Hello, Pieter.
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I'm new to this discussion but not new to the MicroBITX.? I run a version 5 radio on a 16v internal battery bank - two sets of four, with a battery-management board in place.? This yields over 15 watts output on 40m CW, around 10w on 20m & 15m, and about 5w on 12/10m.
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I gather from commentary so far, that your 7805 is overheating from having to regulate down from 13.8 (down to 5v).? I don't have that problem and it may simply be that I have a heat sink on that regulator but I also have (what I believe is) a 10-ohm resistor in line with the B+ supply to the 7805. Thus, the ideas presented above to add diodes to the input of this regulator has merit. Dropping about 0.6v X 3 might help your regulator work less than it done at present.? As diodes are fairly cheap, use 4 if you like, even 5.?
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I've got a few of the and have yet to substitute one for the 7805 - though I've experimented for the radio supply itself and noticed no added RF noise.?
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72,
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Ted
K3RTA
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Ted