Mr. Serious
Senior Member
- Location
- Oklahoma, USA
- Occupation
- Electrical Contractor
Yes.
Photo cell w/o contactor
- Thread starter Isaiah
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Please surf the web. Key words: "contactor", "inrush VA"
Isaiah
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
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.
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MD Automation
Senior Member
- Location
- Maryland
- Occupation
- Engineer
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
Senior Member
- Location
- Baton Rouge
- Occupation
- Electrical Inspector
So what you’re saying is the inrush was really no factor at 1msec
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MD Automation
Senior Member
- Location
- Maryland
- Occupation
- Engineer
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.
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