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

[Bill Wichers <billw at waveform_net>]

> The reed switch for the one I favor has these
> specifications:
>
> 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.

        -Bill




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

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