# Maximum DC supply voltage for USRP

What is the maximum DC supply voltage for the USRP?

I’ve been running it off of a 5V switched supply, but I wanted to move
all power
supplies outside of the USRP cabinet that I have, so I’m using an
external 13.8V
supply, with a LM7805 regulator. The regulator is sagging due to high
current
demand, so I’m thinking of supplying 13.8V directly to the USRP–is
this safe?

Marcus-

What is the maximum DC supply voltage for the USRP?

I’ve been running it off of a 5V switched supply, but I wanted to move
all power
supplies outside of the USRP cabinet that I have, so I’m using an
external 13.8V
supply, with a LM7805 regulator. The regulator is sagging due to high
current
demand, so I’m thinking of supplying 13.8V directly to the USRP–is
this safe?

The LT1085-3.3V linear regulator on the USRP is rated for 30V maximum
input.
However, nominal specs (voltage dropout, max current, etc) in the
LT1085-3.3 data
sheet are only given up to 8V input. My suggestion would be to limit
the input
supply to 8V. The greater the delta between input and 3.3V the more
excess power and
heat you are forcing the LT1085 to dissipate.

-Jeff

On Tue, Jul 31, 2007 at 04:07:14PM -0400, Marcus L. wrote:

What is the maximum DC supply voltage for the USRP?

I’ve been running it off of a 5V switched supply, but I wanted to move
all power
supplies outside of the USRP cabinet that I have, so I’m using an
external 13.8V
supply, with a LM7805 regulator. The regulator is sagging due to high
current
demand, so I’m thinking of supplying 13.8V directly to the USRP–is
this safe?

No, the on board regulator can’t dissipate the resulting power.

You really should feed the USRP with 6.0V, not 5.0V. Some
daughterboards take 6.0V and produce a local 5V analog supply from it.
In a pinch, you could probably feed it 5.5V, but I wouldn’t go any
lower.

Eric

Eric B. wrote:

No, the on board regulator can’t dissipate the resulting power.

You really should feed the USRP with 6.0V, not 5.0V. Some
daughterboards take 6.0V and produce a local 5V analog supply from it.
In a pinch, you could probably feed it 5.5V, but I wouldn’t go any lower.

Eric

I found some 7808s, so I’m feeding the USRP with 8V now, and it seems
happy (much happier than
when I was feeding it with 4.97V!!).

Marcus L. wrote:

What is the maximum DC supply voltage for the USRP?

I’ve been running it off of a 5V switched supply, but I wanted to move
all power
supplies outside of the USRP cabinet that I have, so I’m using an
external 13.8V
supply, with a LM7805 regulator. The regulator is sagging due to high
current
demand, so I’m thinking of supplying 13.8V directly to the USRP–is
this safe?

No, it isn’t. The USRP itself uses a linear regulator, so putting 13.8
V in will result in more than 10 additional watts being dissipated.

Also, many of the capacitors on the daughterboards are sized for 6V
input, and might blow with more than 10 V.

Matt