Photo cell w/o contactor

[Mr. Serious]

Can the integral photocell within the fixture be ‘bypassed’ if desired in order to utilize an external photocell

Yes.

 

[topgone]

Nice to know.
Where did you find the info on inrush and sealed VA? Thanks


Sent from my iPhone using Tapatalk

Please surf the web. Key words: "contactor", "inrush VA"

 

[Isaiah]

Sounds like a good reason to not energize them all at olne time as a single group.

We normally like to see a single photocell facing north so all the lights energize at the same time. I might be wrong here but I think the inrush current is largely mitigated when the AC transitions to DC.


Sent from my iPhone using Tapatalk

 

[MD Automation]

Read up about the inrush of LED lights. For a half a cycle or so, it is extremely high. I don't have time the research it again now, but I remember that it can be hundreds of amps.

I worked an interesting problem about 2 years ago involving 6 small LEDs on an electric tugger we were building, all run on 24 VDC. I will someday create a post about it, and the troubles I encountered with welded relay contacts. But to post here briefly, see the screenshot below for how high (and fast) the current can be when the LEDs were turned on. Charging up the empty DC capacitors in the LED, with little or no current limiting circuitry, is like a dead short / black hole / Scotty, give me everything you can problem.

For reference, the steady state draw after turning on was ~3 amps. But that peak draw was ~140 amps when the contacts closed!! It's all over inside 1 msec. Then the current draw drops to almost zero as the thing gets ready to turn on, which it does after about 10 msec.

Again, this was for a DC device, but the same peak current problem exists in the AC world for sure. And it can cause real problems at switch-on.

 

[Isaiah]

I worked an interesting problem about 2 years ago involving 6 small LEDs on an electric tugger we were building, all run on 24 VDC. I will someday create a post about it, and the troubles I encountered with welded relay contacts. But to post here briefly, see the screenshot below for how high (and fast) the current can be when the LEDs were turned on. Charging up the empty DC capacitors in the LED, with little or no current limiting circuitry, is like a dead short / black hole / Scotty, give me everything you can problem.

For reference, the steady state draw after turning on was ~3 amps. But that peak draw was ~140 amps when the contacts closed!! It's all over inside 1 msec. Then the current draw drops to almost zero as the thing gets ready to turn on, which it does after about 10 msec.

Again, this was for a DC device, but the same peak current problem exists in the AC world for sure. And it can cause real problems at switch-on.

View attachment 2564557

So what you’re saying is the inrush was really no factor at 1msec


Sent from my iPhone using Tapatalk on

 

[MD Automation]

So what you’re saying is the inrush was really no factor at 1msec

The peak (height) of that inrush current was a very real problem, it did not matter how long it lasted. Even at that short 1 msec duration, it was high enough to consistently weld the contacts closed on the relay we had chosen to switch these on and off.

Clearly our fault in the prototype design, it was nevertheless really interesting to dive into exactly why our chosen relay was failing. I was surprised when I graphed, with that oscilloscope, a peak draw that was 35 to 40 times as large as the steady state current. The contacts in the mechanical relay were likely some standard silver tin alloy and rated at 10 amps, which was 3 times the steady state draw. But they never stood a chance with the the peak seen in that graph.

Quite often in a DC circuit, the problem is switching-off a relay (especially w/ inductive loads), since the arc does not self quench like an AC circuit would. But here, it was a switch-on problem, those contacts got punched in the face so hard at closing, they melted together. Even at 1/1000th of a second.

I posted here in this thread just to emphasize comments made by Strathead and others about the inrush of LEDs. It's just something to keep in mind when designing or selecting switching components.

The problem is many LED manufactures are all trying to make the cheapest thing possible, so any kind of current limiting, pre-charge circuit, which cuts down the crazy peak you saw in the graph, is something that is often excluded from the design. All in an effort to shave nickels and dimes off the product.