On Tue, 28 Oct 2003 18:39:14 GMT, Bill Martin
wrote (with possible editing):

Jim is right - DC is much harder to switch as you can't use triacs or
other solid state switches.

Actually you can. They're called switching transistors and they predate
triacs and SCRs and work quite nicely on DC. Anything from tiny integrated
devices to big honking power transistors depending on the load involved.

No argument intended: yes, you can use switching transistors, but they
do consume a bit of power which they translate into heat which must be
dissipated. Switch contacts are largely loss less, but relays do
consume power, although I'd venture to guess that it is less than is
dissipated in switching transistors.

The other problem might be the slight forward voltage loss created by
a semiconductor junction. Probably less of an issue.

One problem with a semiconductor approach is that the simple mercury switch
takes two wires to run -- the +12v and the wire to the pump for example. A
semiconductor replacement switch would typically take three wires -- +12, the
pump and ground. People don't have their boats wired that way and probably
typically just look for a drop in replacement which won't require rewiring --
even though it's a minimal change.

And of course even with a transistor switch you still need a level sensor of
some kind so it's much cheaper all in all to build with a mercury switch.

My main gripe is why in the @^%$#& do they insist on saving 25 cents by under
sizing the mercury switch? They could easily put in a larger unit with
heavier internal contacts and add some rudimentary RLC snubbing circuitry to
minimize the flash and make it much more reliable for an extra buck or two of
manufacturing cost.


... switch a 12 volt relay with contacts of adequate size
for the amperage drawn by the pump, instead of switching the pump
directly...

This is probably good advice if one is having trouble with float switch
reliability. Though instead of a 12v relay one could get a power transistor
and...

Bill -- (Remove KILLSPAM from my address to use it)


--

Larry
Email to rapp at lmr dot com