24v/120v calculations

[mattgorka]

Hello all, I'm a theatre tech, and i ran into a transformer question, and since I'm not very experienced with them, i was hoping that someone could help me out.

I'm using a V transformer on a V circuit to power a string of festoon lights. The string contains 48 individual 5w lamps. The find my amp draw, do I use the circuit voltage, or the transformer voltage?

In other words, does A=240/24, which would be 10 amps
Or does A=240/120, which is 2 amps

I know I'm not including voltage drop or any additional transformer draw, I'm just checking my theoretical math at this point. Thanks!

 

[charlie b]

How did you bring the number 240 into the equation?

I would need clarification: is the transformer a 120 volt primary to 24 volt secondary, and are the lights powered by 24 VAC? If so, you take 5 watts, divide by 24 volts, and you see that each light draws about 0.21 amps. You have 48 of them, so the total current will be 0.2083 times 48, or 10 amps.

 

[charlie b]

OK, I see how 240 figures into the mix. 48 lamps at 5 watts each is 240 watts. A "watt" is the same as one volt times one amp. So 240 watts divided by 24 volts is 10 amps. So that equation is the right one.

 

[petersonra]

Hello all, I'm a theatre tech, and i ran into a transformer question, and since I'm not very experienced with them, i was hoping that someone could help me out.

I'm using a V transformer on a V circuit to power a string of festoon lights. The string contains 48 individual 5w lamps. The find my amp draw, do I use the circuit voltage, or the transformer voltage?

In other words, does A=240/24, which would be 10 amps
Or does A=240/120, which is 2 amps

I know I'm not including voltage drop or any additional transformer draw, I'm just checking my theoretical math at this point. Thanks!

If the lights are in parallel, then the individual current for each lamp is watts divided by volts, and the total current is 48X the individual current.

I am not sure what your 240/24 or 240/120 means.

 

[mattgorka]

Thanks for the help

Thanks for the help

Thanks for the answers. It is indeed a 120v primary and 24 secondary (spell check deleted my numbers!). I was figuring on the 10 amps, just as a precaution.

Thanks again for the assist!

Matt

 

[infinity]

In other words, does A=240/24, which would be 10 amps
Or does A=240/120, which is 2 amps

The answer is both are correct (disregarding transformer losses). The 10 amps is the secondary current and the 2 amps is the primary current.

 

[LarryFine]

The best way to calculate this is to start with the known items: the bulb wattage and voltage. Since they're in parallel, add them up; the bigger numbers make the math simpler. Now, apply Watt's Law: P=E*I.

(Yes, I know.)

5w x 48 lamps = 240w. P=E*I, so I=P/E, so 240/24=10a.

The current ratio is the inverse of the voltage ratio. 120v:24v=5:1=10a:2a.

 

[Physis 3]

The find my amp draw, do I use the circuit voltage, or the transformer voltage?

In other words, does A=240/24, which would be 10 amps
Or does A=240/120, which is 2 amps

I think what you're missing here is that there are two different circuits on either side of the transformer. The primary side, in you're case, is the 120v. and the secondary side is 24v.

Start with the load. That's 240 watts at 24 volts. The current will be 240w/24 or 10 amps.

A transformer can pretty much be considered a straight ratio with regard to voltage and current on either side of it. It's the opposite directions for current and voltage. But the wattage stays the same on both sides. If I explain it right you'll see what I mean and if you do you'll probably always remember it.

120v. (primary) / 24v. (secondary) is the same as 5/1 or a 5 to 1 ratio.

So the voltage on the primary (input) is 5 times what the secondary (output) voltage is.

And, at the same time, the current on the primary (input) is 1 fifth the output (secondary) current.

That's why the power or (wattage) is always the same (ideally) on either side.

Now, given I've done even a fair job of explaining it, (I don't think I did) I'm hoping it's easier to see that the primary (input or line side) will be 240 watts, 120 volts at 2 amps.