Are you protected? This is not about life insurance or about a date with a girl friend. The post for today deals with protecting your circuits and not what you may have been thinking.
Circuit protection is likely the last item on your list or perhaps as often happens not at all. We must strive to be precise and correct for the nit pickers out there so let me bound what I mean by circuit protection.
Broadly speaking the circuit protection involves simple things like your wiring when you try to stuff four pounds in a three pound box. Get a grip, unless you purposefully think about a possible short to the power rail and physically keep wiring away -- IT WILL SHORT!
That was a lesson learned while I was trying to service one of my shirt pocket SSB Transceivers. Similarly think about it when you put the rig in a box -- will there be a short when you screw the top on. Thus, one aspect is the physical hardware.
The other broad category is the hardware functionality. We will start first with reverse polarity protection. Raise your hand if you have never connected a circuit to the wrong polarity. At times you only smoke some of the devices while for other situations there is the giant mushroom cloud over the workspace followed by that awful smell.
We are treading on thin ice here as discussing reverse polarity protection is not unlike the current polar divide between the Democrats and the Republicans. There is an array of solutions for RPP (Reverse Polarity Protection). They range from polarized plugs that only mate with your power supplies to included circuitry to prevent that possibility.
Many hams simply use a power diode in series with the Plus lead to the rig. That works but has one small drawback. You will drop 0.6VDC across the diode junction which if operating portable from batteries as the batteries discharge is significant. When your batteries themselves drop to 11VDC -- you are only seeing 10.4VDC at the rig.
I have used the diode with an added twist. The diode feeds the field coil of a small relay. The source voltage is also connected to a set of normally open contacts on the relay. Thus, wrong polarity and the relay does not close and no juice to the rig. This avoids a voltage drop to the rig caused by the diode BUT does add to the current draw on the batteries. With a bit of searching, you can find small relays whose contacts will handle 2 or 3 amps and their field coil has a low draw. Others might suggest the use of polarized relays thus no diode needed.
I first used this circuit on a project to take the Hallicrafters FPM 300 and rework it to be solid state. (Yeah, I do weird things like that)
Re-birthed FPM300
It was near midnight and RPP was on the list as the last item. Thus, the idea sprang from what I grabbed from the junque box at that time of night. I got several emails about my archaic solution and why wasn't I using a PFET power switch and on and on. Look it was midnight, even Amazon couldn't get me something quicker and it works!
Another piece of protection is to add a fuse, and this is for protecting your finals. In days of old many linear amplifier circuits included fold back current protection. Without this protection and especially on bands like 75/80 meters where having a High SWR will cause an excessive current draw and you could melt the RF devices.
I had a uBitx V3 and when I first got it, I saw there was no fold back protection and that a simple in line fuse set at the right value could save those finals. Now the right value is a high enough amperage, so you don't have nuisance tripping but low enough if you draw more than normal current -- she blows.
I posted my suggestion on the bitx20 reflector where I received a startling comment. The commenter said he read on the Internet that the IRF510 was "bullet proof" and why was wasting their space and time about including a fuse. This lurker must be an Illuminati. I put the fuse in at the get go and sure enough on 75Meters I blew the fuse and not the rig.
Another protective circuit which can be external is the CROWBAR. This has seemed to have lost its popularity but was a staple in many of the Ten Tec Supplies like the Model 262. Essentially the crowbar is a voltage sensing circuit between the supply and your rig. In that circuit is a SCR and a Zener reference diode. As long as the supply voltage is less than the firing point of the SCR -- all is well. The microsecond that the source voltage hits the firing point the SCR goes to ground and the fuse blows. While most solid-state circuits sourced as 12 or 13.8VDC usually will do OK to about 15VDC - after that you have successfully completed a production of smoke test. ZD1 sets the firing point in this case 9.1VDC. For a Ham supply, 14.5VDC would be a good choice.
If the new hardware you are working with has a significant investment in parts, then RPP, a Fuse and a Crowbar are cheap protection. These three approaches however will not provide any protection with the girl friend! It is always a case of the right tool for a proper job!
So, it has been a boring trip but there just might be some piece in there to protect your rig that you just built.
73's
Pete N6QW