Tuesday, December 19, 2017

Why you need an In-Line Fuse on the uBitx Power Cord?

Protect your uBitx Transceiver!

The uBitx package as it arrives has just about everything you need to put the amazing rig on the air --sort of like right out of the box. While not recommended for long term use it can even be hooked up 'al fresco" right on the bench and you are good to go.
But knowing well myself and my propensity to smoke parts, I usually go the extra mile to arrange things to make them "Pete Proof". Reflecting on my earlier statement about having everything there in the package as sent from India, I did note that my Bitx40 acquired just a year ago had one additional item -- an In-Line Fuse assembly. the uBitx kit did not have that piece.
Being somewhat in a hurry to get my uBitx Inhaling and Exhaling RF I simply grabbed the power cord from the Bitx40 and used that for the new rig. That was a fortunate action on my part sometimes known as dumb luck.
So here is the why that you should have an in-line fuse on your power cord:
  1. The uBitx includes a diode that is soldered across the power connector so that when the power is connected properly the diode is reversed biased --no issues there. But if you reverse the leads and have the power being fed backwards the diode conducts to ground and depending on how many amps in your supply you will smoke the diode and hopefully trip power supply --but if you are using a 30 amp supply like in a home station, once the diode is crisped beyond recognition and if it opens up --you will have reverse voltage applied to your beloved uBitx. The in-line fuse using a value like 2 Amps Fast Blow will open as soon as the diode starts to conduct and your rig will be safe.
  2. But the in-line fuse has another important use which I discovered when I tried an interesting maneuver. I have what I call a 40 Meter droopy dipole out in the back yard. Actually that dipole is 98.6 feet long and by using an antenna tuner am able to run that dipole on bands like 75M, 60M and 40M. The magic number of 98.6 has to do with an extended dipole length actually 3 half wavelengths on 20 Meters where it was initially used --yes it is a gain antenna on 20M. That was used before I had the beam. So I thought I would test the uBitx on 75 M and using CW as the tune signal attempted to tune the antenna tuner on 3.805 MHz. As I started the dip and load -- the uBitx went dark. Ouch what did I do now? There was no voltage on the power connector at the back of the chassis yet there was voltage at the supply --well folks that has to be an open fuse. Sure enough the fuse was blown. So important step here! When something like this happens don't put in a new fuse and just recycle things. Take time to ask why did it happen. The issue is current draw on high SWR conditions. Since there is no foldback current protection of the IRF510's that would sense high SWR and thereby reduce the power level until the condition is corrected --you need something else! The fuse will serve that purpose. You might have to put a 2.5 Amp Fast blow in there --but no 15 amp fuses!
This is what I did.
Save your finals --yeah I know they only cost $0.69 a piece but a fuse would cost you less than that.
Pete N6QW


  1. I posted that diode problem on the Yahoo group. Your right about reverse voltage on the tig once the diode blows. Just install it so the anode goes to the 12 v supply.

  2. Thanks for the tip! Would a 2A slow blow fuse be usable in this application? (I have some of those already.) Not sure how quickly those release their magic smoke vs. the IRF510's.

  3. A slow-blow fuse is about never correct for a radio environment. The magic smoke will not run, but STAMPEDE past the gate by the time your fuse blows. Yes, it'll probably do a fuse's primary purpose is: protect your shack or vehicle from a fire due to radio failure, yet, whatever caused the failure will grow many times over before that occurs.

    Just don't.... Fuses are really cheap compared to most parts you may have to replace including the whole rig.

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