I’ve gotten more and more active in mobile and portable radio activities over the last few months. I started off with my KX3 and the internal 8 x AA battery compartment. With this option transmit power is limited to 10W, and transmit time is greatly reduced.
I then moved onto a LiFePo4 battery (7Ah) which gives increased transmit power 15W, and extends transmit time to at least 6-7hours (for voice).
I’ve been meaning to fuse the DC power lines for the KX3. It’s good practice to protect your radio, and reduce the risk of fire.
Fuse both lines?
One question I’ve always had is – do you really need to fuse both positive and negative lines?
If you purchase a commercial radios you’ll see the manufacturer always fuse both lines.
But I never thought about it before. I never knew if they were required, just overkill, or trying to protect the radio. Radios will be used in many unknown situations, potentially unsafe and complex electrical and electronic environments like a motorcar.
Operating from a motorcar, house with mains electricity power supply, means an unknown, potentially unsafe and complex environment.
Electrical circuit for portable operation
But, I’m not operating from a car or location with complex power supply. My electrical circuit is simple – battery, radio, antenna. It’s not an electrically and electronically complex environment. The radio is connected directly to the battery, the antenna is grounded via the radio chasis.
Arguments for and against fusing the negative
Helpfully Bob K0NR has written about this already making the case for and against fusing the negative, in a motorcar environment.
Argument for fusing the negative lead is to protect against return current from other devices that find its way back to the battery through the transceiver’s negative power lead.
For example; the starter could have a fault in its negative cable, causing the current to flow through the chassis to the transceiver and back to the battery.
The negative line fuse will open and protect the negative lead, limiting the damage in this case.
The argument against fusing the negative lead is that if the fuse opens up, it could cause problems.
For example, if the transceiver is completely isolated, the negative line fuse would remove transceiver power.
– Mobile DC Power: One Fuse or Two?
However, if a current from a faulty starter flows, the return path at the antenna coax will most likely allow the radio to continue functioning using the coax as the negative return. Co-ax is not intended to carry significant DC current and may fry under the load.
From reading the interesting comments in Bob’s post, the negative line fuses could be for historical reasons due to some vehicles being positively grounded (i.e. ground was referenced to the positive battery terminal). Positive grounding seemed to be used to reduce the issue of terminal corrosion.
More reading
Bob references a looooong discussin thread on eham.net, and more useful article on the History of Vehicle Accessory Equipment Grounds, and an article from K0BG on wiring and grounding.
What am I going to do?
After all this, I’m going to fuse only the positive line of my expensive radio.
My portable operation electrical circuit is simple – there are no other devices connected to the battery, there is no risk of other devices developing faults and leaking current.
K.I.S.S.
I know I can buy these cables for 12-15 quid, but I prefer to make my own.
I like that I learn more, and get I satisfaction. I’ll make my power cable using inline fuse holders (12AWG wire).
Leave a Reply