48V relay question [RE-wrenches]

[Darryl Thayer]

Hi
Here is what I would do.  
First I would not use the float switch to control the
load, as you suspect it might be short lived.  

All AC relays will work on DC except that you need a
resistor in series with the coil because you need less
DC voltage.  Choose a relay with the contact ratings
you want, find the coil current data, then measure the
coil resistance with an ohm meter.  You want the same
current when operating with 48 volts DC.  Use ohms
law, E=I*R; that is E/I=R so 48volts/coil current=(R
of coil+ R of added resistor)

Daryl

--- Allan Sindelar <allan at positiveenergysolar.com>
wrote:
> 
> Fellow Wrenches,
> Here's a fun one that I could use advice on, please:
> 
> We are installing a 48V full Outback PS2DC system
> for a new offgrid
> residence. The client already had an ETApump 48V
> PV-direct deep well
> installation, with an array of 7.9A @48V nom. The
> well pump is feeding a
> storage cistern, with a separate 48V Booster Pump on
> a DC line from the home
> power system for its power.
> 
> I would like to use the float switch to actuate a
> relay that directs the
> well pump array through an additional pair of
> conductors to boost the home
> array input. This way, in winter when water use is
> low, this array adds to
> battery charging capability once the cistern is
> full. I would like this to
> be automatic, hence the relay. Switching to
> battery-based ETApump operation
> is not an option.
> 
> 48V relays are hard to find, because standard
> coil/contact units have arcing
> problems at higher DC voltages. I can use an MDI
> 48V-coil mercury contactor
> (as used in the old APT Powercenters) but it's a
> single action relay
> (normally open is standard), and I need both NO and
> NC to easily direct
> power between two loads.
> 
> Is there a way I could use the ETA reverse-action
> float switch circuit to
> control this? Can the float switch also power a
> relay coil, so that
> close-on-rise also activates a 48V relay? Could I
> bypass the float switch
> control in the ETApump controller, and simply let
> the float switch divert PV
> input power away from the pump when the tank is
> full? Could I use a
> dual-action float switch (3 wires, both NC and NO
> action in the same switch)
> to control a NO relay?
> 
> Any good ideas would be appreciated. Must be simple,
> reliable, and
> foolproof.
> 
> Thank you,
> Allan @+E
> 
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