[time-nuts] DC-DC converter

Hal Murray hmurray at megapathdsl.net
Sat Sep 26 21:20:25 UTC 2009

> Of course, another problem is that the Z3801A doesn't use 48V. It
> takes more voltage, from K8CU's site: The Z3801A is specified by HP
> (depending upon model) as  either a nominal minus 54 or plus 27 volt
> device. The best bet is to keep the voltage at the nominal value
> specified by HP.  This way any out of specification internal DC to DC
> converters  are satisfied, and the units will perform normally.
> Reports  from other users confirm this. 

I think that's misleading.

The DC-DC units inside the Z3801A are designed to take a wide input range.  
They are happy with 48V.  If they don't work at 48V, it's not an out of spec 
issue, something is broken.

The problem is that they need enough current during the startup transient.

If you are building a simple home-brew unregulated supply, it is probably 
simpler to aim for 54V under normal load so that you will have some room for 
it to droop under the extra startup load.  (Ugh.  You also have to consider 
what happens with low and high line voltage.)

If you are buying a regulated supply, 48V is fine and much easier to find.  
It just needs to put out 50W rather than the 25W in the manual.

I don't think a 25W 54V (regulated) supply will work any better than a 25W 
48V supply.

If your 48V supply is right on the edge of it's current limit, you might 
squeak through by adjusting it up to 54V.  The regulators inside the Z3801A 
really need power not current.  If you boost the input voltage by 12% that 
will drop the current by 12%.

Feeding regulated supplies is tricky.  If you reduce the voltage feeding a 
normal (resistive) load, the current goes down.  If you reduce the voltage 
going in to a regulated supply, the current goes up.

These are my opinions, not necessarily my employer's.  I hate spam.

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