[hpsdr] LPU 5V supply

[Scott Cowling]

Hi Dave,

The recommended solution (well, I recommend it anyway :-) is to short 
out F2. The regulator supplies sufficient overload protection.

When adjusting the value of R6, be sure to use the peak current and 
not the average current that you are drawing. Otherwise you may get 
output voltage dropout, depending on the duration of your peak 
current draw.  Looks like your 2.7 ohm value would work with 11V 
input, even at 2A output current.

Don't forget the tolerance of the resistor when cutting things fine. :-)

GL es 73,
Scotty WA2DFI


At 13:44 2010-07-04 -0800, 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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