# Do globes care about AC or DC?

Discussion in 'Electronic Basics' started by RR, Oct 24, 2005.

1. ### RRGuest

Hi,

If I have a 12V globe (or any voltage for that matter), does the globe
"care" whether it is run on AC or DC?

"Care" in the sense of:
- will it work?
- will it last as long (or maybe longer)?

In particular, we're thinking of moving out of town and building a house
without connecting to the electricity grid.
We like the 12V lights that have now become so popular. These run from
mains via a transformer. The transformer converts mains AC to 12V AC.

If we ran 12V batteries for the house, it would make sense to just run 12V
DC to all our lighting rather than convert to mains AC via the inverter and
then back to the 12V with the above transformer(s).

tia,
RR

2. ### Ralph MoweryGuest

AC or DC is fine for the globes. Just use the same voltage.

3. ### Don BruderGuest

If it's a standard incandescent type, AC versus DC means exactly zilch.
If it's an LED type, or a flourescent, then it's going to be picky about
what it eats. In theory, a flourescent COULD run from either AC or DC,
but the use of transformers to play with the starting voltage/current
and such means that AC must be used to power it unless you go with
something that's built to deal with DC. (which will more than likely
simply turn the DC into AC or pseudo-AC via some form of inverter, do
the transformer work on it that's needed, and run the light from that)
LED-style, of course, will need to run on DC, and will expect the proper
polarity. If fed the wrong juice, or the wrong polarity, they tend to
leak out all the magic smoke rather quickly.
Again, if it's an incandescent, it should work fine, regardless of
whether you feed it AC or DC of the proper voltage. It *MIGHT* last
longer if run on DC, or it might not make any measurable/significant
difference, depending on the exact filament technology it's built
around. I would think it extremely unlikely that the lifespan would be
any shorter due to being run on DC.

5. ### Don BruderGuest

Feed 'em 12 volts of either AC or DC, and they'll work just fine. Going
back to your previous message, I would say these guys should last at
least as long when run on DC as AC, and likely a bit more besides. But
that's just my hunch, with nothing scientific to back it.

6. ### John LarkinGuest

DC can cause electromigration, but the effect is minor here.

John

7. ### quietguyGuest

I wonder if the length of the cabling to your lighting will be an issue - long
runs at low volts and 'high' amps mean thick(er) cabling to avoid excessive
voltage drop.

Worth thinking about as 32v systems were generally the go in the old days to
reduce cable losses

David

8. ### Roger DewhurstGuest

Your 12v wiring will have to be much heavier than the wiring for 230v.
Perhaps you might give some thought to an inverter to convert 12v DC to 230v
AC. This will make consumer electrical goods easier to operate. Try a
ship/yacht chandler for a suitable inverter and a wind generator to mount on
the roof.

R

10. ### DavidGuest

The main thing to worry about here is cabling and switching. If you are
using 12V, 50W dichroics thats about 4A per bulb to switch. If you have four
that's 16A to switch. Obviously switch will have to be rated higher than
that. Or maybe use relays and then there's dimming to consider. None of
these problems exist when used with mains transformers/SMPS's as the primary
current is small and dimmers are easy to add.

Anyway in just a few years LED's will start to take over and they are about
35% efficient as against 7% for your halogen globes. I would think about
using LED's now and just a few halogens where you need good light. More
expensive yes, but you will save on cabling and switching costs. Oh there is
one drawback with LED's at the moment and that is colour/color temperature
and CRI but thats another story.

Cheers.

11. ### Andy BaxterGuest

David said:
Do you know how LEDs compare to fluorescents? From my own testing they
seem about the same or a bit better (maybe 10-20% better?) but I've never
seen this properly tested anywhere.

12. ### DavidGuest

Depends on what you mean by "better". Fluoro's produce about 40 lumens/watt
against 25 lumens/watt for LEDs. But LED output is constantly improving.
for LED is when the light output reaches between 70 - 50% of original. Light
from LED is concentrated, whereas light from a fluoro is uniform.
Colour/color temperatures and CRI vary widely.

Cheers.

13. ### Jasen BettsGuest

Not really.
AC may be slightly more stressful on the filament, so possibly slightly
longer on DC.
yes, IMO, that would work. check with the maker of the globes.

but if you're making your own electricity you may find that LED or CCFL use
less electricity, and so can run for longer.

Bye.
Jasen

14. ### RRGuest

Thanks to all who replied! Very helpful.

Quietguy: yes, I'm just looking into the issue of cable thinkness.

Roger Dewhurst: it just doesn't seem to make sense to have 12VDC in my
batteries, invert it to 230VAC and then run transformers to bring it back
down to 12VAC. That's a lot of conversion and surely a lot of loss?

David said:
Thanks, but aren't switches rated on power? So a 240V switch rated at 10A
will be quite happy to switch 12V at up to 240*10/12 = 200A?

Maybe I've still got a lot to learn?

15. ### Randy DayGuest

RR wrote:

[snip]
Not exactly. The power needed to melt the contacts is
calculated by I squared R, where R is the resistance
of the switch contacts. If I is constant (16A), R is
constant, then power dissipation across the contacts
is constant.

The 240V only comes in to play when worrying about
arcing.

16. ### Don KlipsteinGuest

Most white LEDs have color rendering index rated to be 70-85, and old
tech cool white fluorescent rates usually 62, sometimes 66. In both cases
the main color distortions are making most colors - especially reds and
greens - darker and duller than "proper". White LEDs are not as bad as
cool white fluorescent.

Triphosphor fluorescents, which includes most compact fluorescents, are
different. Most compacts, Philips "Ultralume" and the better of the two
common grades for T8 (GE SPX, others with color code of 8 followed by a
2-digit abbreviation of the color temperature) have rated color rendering
index of 82-86 (82 for compacts). The main color distortion is making
most colors brighter and more "vivid" than proper, although reds come up
orangish.

Most white LEDs have color temperature around 6000K (icy cold slightly
bluish). Most higher power white ones appear to me to be around
5000-5500K - not as bluish to sometimes not looking bluish at all but
still icy cold.
There are some "warm white" LEDs out there, with color temperature
mostly around 3500K. However, I see a tendency for those to be a little
less efficient than the higher color temperature ones.

- Don Klipstein ()