circumvented the vehicle ground strap. Also, there is no way that any potential of any significance will develop over the ground strap. It is nearly the equivalent of attaching to the battery negative post.
You will probably be fine if you follow the manufacturer’s instructions. It is always safer to follow the manufacturer’s instructions.
What you Have Witnessed
The author hates cookbooks. This is because the author is tired of hearing people “say so” for varied conclusions. You might compare it to a cliche that is currently in the 2022 mainstream where you will hear, “You have to follow the science.” When you hear that, the speaker is probably telling you to follow the scienTISTS and not the science. They want you to listen to what people say and then take a vote on what is a correct conclusion.
A better course of action is to reduce a problem to its elements and adjudicate on that basis. That is what you have witnessed in this web page. If we have presented incorrect information, all one has to do is point out the source of the error in the page and present an alternative, something we have missed or an oversight.
But in any case, we advise you to hate cookbooks. I thought everybody hates cookbooks. Don’t you?
Dave Casler (KE0OG) Weighs In
Dave has never let us down with his authoritative presentations on amateur radio. Dave has a regular column in the ARRL QST magazine and is known for being clear and concise in his presentations.
Dave has put out a video on this very subject of double-fusing. His conclusion is to keep the double fuse.
Dave points out one of the possible problems we outlined above in our “Possible Reasons For” using a double-fuse. He points out that if the vehicle ground strap becomes disconnected, all of the electrical modules and devices within the vehicle will then try to find a path home through your dedicated return wire. But, as we noted above, you installed that dedicated return wire for the very purpose of not allowing any sharing of return paths to ground. Any sharing facilitates the possibility of engine or other noise showing up in your transceiver’s audio output. The only possibility for that would be through the capacitively coupled antenna counterpoise as we already noted above.
I found another YouTube video (11 minutes) by StealthGTI which gives an excellent presentation on a very large mobile aux power configuration. In this mobile rig, he has an amateur radio but at the same time a much more powerful 100 Ampere entertainment system. He shows you all sorts of detail in a very professionally done mobile power distribution system.
And guess what… He has not fused the return wire and makes no mention of a need or non-need for one.
But StealthGTI goes beyond simply showing you his rig. He has carefully block-diagramed out the electrical configuration and uses that block diagram referring back to it as he takes you through his vehicle. As one who has worked an entire career in automotive power distribution design at Ford, I can say that StealthGTI’s work is something to respect.