On 28 Oct 2003 05:31:53 -0800, (Jim Woodward)
wrote (with possible editing):

I'm afraid this is a general problem. The vast bulk of the market is
so price conscious that those of us who would pay more for quality
can't find it.

It's particularly pervasive in the rec.boats market because many
boaters use their boats only ten or twenty days a year, so a product
with a life of 360 days, which would be a disaster in a home, does
fine on most boats.

DC switches are a prime example of this. DC is hard to switch, so
good switches are much more expensive than AC switches. We've
discussed the Rule switch, but their competitor, Jabsco, does the same
sort of thing with their pressure water control switch. It's a
Honeywell MicroSwitch (tm) with a good AC rating, but, IIRC, a 0.5A DC
rating. Guess what -- the pump draws 5 times that, so the switches
fail predictably after a couple hundred days of use. This is a long
time for the typical rec boat, but a nuisance for the live-aboard. A
satisfactory substitute is hard to find.

Since those of us who live-aboard are a very small market, our only
solution is to share experiences and solutions here and elsewhere.
You're probably dreaming to hope that Rule will do a re-design -- they
have their cost of manufacture, their MTBF, their average customer
use, and their warranty life calculated very carefully and their
business model calls for being market leader, not quality.

Jim Woodward
www.mvFintry.com

Jim is right - DC is much harder to switch as you can't use triacs or
other solid state switches. FWIW, what I would do is to have whatever
switch you use, switch a 12 volt relay with contacts of adequate size
for the amperage drawn by the pump, instead of switching the pump
directly. I would use a pluggable relay and mount that up where it
was easily accessible, then keep a spare on hand.

--

Larry
Email to rapp at lmr dot com

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.

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)

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

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.

Actually MOSFET power transistors are readily available with 0.01 ohm of on
resistance, no junction drops and only microwatts required to drive the gate.
They would consume less power than the wires to the pump do -- very nearly as
little as the mercury switch does.

Using them would just cost more to build however and the manufacturers
apparently don't think they successfully could charge more for a more
reliable pump switch.

Sigh....

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