Larry wrote:
"Offbreed" wrote in
oups.com:

I cannot relocate the circuit,


It's quite easy, actually.....

SCR cathode to 12V negative bus

SCR gate to 10K ohm 1/4 watt resistor to limit its gate current to a
safe level. The other side of the resistor hooks to the higher float
switch, whos other lead is hooked to +12V. The resistor limits gate
current to 12/10000 = 1.2ma. The anode of the SCR is hooked to one lead
of the lower float switch. The other lead of the lower float switch is
hooked to the negative lead of the pump motor.

Here you have the entire load going through the lower float switch, and
I was hoping to avoid that. One of the advantages of the original
circuit was that the lower switch was not exposed to the sparking that
would cause.

The positive lead of the
pump motor is hooked through the appropriate inline fuse directly to the
house battery + post to prevent some idiot from turning off the bilge
pump! NO BILGE PUMP SWITCHES PLEASE!

As the water rises, the lower switch closes, powering up the SCR anode,
but without a gate + the SCR doesn't fire. As the water flips the upper
switch, the +12V applied to the resistor brings the gate current up
above the microamp-level firing current and the SCR gates on. Once
fired in a DC circuit, the SCR will not open just because the upper
switch opens and it loses its gate current. Current runs the pump until
the water level in the bilge drops to open the lower switch which shuts
down the pump, and ungates the SCR because it has no Anode current, now.

The circuit won't cycle, again, until the lower, then the upper switches
both close to gate the SCR, again, as the cycle repeats at a much slower
rate.

Use a powertab 8A SCR and just screw it into the wood somewhere way
above the waterline, even out of the bilge if you can to prevent
corrosion. Run a gate lead out of the bilge to the 10K resistor
soldered directly to the SCR gate lead to support it. As this gate lead
will be hooked directly to the 12V plus, make sure the power to the
upper switch hooks to the motor side of the bilge pump fuse for safety.

Now you have a true hysteresis bilge pump. AT 4A, the SCR barely gets
warm

In an emergency, just jumper out the SCR and you have a simple bilge
pump with the lower switch.

Okay, many thanks. I added the technical part of what you added in the
correction to make it a single post (that part about the DC motor
shutting it's self off would have driven me nuts). :

Oops....I forgot to mention a problem with DC motor driving with this
latch. DC commutator motors don't draw current through the whole
rotation, just when the commutators make contact. So, you'll be feeding
pulsating DC to the SCR, which WILL ASSUREDLY make it unlatch! The cure
to the problem is to put a 100 ohm, 5 watt resistor right across the
motor, so that when the motor commutator opens, the SCR has a holding
current through the 100 ohm resistor to keep it latched, insuring a
steady flow of holding current until the lower switch opens.



Eh, well, sexy voice, sexy body, and sense of humor are a combination to
be treasured. Your friend was very lucky.