Wednesday, March 14, 2018

Beyond Astron Revitalization: Protection..

So we got here because the genius that I am thought it would be cool to re-purpose an Astron supply to charge batteries as well as provide power for radios I was checking out. Sadly, when the mods were done, I zapped the PS and launched the series of posts below to restore it to normal. Still faced with the prospect of making my contemplated mods non-lethal, I sent the following email to my dear friend and trusted resource, Dick, WB6JDH:

I finally got this beast squared away -- as in back to normal before I modded and zapped it. But I have a question regarding my mods and how to prevent a re-occurrence on the problem; Here is the product I am using to display the battery charging aspect of the PS:
As you can see, it has a schematic on the back of the device. Also, it is shunted for 20A -- hence the circuit shown above. However, I am not putting this "in series" with the whole power supply in order to have it show current draw as I am downright nervous about running an actual 15-20 amps through this meter; I am not willing to push it.

I will be using the PS to do two things:

(1) Power 12VDC radios that I am testing -- which I will do on reduced power and well under 15 amps.

(2) Recharge my gel cell batteries -- which draw only about 2 amps max.

The gel cells will plug into the front of the PS as shown below. The normal back post terminals will power radios being tested and the meter will be switched out of the circuit by the front panel switch.
So here's the question: is it sufficient to put a diode on the positive line to the banana jack in front to prevent battery back washing and protect against reverse polarization and frying my pass transistors as I did before? Or should I also add a reverse polarization diode across the lines as well?

Some guys on the repeater builder's site have described putting a diode of sufficient size across the C-E junctions of the pass transistors but I felt that since the battery charging part of the circuit would be switched in when used, that might be overkill.
Your faithful servant, etc.
Anyway, in preparation for Dick weighing in on this, I did some research and came up to the realization that merely slamming a diode into line solves the problem by incurs a 0.7 VDC drop and, in the case of the charger, would force me to run the PS at about 14.5 VDC to effect gel cell charging. This would, of course, might not be so peachy for the 12 VDC radios I want to bench test. So, in studying alternatives, I came across this video:

Now, the first and third methods are flat out unworkable as I want to keep it simple. The diode across the power leads with the fuse in line appeals to me as being a direct solution. The downside, of course, is the prospect of mounting a fuse somewhere. If it is inside the PS then I'd have to unscrew the case every time I sinned, But, then again, that would teach me a lesson.

This video explains a more sophisticated solution and I might consider that:

..stay tuned, still pondering.

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