[hpsdr] LPU 5V supply

[Graham / KE9H]

David:

As long as you have some protection (a 5 A fuse, or a current limited power
supply) upstream to protect the feedwire and PCB, then it would be 
reasonable
to just short out the 2A fuse, as you described.

--- Graham / KE9H

==

David McQuate wrote:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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
> wa8ywq
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