In article ,
Offbreed wrote:

Well, yes, that can be a problem, but I think SCRs can be shut off by
changing the bias of the gate circuit.

I don't think the above is the case for SCR's. Once triggered into
conduction, they stay in conduction untill the current flow thru the
device is interupted external to the device. All the SCR's that I
have ever used seem to work this way. You mileage may vary.....

Me

Again, this won't help your particular situation, and I do not have a
schematic, and lastly, it's an AC motor, but:

My latest (home) sump pump doesn't use float switches, but merely
sensing wires, one high level and one low level.
When water level reaches the tip of the high level wire, the pump is
started, and after the water level drips below the low level wire, an
adjustable timer is started to run the pump for a specified time longer.
Right now, I have the adjustment set for the pump to run about half a minute
after the water drops below both wires.
This way, the back wash down the discharge hose doesn't have volume
enough to start the pump again.
Again, an AC system, and would be simpler to design than your 12 volt
DC situation, but the water just contacting the wires to toggle flip-flop
IC's is a good start at eliminating float switches entirely.
Old Chief Lynn

Me wrote:
In article ,
Offbreed wrote:

Well, yes, that can be a problem, but I think SCRs can be shut off by
changing the bias of the gate circuit.

I don't think the above is the case for SCR's. Once triggered into
conduction, they stay in conduction untill the current flow thru the
device is interupted external to the device. All the SCR's that I
have ever used seem to work this way. You mileage may vary.....

I misread what someone wrote about a home brew phone circuit, and got
the gate current mixed with the holding current. The holding current in
his circuit was not enough to keep the SCR conducting so he had to keep
a bias on the gate, with the first SCR he tried, one I have on hand, so
I thought I might be able to use it on the pump.

My error.

Interesting discussion .

But why not simply use a Johnson float switch, the type with a rolling ball
inside the float. These are designed to do exactly what Offbreed wanted,
with less to go wrong.

Alec


"Offbreed" wrote in message
oups.com...
A long time ago, I saw a circuit using a silicon controlled rectifier
as a latching relay to control a bilge pump. It used two float switches
(one mounted higher than the other) and both had to be up to start the
bilge pump. The pump continued to run until the lower float switch
fell.

The advantages were that the back flow from the hoses did not cause the
pump to start again, and the low current demand allowed the switches to
last longer.

I cannot relocate the circuit,

Does this ring any bells? Anyone know where to find the diagram?

The only circuits I can come up with have the full pump current going
through the lower switch, unless I go to a mechanical relay.

Alec wrote:
Interesting discussion .

But why not simply use a Johnson float switch, the type with a rolling ball
inside the float. These are designed to do exactly what Offbreed wanted,
with less to go wrong.

Mail order allergy, and never heard of them.

"Johnson float switch", eh? Thanks, I'll look into it.

Offbreed wrote in news:3MqdnaLc4IVazCLeRVn-
:

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.



Won't matter. You'll use a Rule float switch that has a mercury switch in
it so it won't ever pit up or corrode. Mine never has....

There's not that much current, only 4-5A, anyways.

Rule switches I got. Mercury, eh? Thanks.