About a year ago I purchased several high quality outdoor motion detector/spotlights. I had a minor issue with one and called the mfg.’s tech support. After resolving the issue somehow I had mentioned that I was using LED bulbs. The tech support guy advised me the warranty may be voided if I continued using LEDs because the internal switching circuit for the light bulbs (of the motion detector) could not handle the repeated high inrush currents from LED bulbs. He suggested I use halogen or incandescent bulbs. They were expensive motion detectors and had five year warranties so without much further thought I just went out and bought a six pack of Gx halogen bulbs (I’ll leave it up to you to figure out the brand!).
Fast forward to just recently. Of the original halogen bulbs, one burned out after about eight months, its replacement burned out one day after I installed it, and one was dead on arrival (I ohmed it out first).
Disgusted with halogens, I decided to go down the path of seeing what I could do to limit the inrush current to the motion detectors so I could use LEDs. I ended up at Ametherm’s website and started to go through some of their preliminary calculations for selecting an NTC (thermistor) as a possible solution. Then a light came on in my head (no inrush current here!).
The OEM specs for the motion detector says they are rated at 1000 watts. From reliable sources on the internet (Fluke and others) an incandescent light bulb has an inrush current of about 15x its steady state current. So, two 500 watt bulbs have a steady state current of 1000 watts/ 120 volts = 8.3 amps with an inrush of 15 x 8.3 amps = 125 amps. I didn’t have a 500 watt incandescent bulb but I did have a100 watt bulb that read about 10 Ω (cold), so this all sounded reasonable.
Ametherm’s website indicates typical LED light drivers have an inrush of about 100x steady state current. The biggest PAR38 LED lamps I could find for my motion detectors were 250 watt equivalents that actually only consumed 32 watts. Each motion detector has two sockets so 2 x 32 watts = 64 watts ⇒ 120 volts/64 watts = 0.53 amps steady state ⇒ 0.53 amps x 100 = 53 amps inrush.
53 amps of inrush for LED’s is much less than 125 amps for a 1000 watt incandescent load inrush. So I should be ok using two 32 watt LEDs, right? I didn’t call the OEM back after doing these calculations but thought I’d first reach out to see if anyone could find if I was missing something or I made some other error. Opinions, thoughts?
Fast forward to just recently. Of the original halogen bulbs, one burned out after about eight months, its replacement burned out one day after I installed it, and one was dead on arrival (I ohmed it out first).
Disgusted with halogens, I decided to go down the path of seeing what I could do to limit the inrush current to the motion detectors so I could use LEDs. I ended up at Ametherm’s website and started to go through some of their preliminary calculations for selecting an NTC (thermistor) as a possible solution. Then a light came on in my head (no inrush current here!).
The OEM specs for the motion detector says they are rated at 1000 watts. From reliable sources on the internet (Fluke and others) an incandescent light bulb has an inrush current of about 15x its steady state current. So, two 500 watt bulbs have a steady state current of 1000 watts/ 120 volts = 8.3 amps with an inrush of 15 x 8.3 amps = 125 amps. I didn’t have a 500 watt incandescent bulb but I did have a100 watt bulb that read about 10 Ω (cold), so this all sounded reasonable.
Ametherm’s website indicates typical LED light drivers have an inrush of about 100x steady state current. The biggest PAR38 LED lamps I could find for my motion detectors were 250 watt equivalents that actually only consumed 32 watts. Each motion detector has two sockets so 2 x 32 watts = 64 watts ⇒ 120 volts/64 watts = 0.53 amps steady state ⇒ 0.53 amps x 100 = 53 amps inrush.
53 amps of inrush for LED’s is much less than 125 amps for a 1000 watt incandescent load inrush. So I should be ok using two 32 watt LEDs, right? I didn’t call the OEM back after doing these calculations but thought I’d first reach out to see if anyone could find if I was missing something or I made some other error. Opinions, thoughts?