[hpsdr] LPU 5V supply

[David McQuate]

When running Mercury 3.0 (Multi-Rx) the LPU 5v supply drops out after a 
minute or so.
Mercury 3.0 + Ozy 18 requires 1.4A (on my system)
Mercury + Ozy + Penelope + Excalibur requires 1.6A.

The stock fuse, F2, is a Bourns MF-MSMF110/16-2, rated at 1.1A holding 
current at 23C and a maximum voltage of 16.  Bourns two higher current 
resettable fuses--
model                         Vmax  Holding current
MF-MSMF150/24X    24     1.5 A
MF-MSMF250/16X    16     2.5
--These are larger, but it seems to me that they could be soldered onto 
the LPU's pads for F2, with careful placement.

The alternative is to simply short out F2.  The 5v regulator, U2, has 
built-in protection against overcurrent, overvoltage, or a short 
circuit, and thermal shutdown, so, as long as U2 is working, the fuse is 
not necessary.

The value of resistor, R6, in series with the input to U2 can be 
adjusted, if necessary, to reduce power dissipation in U2, while 
maintaining adequte "headroom" to prevent dropouts.  U2 needs only 0.5 
volts input-output differential at full rated output current (2A).  If 
the LPU input were to sag to 11V--say because it was running from a 
nearly discharged battery-- and the 5V regulator current were 2A, R6 
(2.2 ohm) would have 4.4 volts across it, and the U2 input would be 6.6 
V--well above the necessary 5.5 V.  Seems to me R2 could be as high as 
2.7 ohms.

Dave
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