BOB Radcom

BOB pcb version 1.1

Bernard’s Overvoltage Board (named as such by my friendly local amateurs) is a module designed to sit between a 13.8V PSU and the equipment that it serves. It’s purpose is to react to sudden overvoltage situations by shutting down within microseconds and thus preventing subsequent damage to expensive equipment.

To read my original article you will need to get your hands on the RADCOM journal. The best way to receive RADCOM is to ensure you are a member of the RSGB.

A blank PCB and components are available from the author on a first come first served basis. You will find them here. When they’ve gone, they’ve gone !

The original BOB was enthusiastically received by members of the local radio club and it would make an ideal club project. The module can be built in to a PSU, or can go inline in the DC cable to your amateur radio transceiver.

To construct it, surface mount skills are needed but there are not too many parts and given the guidance of a suitable mentor, it might make an excellet project for those who want to learn those skills. There are two power MOSFETs on the board and they have solder pads underneath them which means that either a hot air rework station (available for £30 from Ebay), a hot plate (I’ve used a domestic £20 single plate one from Argos in the past) or oven are required for proper assembly. All other components can be soldered with care using a soldering iron.

As an aid to the constructor I have packaged all of the CAD files in a zip ready to be sent to a PCB fab house of your choice.

If you would rather that I send you a blank PCB then you can get them here. You’ll also want this project bill of materials:

BOM
** These resistor values can be varied to change setpoints

As outlined above, several resistors should be chosen by the builder to select the Undervoltage Setpoint (below this input voltage there will be no output voltage) and the Overvoltage Setpoint (above this voltage there will be no output voltage). It is recommended to use 1% tolerance resistors.


Selection of resistors for overvoltage setpoint

Selection of resistors for undervoltage setpoint

Fuses and the Bob Overcurrent Set-point.
 
A 20 amp power supply is usually capable of supplying three times that current for at least a few milliseconds if the load demands it.  That is why without protection, load equipment circuit board traces can evaporate and semiconductors rip themselves apart when faults arise.  

We typically protect a 20amp transceiver with a 25amp fuse.  But what does that actually mean?   Well a 25amp fuse when subjected to 50 amps will typically blow after anything between half a second and several seconds.  See the table below. 

As many of us have found out to our cost, that can be quickly enough to protect the power supply and avoid a fire, but not quickly enough to protect the load i.e. our transceiver. 

BOB's prime purpose is to provide super-rapid overvoltage protection but the LTC4368 overcurrent protection circuitry can be useful too - if we understand how to use it.  

Current exceeding the set-point causes power shutdown. But the LTC4368 is fast - very fast.  The set-point must take into account inrush current of the load equipment or the very act of turning the equipment on might cause the LTC4368 to shut down.

Primarily the inrush current comes from the initial charging of the many capacitors in parallel found inside a typical HF rig.  They may not be high value capacitors, but they are of low ESR and they are in parallel.  The overall ESR can therefore be miniscule. Inrush currents can be very high, even if only for a few microseconds.

Let's look at an interesting experiment that I have conducted several times. 

Take a good quality power supply capable of sourcing 20 amps and that contains a 25 amp fuse.  Connect it to a dead short.  Unsurprisingly the 25amp fuse blows (and we hope the power supply survives but don't carry out this test yourself!). 

Now try it again, but short it out with a forward biased 1n4148 signal diode.  Guess what?  The diode blows, the fuse doesn't.  The diode "became" the fuse.  That diode may well have been a component inside your rig.  Again, it is better that you don't actually try this one but you can easily understand how it works. 

Now repeat these tests with a BOB having a set-point of 50 amps.  Connect a dead short.  Boringly, nothing happens other than BOB switches off the power. 

Now try the experiment with the 1N4148 small signal diode.   Short out the BOB with that.  Guess what?  Boringly still nothing happens.  BOB switches off the power before any damage occurs to the diode i.e. phenomenally quickly.  

If the BOB setpoint is made 50 amps we provide both  immunity to most inrush current tripping yet we also make it unlikely that internal rig faults can propagate to cause trace burnouts and other irreparable damage.

The problem with traditional fuses. In a nutshell they are slow and unreliable compared t othe circuitry that they are supposed to protect. They often save the power supply but not the load.

Fuse under testFuse Rating (A)Time to Blow at stated ratingTime to blow at 200% ratingTime to blow at 400% rating
SinglFuse™ SF-3812F-T Series 25A Possibly Never 60 Seconds Not Stated
Multicomp Fastblow25 APossibly Never4 minutesNot Stated
Cooper Bussman 500 Fast Acting15APossibly Never2 minutes3 seconds
BEL SMD Fastblow25APossibly NeverNot Stated5 Seconds
LittelFuse ATOF® Blade Fuses 25APossibly Never5 seconds500 milliseconds
Overcurrent Setpoint Resistor Choice. See text.