parallel vs in series

1735 Views 6 Replies 4 Participants Last post by ddawg16, Dec 22, 2012

Kelly Durfey

Discussion Starter · Dec 22, 2012

Hello,

I have a question regarding the effects of wiring up a pair of flood lights in series rather than parallel.

I have two sets of floodlight hanging off my back porch, each with two 100 watt bulbs to light the yard. They're currently wired up in parallel, each drawing from the same hot wire coming from the box.

I wanted to replace one of the lights with a new lamp that has a photo/motion sensor and then rewire them in series so that both laps would operate from the sensors. But, when I got them wired up in what I thought was "in series", I noticed a huge drop in intensity from the lamps.

My question is whether this would be a consequence of miswiring the lamps or the possbility that I was simply sending the current through too many bulbs to get maximum output.

(When wiring "in series", I simply took the neutral of the first lamp and connected it to the lead of the second and the neutr of the second went back to the box.)

Any input you guys (or gals) can offer would be greatly appreciated. You've been a great help in the past.

Thanks,
Kelly

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ddawg16

· #2 · Dec 22, 2012

When you wire them in series, your basically doubling the resistance of the ckt....hence, those 2 100w bulbs are only pulling 100w total instead of 200w.

In other words....it's only going to be half as bright.....

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Kelly Durfey

Discussion Starter · #3 · Dec 22, 2012

Thank you for the prompt response.

My biggest concern was that a wiring mistake had led to a fire risk. but it sounds like this is something you would have expected. It also sounds like something that's unavoidable when wiring in series, is this correct?

In other words, I'm on my way to by another lamp.

thanks again,
Kelly.

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brric

· #4 · Dec 22, 2012 (Edited)

You don't wire them in series. Both lamps are powered from the load connection on the motion sensor. The neutral goes to the motion sensor and to both lamps. The incoming power connects to the motion seensor, usually a black wire. The motion sensor load wire, usually red, goes to the black of the first lamp as well as the black of the second lamp.

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AllanJ

· #5 · Dec 22, 2012 (Edited)

You do not want to wire the lamps (light bulbs) in series.

One thing to stop now and examine is to be sure that the motion sensor can handle the power for all (4) lamps. (400 watt in this case).

Electrical idiosyncrasies:

With two lamps in series (or 4 lamps as two series pairs) the current is indeed going through too many lamps to get maximum (light) output. Each lamp is getting about 60 volts and the two lamps (each 2 lamps) account for all 120 volts. For each two lamps the current draw is roughly 100 watts. The lamps will not try to draw more power and achieve more brightness.

But with all of the lamps in parallel the current is not going through too many lamps to get maximum output. Each lamp gets 120 volts. Each lamp draws a full allotment of 100 watts and the circuit dutifully serves up the needed amount of current (amperes) until a breaker trips or some other component in the circuit is overloaded and overheats and burns out.

In order to understand why these idiosyncrasies exist you need to understand Ohm's Law: at all times in any portion of a circuit, the volts across the "start" and "end" of that circuit portion equals the amperes flowing through times the resistance. Each lamp has a given resistance (that's really temperature dependent) but you can ignore the temperature part to get the jist of the difference in behavior of series and parallel circuits of lamps. A 100 watt 120 volt incandescent lamp has a resistance of about 145 ohms at full brightness, and draws about 0.83 amps. You can assume zero ohms for the wires.

(Measuring the resistance of the cold unlit lamp using an ohmmeter gives a much lower value, which is why I mentioned temperature dependence.)

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Kelly Durfey

Discussion Starter · #6 · Dec 22, 2012

The motion/photo sensors are built into the lamp i already have. I didn't want to mess with external ensors as they cost more independently than a new lamp and I'd be responsible for the extra wiring, a responsibiliy I'm trying to minimize.

I hadn't actually considered the possibility of tapping into the wiring of the included sensors. While it is an interesting possibility, I think the coment about the limits of the sensors is a valid concern.

In the end, I think the diminished output when wired in series makes a lot of sense. But, without a more complicated setup, it would be easier and not much more expensive to just buy an indpendent sensored lamp.

If I've misinterpreted something, please let me know.

As always, I appreciate your input and willingness to help.

Kelly

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ddawg16

· #7 · Dec 22, 2012

Kelly Durfey said:
The motion/photo sensors are built into the lamp i already have. I didn't want to mess with external ensors as they cost more independently than a new lamp and I'd be responsible for the extra wiring, a responsibiliy I'm trying to minimize.

I hadn't actually considered the possibility of tapping into the wiring of the included sensors. While it is an interesting possibility, I think the coment about the limits of the sensors is a valid concern.

In the end, I think the diminished output when wired in series makes a lot of sense. But, without a more complicated setup, it would be easier and not much more expensive to just buy an indpendent sensored lamp.

If I've misinterpreted something, please let me know.

As always, I appreciate your input and willingness to help.

Kelly

At the end of the day.....your almost always better off using stuff as the manufacture intended.....

So....getting a second light is a good idea.....besides....if the sensor on one fails...at least the other one should still be working.

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