There’s another case where a fuse in the negative lead is useful: imagine having a linear amplifier powered from the same source as the radio, and the amplifier negative is shared with the RF ground (which is common for most amateur linear amps). Now, what happens if for some reason the PA negative lead becomes disconnected - now all the amplifier supply current runs through the shield of the coax connecting amplifier and radio, through the radio PCB and finally back to the power source through the radio’s negative lead. A fuse in the radio’s negative lead will then prevent the radio from being fried .
Hi all,
Wouter you made me think. I agree, it is more complicated to provide protection for batteries and a radio with a linear. Once again Kirchhoff’s laws are our guide.
With a linear in use I think that each battery should have a high rated fuse to protect it and the linear. Then the radio should have 2 lower rated fuses, one in the positive and one in the negative. The positive line fuse limiting the rig fault current to protect against PA faults etc… The negative line fuse protects the rig circuitry if a disconnection of the linear negative line causes high currents to flow through the coax and into the rig.
My head hurts, clearly every case needs to be looked at very carefully, adding an auto ATU??
More than a fuse if it is placed in a right place.
I learn this trick from the HF-90 QMac, a 1A resettable fuse is inserted between the Source of the PA MosFet and ground. When the current approaching 1A, the fuse internal resistor rises up (of course, will open circuit at 1.5A and above). This rising value will force Vs Source voltage to rise, compared to ground, and as a consequence, it will decrease the Vg Gate voltage.
As Vg Gate is decreasing, power output will decrease too as how the MosFet works. So it is an auto bias, to keep power output in control. In case of malfunctioning, the fuse will open circuit, shut down the PA.
Andy,
Understood. It happens sometimes. Not all the Gremlins retired after the war.
To clarify for some readers:-
The problem with a vehicle arises when the starter motor is engaged. There is a significant volt drop across the cable from the battery negative to the car chassis and or engine block. The 200 to 400 A cranking current through maybe 10 milliohms is 2 to 4 Volts. The coax screen connects from the car chassis to the rig PCB which connects to radio’s negative lead from the battery. If the resistance in this parallel arm is 100 milliohms then maybe 18 to 36 A flows. If the resistance is less more current flows. There are various ploys used to avoid this and negate the need for the fuse in the negative line. Having a seperate battery for the radio for example.
It’s good pactice to have a heavy wire bonding an amplifier and/or ATU to the rig whether at home or in the field. Something I sometimes forget to do I admit.
We are all much smarter just after the smoke gets out.
In general, any “ancillary” equipment would need to have a fuse in the negative lead as well to protect from a “negative lead going open” scenario. I assume this is also one of the reasons for Elecraft to have the KX3 power leads (which do not come with any inline fuse!) connect to the KXPA100 in case both are used together.
Welcome to the SOTA reflector! I have a FT818 with a Bioenno 4.5a battery attached via power poles. This has been working great. I do not have a fuse. I believe the Bioenno has built in circuitry to prevent malfunction. I’m interested to see what others suggest in this setup.
Looking at my 817 power cable, (maybe a 3rd party one or Yaesu) it says 24AWG on the cable. If you go and check using voltage / current calculators it may not be possible to pull 24A down the full length of provided 817 cable. i.e. a 6ft long cable has a 12ft go and return length. Trying to pull 24A along that length suggests a voltage drop of 15V i.e. you can’t get 24A down the cable and so it may not be enough to trigger the protection circuit in Bienno battery.
You could use a shorter length of cable rather than the full length, you probably don’t need the full length. You could use a thicker cable run. But you still rely on being able to pull 24A down the thin cable for at least 2secs to cause the battery to trip out. That is questionable that it will work unless you have done some trip tests with the cell.
Also according to the Bienno manual, to reset the overcurrent trip you need to remove the load then connect up the charger momentarily. This doesn’t sound feasible when you’re up a mountain. A normal fuse that fails can be replaced with a spare on site. As I now have 2 fuses I carry 2 spare fuses sticky taped to the 817 case flap. And if I do something completely clueless and blow 3 of my fuses I can still short out a fuse holder with a bit of wire (or cut it out and twist the wires together) assuming I have found what I was doing that was toasting the fuses.
This may seem contrived and complicated and you’re right it is.
I missed seeing this after you posted. Thanks for the explanation. I have since ordered a double ended power pole connector with a fuse on pos and neg leads. I am using 4a blade fuses and it is working well. As you know the max FT818 output is ~2.5a TX so I thought 4a was the best amperage to select but I am only guessing. When selecting fuses for a radio is the proper way to select fuse size for protection 1-2a above max TX amps?
In most cases, yes you should run a fuse and in most cases yes I do run fused power cables. Having said that, I don’t presently have a fuse for my FT-817 as the battery is strapped to the outside of the radio and has only a couple of inches of power cable from the battery to the radio (with Anderson plugs). Calculated risk you could say.
I have had one scare with an unfused power cable - my very first SOTA activation (when VK1 association went live 9 years ago) I had a 7Ah SLA in a shoulder bag. The unfused power cable was connected to the battery terminals with the cable stuffed into the top of the bag and the plug unprotected. As I walked out and back into the activation zone (silly interpretation of the rules from back then), the plug was flopping around inside the carry bag and the centre shorted out on the negative terminal of the battery and I started to smell something burning at the same time that the bag became rather warm against my hip. I did the standing on an ants nest impersonation,dropping bags of gear, antennas, masts etc to get the battery bag off me and then managed to open the bag and pull the cable off the battery.
Result of that was a badly melted cable but fortunately I had half the shack with me (first activation where we all take too much gear) and was able to make up a power cable to enable a successful activation.Batteries have a lot of stored energy in them - that old 7Ah SLA was bad enough, I would hate to see what an unprotected lithium battery would do given the much higher instantaneous amperage they are able to deliver.
After a couple activations using the Bioenno (which is so far a great battery) I am thinking of going with a Windcamp internal battery. The Bioenno attached to the power port at the rear of radio prohibits easily moving around with the ft818 without putting stress on PowerPole connections. I removed the original NiMh battery so putting the Windcamp in should be relatively easy. Interestingly the original and Windcamp batteries do not use fuses. Does putting a battery inside the radio change the dynamics with respect to fuse use? Or is it that there is not enough room for a fuse and you have to just take your chances?
The Windcamp has a reasonable reputation in the 817 community - seems the knockoff copies have issues, but if you buy from the original sellers they are good. I use a 3S LiPo (2500mAh) from Hobbyking and it is way cheaper and much easier to swap over if I need a second battery. My 3S LiPo is tiny and I have used 2 elastic bands to hold it on the top of the radio - hasn’t fallen off in the 4-5 years I have had it setup that way. So for me, the much greater cost of a Windcamp is not justified but everyone has different needs I guess.
Some internal batteries will have small fuses, but most will not as there are a lot less things that can go wrong, so as a generalisation they are less necessary.
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