Can you help shed some light ?

chim · May 12, 2002

Yup, in a series circuit, all individual lamps (as long as they're equal) see the same voltage. A string of Christmas lights with 24 lamps sees 5 volts dropped across each lamp when plugged into 120V. This can actually be measured. They also see the same current regardless if equal or not - only one path to follow. Putting two lamps in series with each other cuts the current by 1/2 and the power by 1/4. This is due to doubling the resistance in the circuit which reduces the current by 50%. Since power is equal to the current squared times the resistance, the power at the individual lamp would be that much less.

Just for fun, assume you have two lights with a hot-filament resistance of 3 ohms. If you connected one across 12V, the current would be 4A, and the power would be 48W ( 4 X 4 X 3 = 48). This is close to some of the "work lights" we use on our tractors. If you hook two of these lights in parallel (normal) each light operates like the example above, drawing 4A and producing 48W. If you take one wire to feed both lights (T-tap the +) that one wire will carry 8A. Total power for both lights is 96W.

Now, if the two lamps are connected in series, the 12V is being applied to a total resistance of 6 ohms. This will result in 2A flowing through the circuit instead of 4A. The power at each light now is 2A X 2A x 3 ohms = 12W. Total power for the two lights is now 24W.

Sorry if you found this boring................chim

billfires · May 12, 2002

EarPlug, I don't know about the definition but I do know that dielectric grease is used on bulb sockets to prevent corrosion and assist with electrical conduction.

Bill C

billfires · May 12, 2002

Chim and Ed King, in 12v wiring in series you would run the 12v to the pos. wire on the light and then from the ground wire (which wouldn't be grounded) you would run to the pos. wire of light #2. This would provide approx. 6v to each light. Also the wattage is the voltage (12) times the amperage (4) to equal 48 watts.

Bill C

beenthere · May 12, 2002

I don't mean to wade in here and cause more confusion, but seems Billfires and Earplug are both right. However, the addition to the statement of Billfires that "and assist with electrical conduction" makes it sound like the di-electric grease conducts electricity. If it did, then seems to me there would be a direct short to ground. I think it might "assist the electrical connection" and keep out water and thus reduce corrosion, but seems it cannot be a conductor of electricity.

chim · May 12, 2002

Bill, both your description of a series connection and formula are correct, but the current is only 2A because the two lamps together equal 6 ohms across a 12V source. So the 2A times 12V = 24W for the circuit, or 12W per lamp. Taking the current squared times the resistance gives the same answer. 2A X 2A = 4, then 4 X 6 ohms = 24W.

The more items you connect in series, the lower the current goes. If you hooked four of these lamps in series, the current would drop to 1A, and the power for the circuit would be 12W, or 3W per each of the four lamps...............chim

Soundguy · May 14, 2002

Bill C, Dielectric grease is by definition a non-conductor of direct current. And therefore has some insulative properties. Actually though, the dielectric grease is specifically good for the following reasons. Most greases ( dielectric included ) act to shed and displace water, and have a debouncing effect. Think of a relay contact, or switch contact. As the contact points get closer, and are almost touching, they will reach a point where they are still not touching, but arcing is occouring. This arcing makes micro molten spots on the contacts, and shortens the contact points life. Over time, the contacts become more pitted, and have less surface area contact, and thus less current capability, leading to a more resistive connection, that heats up, and eventually fails. The grease prevents the arcing, only allowing current to flow with the direct contact to contact connection.

I've used dielectric greases for years.. also silicone based general purpose lubricants, even vasolene, and even automotive grease, with nearly equal results. I do wonder about possible flamibility concerns of the regular greases.. but in the last 17 years, have yet to have had a fire due to a greased connection. As a side note... anything that keeps oxygen or other corrosive agents off of the connections ( battery terminals, etc ) is a plus.

Soundguy

""One last thing, di-electric grease on the bulb sockets and any connections will prevent corrosion( regular grease acts as an insulator and shouldn't be used). Hope this helps and any other questions just ask.""

Soundguy · May 14, 2002

Yeah.. his idea was close.. and perhaps what he should have said was that the voltage drop across each light was 3v etc.... not tat each light saw 3v less than the predecessor, etc.

Soundguy

"Yup, in a series circuit, all individual lamps (as long as they're equal) see the same voltage. A string of Christmas lights with 24 lamps sees 5 volts dropped across each lamp when plugged into 120V. This can actually be measured. They also see the same current regardless if equal or not - only one path to follow."

Soundguy · May 14, 2002

Yep... when in doubt, hit the dictionary. Again.. by definition, a dielectric is a non-conductor of direct electrical current. For those in doubt, hit the webster new colegiat edition, etc.

And yes, the dielectirc grease assists the electrical connection.
As it sheds moisture, air, and debounces, etc.

Soundguy

"I don't mean to wade in here and cause more confusion, but seems Billfires and Earplug are both right. However, the addition to the statement of Billfires that "and assist with electrical conduction" makes it sound like the di-electric grease conducts electricity. If it did, then seems to me there would be a direct short to ground. I think it might "assist the electrical connection" and keep out water and thus reduce corrosion, but seems it cannot be a conductor of electricity. "

Soundguy · May 14, 2002

"The more items you connect in series, the lower the current goes. If you hooked four of these lamps in series, the current would drop to 1A, and the power for the circuit would be 12W, or 3W per each of the four lamps...............chim "

Just throw them out hte ohm's law formula.. e=IR, and the various permutations P=IE etc.

Seems most have a general misunderstanding of series / parallel connections, especially dealing with dc resistance in parallel, etc.

I hit your bio, and expected to see an EE or tech or something. Nice to see another electronics oriented person online here. Seems that even though we are mostly talking about a diesel or gasolene engine problem, and electrical one tends to come up somewhere.

Soundguy

EdKing · May 14, 2002

Yeah.. his idea was close.. and perhaps what he should have said was that the voltage drop across each light was 3v etc.... not tat each light saw 3v less than the predecessor, etc.


Each light will actually "see" 3 volts less than the previous, although each light will only be using or dropping 3 volts. If you measure the voltage across each bulb, you will read 3 volts, but if you read from the positive side of each light to ground, you will read 12 volts at the first light, 9 volts at the second, etc. All this assuming all the bulbs are the same. All basic Ohm's Law as it applies to a series circuit. I thought of "throwing out a post on Ohm's Law" but didn't want to get into explaining the way current and resistance are measured in serious vs. parallel circuits.