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[APD] Re: What is a contact rating and what does it mean to me?

The reed switch for the one I favor has these

Contact Rating: 66 vac/dc
Type: SPDT
Max Switching Voltage: 220 vac/dc
Max Switching Current: 1.0A ac/dc

What I want to switch is 110 house current running
a 15 watt UV steralizer and a 12 watt Fluval #4
filter.  I calculate the amps (A=W/V) as 0.225, so
I look good on Max Switching Current.  Max Switching
Voltage looks good.  SPDT, well, I want one to be
normally open and one normally closed, so that all
seems okay.

What does the contact rating mean and how does it
apply to my situation?  For twice the money I could
up it to 100 vac/dc.

That's a somewhat strange rating system, but most likely what they mean is that the switch is RATED for 66 vac, but can break (interrupt) up to 220 vac. This would be needed to switch an inductive load for example that will spike the voltage when interrupted.

Unfortunately, I don't think either unit (66 volt or 100 volot) will work for you directly. Reed switches are notoriously delicate, and can't really switch a motor directly since they can't handle the large startup currents well (the contacts burn out or 'weld'). I think you have two options:

1 - Look for a *mercury* float switch. Mercury is great stuff for inductive switching -- the high current makes little mercury droplets inside the switch which then run back together into one big drop again. It's kind of like a switch that is self-fixing :-) You can get these for sump pumps and they *might* work in your application.

2 - Use the float switches you found, but set them up to operate a relay or contactor instead of running your load(s) directly. The easiest way to do this is to use some 24 volt AC relays (a very common industrial control voltage with lots of parts easily available), and let the float switch control the 24 vac line that runs the relay. The relay then gets to switch the actual loads you want to run.

The problem with the switch you found is that while the steady state ratings are suitable for your equipment, the thing that destroys relay contacts are the transients that occur at the instant of turning on or off. With inductive loads like the motor in your pump and the ballast in your UV filter, chances are that your reed switch won't last for more than a few cycles -- and it might weld on the very first "turn-on" instant. Inductive loads draw very large currents (many times the steady-state currents) when they are switched on, and produce large voltage spikes when they are switched off. That's all bad news for the contacts of the device doing the switching.

The *best* thing to use would be a solid state relay since they need very little current or voltage to operate and can typically switch large inductive loads easily. http://www.allelectronics.com usually has some available for around $10/ea. With a solid state relay, just find a small power supply that produces a voltage somewhere around the middle of the range the relay needs and let the float switch control that. The relay itself can then switch your load.


***************************** Waveform Technology UNIX Systems Administrator

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