1000W Motion Sensor Can't Switch LEDs??

al hildenbrand

Senior Member
So, this one surprised me.

Virtually all wall mounted 1000 Watt motion sensors, that I have experience with, switch their lighting load (the controlled load) on and off with a mechanical relay. You can hear it snap. The actual switching of the controlled load is being done, not by a "silicon switch", but, rather, by make and break physical contacts.

I just learned that a 1000 Watt RAB STL360 (incand. and fluor. approved) is not approved for switching 120 V AC LED light fixtures.

If a 120 V AC LED light fixture can be controlled by a vanilla wall mounted AC snap switch, what could make it incompatible with a mechanical relay switched motion sensor?
 

al hildenbrand

Senior Member
181030-1332 EDT

al hildenbrand:

Is there a mechanical switch in the STL360?
Yes. The STL360 has a mechanical switch, a coil and contacts relay, that turns the controlled light fixtures downstream on and off (per RAB tech support staff).
 

al hildenbrand

Senior Member
Are some (so many as to cause RAB to not support the entire class) LED drivers such virulent noise sources as to be able to damage "nearby" electronics?
 

JFletcher

Senior Member
I wonder if it is just not been tested with LEDs, thus it is not listed for them, or if there is a mechanical or electrical problem with switching on the LEDs, like too much in rush or incomplete opening of the circuit resulting in leak through current and blinking of LEDs? Just spitballing some possible reasons here...
 

al hildenbrand

Senior Member
. . . or if there is a mechanical or electrical problem with switching on the LEDs, . . .
Their tech support said they hear about the LEDs that damage the sensor, but they are sure there are LEDs that are out there co-existing just fine with the STL360.
 

Jraef

Moderator
Staff member
It's likely that the problem is what's referred to as "wetting current" through the relay contacts. It's probably too low on an LED fixture. Not having enough current through the contacts means there is no arc when they open and the relay contacts actually NEED a little bit of arcing in order to "burn" off any oxides that build up on the surfaces. So it will work at first, but in a relatively short time, the contacts will appear to fail.
 

Strathead

Senior Member
It's likely that the problem is what's referred to as "wetting current" through the relay contacts. It's probably too low on an LED fixture. Not having enough current through the contacts means there is no arc when they open and the relay contacts actually NEED a little bit of arcing in order to "burn" off any oxides that build up on the surfaces. So it will work at first, but in a relatively short time, the contacts will appear to fail.
I have a motion sensor 2-Par 38 over my garage. I changed the lamps to LED last year. Most of the time the lights won't come on. Once in a while they will. I hadn't thought of this, maybe that is the problem. I might screw in an incand for a day and then remove it again and see if the problem goes away for a while. Interesting.. We need a scratch head emoji!
 

al hildenbrand

Senior Member
I have a motion sensor 2-Par 38 over my garage. I changed the lamps to LED last year. Most of the time the lights won't come on. Once in a while they will. I hadn't thought of this, maybe that is the problem. I might screw in an incand for a day and then remove it again and see if the problem goes away for a while. Interesting.. We need a scratch head emoji!
As you consider your 2-PAR 38, check to see if it actually has a mechanical relay. The self contained units tend to be a solid state silicon switch that lets the current through to the lamps.

Apples to apples.
 

Strathead

Senior Member
As you consider your 2-PAR 38, check to see if it actually has a mechanical relay. The self contained units tend to be a solid state silicon switch that lets the current through to the lamps.

Apples to apples.

This is a round bell box cover with two 1/2" threaded lampholders and a 1/2" threaded motion sensor/photocell about 2" by 3" in size.
 

gar

Senior Member
181030-1457 EDT

al hildenbrand:

Most electro-mechanical relays use pure silver or silver-cadmium-oxide contacts. Silver is pretty good even at low voltage, 5 V, and not too low current, even milliamperes, until contaminated. Should never SCdO at low voltage and current. In the 120 V application range SCdO has about twice the current rating of silver.

LED drivers usually consist of a rectifier directly driving a capacitor input filter. This can produce some very large short duration inrush currents when random turn on occurs on a voltage peak. This is likely the concern. A series inductor or resistor can reduce the peak. A current limiting thermistor appropriately selected can greatly reduce this peak inrush. A wide variety of these thermistors are made for the peak inrush current problem.

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