Fix: Reverse polarity


Reverse polarity is damaging to your uBITx.  You may think that you will never hook up 12v the wrong way round, but none of us is perfect.    It is also helpful to protect your rig from shorts (when modding the rig) and from IRF510 thermal runaway or parasitic oscillation.

There are number of ways to increase reverse voltage and current overload protection:

  •  Install the 1N4007 diode as a crowbar for reverse voltage protection that comes with your uBITx.

This relies on the fuse in your power supply to burn out if the power supply is connected the wrong way round.   The diode will conduct if the voltage is reversed.   Within seconds the diode will burn out, followed by every active device in your rig.  If this fuse is more than about 3A, you won’t get any real protection as the fuse in the power supply may never blow.  If the power supply is a 20A supply with a 25A fuse, then you probably have no protection with the default solution supplied by HF Signals.

  • Install a fuse in your µBITx before the crowbar diode (diagram thanks to MVS Sarma)


In fact, it may be a good idea to install two fuses: a 0.5A fuse in the supply line to the rig, and a 2A or 3A fuse in the supply line to the finals, since these are separated on the power connector.  The centre wire of the 3 way connector provides power to the finals.   A range of options exist for the fuse holder (in line or panel mount).  These are so cheap out of China, order 10 of them as you will then use them in every project and a box of fuses of different amperages.  

An alternative to a traditional fuse (fast blow preferred), is a resettable fuse.  You don’t need to replace the fuse.  Just turn off the rig and turn it back on again.  These may, however, take a while to “blow”, and damage can be caused in the mean time.

  • Use a polarised relay

Raj VU2ZAP recalls, “In some Yaesu rigs of the 80s the power was switched on by a POLARISED relay. It will not latch if the  polarity is reversed. This is a possible no voltage loss solution!    A diode can be used in series with a regular relay too.”

The downside of using a relay is that it will draw around 25-40mA on receive.  This may not be the right solution for a portable rig.

A relay can also be used to switch between “mains” and “battery” power (using two separate power supply jacks).   The relay turns on when mains power is connected, otherwise the battery supply is used.

Use a standard relay

With four components, and the on/off switch, Bill K9HZ provides the ultimate reverse voltage and over-current protection system for your µBITx.  The fuse protects the circuit from excessive current draw – so you won’t blow your finals when you wind up the bias too far and they try to go into thermal runaway.  The relay must be powered on to power your µBITx (and the switch must be turned on).  With the series diode in place the relay cannot turn on unless the power is wired up correctly.

Use a P-FET

Use a P-FET like the IRF9540 for a nice simple solution (with thanks to PH2LB).