Float Switch/ Electric Question

[Dyraxe]

I have this float switch.

http://www.aquahub.com/store/ifloatfloatswitch.html

I want to make "Meleve's auto top off" and run the power cord 20 feet away hooked to bulk reef supplies 50ml doser. My question is: is this safe can it be done? What size wire should I use? Will the doser have too many amps? etc. etc. etc.

I really do not want to burn my house done. Thanks for looking!

 

[Psychographic]

I personally think it's unwise to run household current through one of these switches, the contacts on them are just too weak for it. I would use a relay and a 12vdc power source for safety. Nothing worse than a flood should the contacts stick.

 

[billsreef]

According the specs it is rated for up to 220VAC and 50watts. So it shouldn't be a problem running directly on 110VAC and the low wattage of the doser. Besides, it's the wattage and the amps that heat up the contacts and cause them to burn up, not the voltage For a 20 foot run, I would bump up the wire gauge to 16ga.

 

[Dyraxe]

Thank you guys. I used both your ideas and ran with it. I got an auto top off premium kit to reduce the volts in the relays from the same place,
http://www.aquahub.com/store/product26.html
as well as larger wire.

 

[Psychographic]

According the specs it is rated for up to 220VAC and 50watts. So it shouldn't be a problem running directly on 110VAC and the low wattage of the doser. Besides, it's the wattage and the amps that heat up the contacts and cause them to burn up, not the voltage For a 20 foot run, I would bump up the wire gauge to 16ga.

I have never heard of a switch sticking when run 12vdc through a relay, I have on the other hand read of them sticking when 110vac running through them. The problem with these switches is how tiny the contacts are, think about how thin they have to be in order for a small magnet to attract them to complete the circuit. Now have them arc upon contact multple times a day over a period of years.

Would you risk your tank flooding over an extra $10-$15 to add a relay and power source? Also if you look around, more than one vendor recommends the relay and low voltage setup even though their switches are rated for 110.

 

[billsreef]

I've had to replace numerous 12VDC relays over the years Even they can burn out. Typically it's the high amperage side that either welds itself closed or looses connectivity from burnt contacts.

 

[BeanAnimal]

Float switch (or relay) contacts are damaged due to the electrical spark (called an ARC) that forms when the contacts are pulled apart (or before contact, as they are pushed together) while energized. The arc intensity and duration of the arc are directly related to the voltage and current available.

120V at 50W is .42 AMPS... not that big of a deal. The possible problem is that we are not breaking a resistive load but rather an inductive load (a motor). The stored energy in the motor's coil needs to dissipate when the power is turned off, it does so by attempting to travel backward through the float switch (or relay) and intensifying the arc created as the contacts are pulled apart. Also note that the 50W motor may easily take 2x that current to start spinning, or well over an Amp, intensifying the ARC when the reed switch closes to start the motor. All of this arcing can be fairly hard on the tiny reed contacts in a typical hobby float switch.

The preferred method is to use the float switch in a 12V or 24V AC or DC circuit that trips a suitable relay for the desired load. The relay contacts are designed with a much higher duty cycle and arc tolerance. The coil of a typical "ice cube" relay is 130mA or so. With a simple diode, we can almost eliminate any inductive kick directed towards the tiny reed switch in the float and therefore significantly increase its service life and decrease the probability of failure.

From a safety standpoint, the 12-24V float is safer in your sump, but then again you should be using GFCI protection on anything attached to your system anyway.

As Bill has pointed out, many people use the float switches with small loads and have no problem, even if it is not the ideal solution. Just remember, as the motor (pump) size increases, so does the wear (and probability of failure) of the float switch. Any reasonable system should have some real world fail-safe to prevent catastrophic failure should the float fail in either the closed or open position.

 

[Dyraxe]

Thank you bean for more info. I put everything on a GFCI with my fish tank. What do you guys think of the aquahub 12v relay? What kinda of life should I expect from it?

 

[cabrego]

You are asking for problems if you run the float switch without a relay. The switch by itself can not handle current spikes due to voltage surges. I use the deluxe kit from aquahub, for the diy'er is is the best deal. Pony up the extra 20-30 dollars and get your self the kit.

If you would like to run a doser, you could use an extension with two plugs to power your second pump.

GL