Amperage of series circuit MR16 EYC Lamps.

[jglodeklights]

Electrical engineering is not my forte. That said, I'm thinking about doing a project involving MR16 EYC lamps for my photography hobby. I know that, as most households are rated at 120Volts (depending on how happy the grid is in your area), this requires serial circuiting the lamps in groups of 10 to get down to 12 volts at each lamp. However, how does one calculate the amperage they are drawing? Is it the combination of each 75watt lamp at 12 volts? Or is it the 750 watts at 120 volts? Just trying to figure out how much draw I would actually be putting on things. Also, to be fair, I've been completely blanked mentally on how mini strips worked until I reread about serial wiring today. DUH!

Thanks!

 

[epimetheus]

Since the lamps are wired in series, all elements have the same current running through them. An EYC lamp is rated at 75W at 12V; that's 6.25A each. A series string of 10 EYC lamps is 750W at 120V which also equals 6.25A.

 

[JD]

One of those odd questions where both answers are actually the same!

Anyway, one caution is that the lamps should be from the same batch and about the same age. Lamps vary a bit in resistance from batch to batch as well as age. Balance becomes very important as any lamps that are a bit higher in resistance will end up with more voltage across them. Too far out of balance and you will end up with a string of 10 flashbulbs! Unfortunately, this may also happen near the end of life; One goes, they all may go.

 

[derekleffew]

...until I reread about serial wiring today. ...

Serial wiring* is work by an electrician who can't stop himself. You mean series wiring.


*A cereal killer took the lives of Cap'n Crunch, Count Chockula, Frankenberry, Toucan Sam, Dig'em Frog, brothers Snap! Crackle! and Pop!, Tony the Tiger, and others.
-----
Here's a related question (which should look familiar to anyone who has prepared for the ETCP exam):

Four 50W, 12V MR16 lamps are to be connected to a 48V transformer. Which of the following
best describes the current required from the transformer?
(Content Outline 1.A.1)
A. 1A
B. 4A
C. 8A
D. 16A

 

[mrb]

wire a neon indicator light across each bulb, when one burns out its indicator will light. That saves a ton of trouble finding the burned out lamp.

 

[sk8rsdad]

Too far out of balance and you will end up with a string of 10 flashbulbs! Unfortunately, this may also happen near the end of life; One goes, they all may go.

Ummm... no. When one goes it acts as a fuse and the rest no longer have current flowing through them thanks to the now-open circuit. It would take a strange and exceedingly rare quirk of fate for all 10 lamps to fail and form a short circuit to achieve the flashbulb effect.

 

[zmb]

Here's a related question (which should look familiar to anyone who has prepared for the ETCP exam):

Is the correct answer B. 4A?

 

[ship]

be cautious about wiring and retaining this experiment. Just because it's low voltage doesn't mean that they don't run really really hot.

Can you do the same with say 10 or 20 Watt lamps that run a little cooler?

 

[derekleffew]

Is the correct answer B. 4A?

Yes. (extra characters) Yes it is.

 

[JD]

Ummm... no. When one goes it acts as a fuse and the rest no longer have current flowing through them thanks to the now-open circuit. It would take a strange and exceedingly rare quirk of fate for all 10 lamps to fail and form a short circuit to achieve the flashbulb effect.

Don't bet you life on that one! I have seen them fail and not blow, but I have seen them fail and blow the whole string!

Here's why:

If they "really" operated at 12 volts this would not happen, but there is 120v potential across any open lamp. If hot (fresh blown), this is enough (at times) to sustain an arc, bringing the lamp resistance to 0 ohms and resulting in an over-voltage on the remaining lamps. If any remaining lamps are weak, they may go as well, spiking the volts-per-lamp even higher. The whole thing can cascade in less than a second!

Of course, ACL's are most famous for this as there are only 4 in the loop. 10 is more stable, but as I said, don't bet your life on it!

EDIT: Lamps designed to run at 120 volts have an anti-arc fuse links built into their bases. Low voltage lamps do not. Unfortunately, when we chain lamps, we drop through this loophole. (Not that the fuse would stop a cascade, but they usually limit damage to auxiliary equipment by stopping the runaway current demand of an unballasted arc.) This arc is why we see that beautiful flash when some lamps blow, and sometimes take the SSR in the dimmer with them!

 

[jglodeklights]

Serial wiring* is work by an electrician who can't stop himself. You mean series wiring.

Ah! You're right! Thank you for the great info everyone. Ship, I may choose to go down to 50 watt lamps, but 10 or 20 watt lamps would be too low output for intended purpose, especially since the output will be bounced and likely heavily diffused. I'm planning on using materials that don't go up in flames (read metal) and other materials that will insulate the lamp holding elements. mrb, that was actually already in my plan. Anyone have a recommendation on where to find these?

So a little bit about what I'm planning (really thinking at this point) on doing:

I'm creating my own ring light! While there are lots of DIY versions out there utilizing A lamps, these usually are A. of poor output as they tend to be bare bulb with no "reflector" to focus the light forward and B. unwieldy if you intend to add material to create a perfectly even source around the entire ring as you would need to get the diffusion somewhat aways from the lamps (otherwise you end up seeing, in the subject's eyes, a less than desirable spotty reflection of the lighting fixture versus a completely even ring).

MR16 lamps bounced in the ring is just one solution I'm looking at due to size of the lamp and the relatively bright output I'd be able to get out of them. The other option, which I may go with, is some higher density LED strips wrapped around and around. I'm not sure this would be bright enough, though. With enough density, maybe. But CRI could be a problem.

 

[jonliles]

Sounds overly complicated. Why not 3 sources of light, such as 3 fresnels or Scoops with 1/4 Hamburg and whatever color correction is needed?

 

[jglodeklights]

Ring Lights have a completely different look than 3 or even 5 or 7 point sources arranged in a ring formation. You'll also note, I wish to create a circular reflection in my subjects' eyes; it looks cleaner than multiple point reflections. Ring lights of the size I'm looking for are prohibitively expensive in either continuous output or strobe form. Smaller ones that fit on my camera or lens are not of the correct size to create the modeling I wish to have, and in the price range I wish to keep it to are ridiculously underpowered for portrait work.

 

[shiben]

Ring Lights have a completely different look than 3 or even 5 or 7 point sources arranged in a ring formation. You'll also note, I wish to create a circular reflection in my subjects' eyes; it looks cleaner than multiple point reflections. Ring lights of the size I'm looking for are prohibitively expensive in either continuous output or strobe form. Smaller ones that fit on my camera or lens are not of the correct size to create the modeling I wish to have, and in the price range I wish to keep it to are ridiculously underpowered for portrait work.

Then you are right, the CRI on LEDs will be a problem. What you COULD do is wire it up with strips of RGBAW LEDs and make it so you can control variable color... The electronics on this would be a pain in the rear, but might be fun to figure out. Who knows, maybe there is a way to hack it from a couple of LED PARs? That might actually be fun... Really depends on how big you need it...

 

[jglodeklights]

Using RGBAW would be interesting.... however, I'm not looking to get that complicated with things (or that expensive for how much crap I would need to control it with) And this will be fairly large. Finished internal diameter will be about 18" and the outside diameter will be about 24-27". I hope. Cost is hoping to be under $150.

I did some more looking/thinking at I'm realizing MR16's may not be the way to go, but something with a similar bi-pin base and 12V possibly is. Thinking 10 JC-50 or 75's distributed evenly around the ring with little reflectors (made out of sheet metal flashing maybe?) to bounce the light to the inside of the ring where it can then bounce to the outside ring and forward to the subject. Done with cheap materials, socket cost would be $48, lamp cost would be $27, and materials, including those to make some form of stand could probably come in under $50. CRI would still be slight off at a 3000K color temp.