Running at half the voltage

Discussion in 'Electronic Basics' started by john stone, Dec 11, 2012.

1. john stoneGuest

Have a 10Watt, 12Volt Hologen lamp for a christmas decoration. But the
mains tranformer I have is only 6 Volts.

Apart from being a bit dimmer, would there be any other consequences running
at the lower voltage? Thanks.

2. Leif NelandGuest

john stone forklarede:
A bit dimmer?

If the resistance of the lamp was independent of the temperature, which
it is not, the lamp would draw 1/2 the current at 1/2 the voltage,
giving 1/4 the wattage, 2.5w.

However, the resistance _is_ dependent of the temperature. Se more at

http://en.wikipedia.org/wiki/Halogen_lamp#Effect_of_voltage_on_performance

But the bottom line: Nothing will fail catastropically, but you may get
much less light than expected. But it may look nice and christmassy

Leif
(with a lot of but's ;-))

A halogen lamp on the correct voltage runs the filament hotter than a
standard incandescent tungsten lamp. Without the halogen gas filling,
this would soon cause the tungsten to migrate from the filament to the
inside of the bulb, where it would blacken the glass. The filament
would rapidly become thin and then would break.

At a certain temperature, the halogen will combine with the tungsten on
the inside of the glass. It then floats around until it meets the hot
filament, where it deposits the tungsten back where it came from
(approximately). This depends on having the glass hot enough to allow
the halogen and tungsten to combine, which occurs at near dull-red heat
(which is why quartz glass is necessary).

If you under-run the lamp, the filament will not blacken the glass as
quickly, but the glass will not reach recycling temperature, so any
blackening will remain. The relative temperatures of the filament and
glass will determine whether the life if the lamp is extended or
shortened. The light output will be very muchreduced and lacking the
blue end of the spectrum.

4. William SommerwerckGuest

To put it a bit more simply...

A /slight/ reduction in voltage will cause a halogen lamp to burn out
prematurely, because the bulb doesn't get hot enough to initiate the
"recycling" process -- but the filament is still extremely hot.

Half voltage is a huge drop. You should have no problems.

5. Guest

At 6V the halogen cycle won't operate so you'll probably get
discoloration of the bulb after some time and perhaps the bulb won't
last as long (to the degree that it's a trade-off of lower power vs.
the elimination of the halogen cycle). Other than that, it'll
probably be a very yellow light at pretty low intensity.

6. Guest

The envelope is probably black by now. Incandescent life goes with
something like the 12th power of, er, power, so it's not surprising.
You'll probably find that it won't last all that long, now, at full
power. It's filament is now on the inside of the envelope.

7. William SommerwerckGuest

We run a halogen (over our dinner table) from a dimmer.
Actually, it's the 12th power of voltage.

Even a slight drop in voltage causes a big increase in life. For example, a
5% reduction almost doubles the lamp's life (1.85 times). At a 10%
reduction, the lamp lasts 3.5 times as long.

Of course.

9. Sjouke BurryGuest

Halogen has a build in cycle to keep the filament healthy.
That does not work at a lower temperature.
So running it at the wrong voltage will cause it to fail
early.
Other then that, for a decoration a dim bulb is no problem,

10. Phil AllisonGuest

"John Larkin"

** The formula that predicts lamp life is only valid for a small range
around the nominal voltage, about 10%. The simplifying assumptions it is
based on are true only in this range.

Reducing the voltage by large percentages extends lamp life by factor of 10
or possibly up to 100 - the halogen cycle has SFA to do with it.

.... Phil

11. William SommerwerckGuest

Even a slight drop in voltage causes a big increase in life. For example,
I'm guessing yes. A friend had halogen lamps on a dimmer, which he dimmed
only slightly -- and they burned out pretty quickly.

The halogen cycle requires a very high temperature. Ergo...

12. Guest

Because you DON'T run it at full brightness the envelope isn't cleared
by the halogen cycle. This is a common failure of halogen bulbs. They
get black, reducing their light output and fail early.
But you just said you rarely run it at full power. It can't if you
don't.
s/power/voltage, as pointed out earlier.
12/16, who's counting.
The halogen cycle stops.
There are carbon filament bulbs still operating from a hundred years
ago. <shrug>

13. Guest

A faulty assumption doesn't bode well for any conclusions drawn.
It must be *really* dimmed, unless it's also run for some hours at
full power. Every halogen I've dimmed has turned black after some
time.
It makes a difference.

14. William SommerwerckGuest

"John Larkin" wrote in message
Does anyone ACTUALLY READ WHAT IS POSTED? And THINK about it? Why is it
necessary to explain something in excruciating detail over and over and over
again?

15. Phil AllisonGuest

"William Sommerwerck"

** John has done that and written an excellent reply.

instead of childishly stamping your stupid fat feet all over the place.

..... Phil

16. William SommerwerckGuest

"Phil Allison" wrote in message "William Sommerwerck"
"William Sommerwerck"

** John has done that and written an excellent reply.
instead of childishly stamping your stupid fat feet all over the place.

If you'd been paying attention, you'd have seen that his most-recent remark
(above), though correct /out of context/, has nothing to do with the issue
at hand.

17. Phil AllisonGuest

"William Sommerwerck""
** What "issue" is that ??

18. William SommerwerckGuest

"Phil Allison" wrote in message "William Sommerwerck""
** What "issue" is that ??

There are actually two issues.

One... A halogen lamp has to run at or above the temperature at which the
tungsten is redeposited on the filament more rapidly than it evaporates.
This temperature is presumably well-above the temperature of a conventional
incandescent lamp. It's reasonable to assume that reducing the filament
voltage some unstated amount will lower the temperature below the critical
recycling temperature, but still high enough to cause the filament to

Two... "Obviously", if the filament voltage is "low enough", the rate of
tungsten evaporation will be so low, that it doesn't matter whether the lamp
is conventional or halogen.

These have nothing to do with each other, as the temperature for One is
almost certainly well above the temperature for Two.

This is what Wikipedia has to say. (The following is 100% accurate and
unimpeachable, of course.)

"Halogen lamps are manufactured with enough halogen to match the rate of
tungsten evaporation at their design voltage. Increasing the applied voltage
increases the rate of evaporation, so at some point there may be
insufficient halogen and the lamp goes black. Over-voltage operation is not
generally recommended. With a reduced voltage the evaporation is lower and
there may be too much halogen, which can lead to abnormal failure. At much
lower voltages, the bulb temperature may be too low to support the halogen
cycle, but by this time the evaporation rate is too low for the bulb to
blacken significantly. There are many situations where halogen lamps are
dimmed successfully. However, lamp life may not be extended as much as
predicted. The life span on dimming depends on lamp construction, the
halogen additive used and whether dimming is normally expected for this
type."

The article says that the first halogen lamps (for a carbon filament) were
patented in 1882.

19. Phil AllisonGuest

"William Sommerwerck"

** Irrelevant.

The TRUTH is that the re-deposited metal does NOT repair the damage done to
the filament - there are many pics that show this.

The halogen cycle merely keeps the quartz glass clean !!

** The simple fact is that halogen lamps do NOT have especially long lives -
any more than non halogen lamps with the same filaments.
** See - it is all about the darn glass.

** Note weasel words.
** Still all about the darn glass.

** Not many - ALL !!
** The predicted life extension ( power of 12 or 14 ) is not usable beyond
about 10% voltage reduction as the numbers become huge.

** Yep.

And the one thing that matters most is the GAUGE of the wire in the
filament.

Most halogen lamps are LOW voltage, hence THICK filaments - leading to
longer life than for high voltage ( ie 120V /240V) lamps of the same power.

It is sooooooo simple - if the surface temp is the same but there is way
more material then it takes longer for the filament to wear out.

..... Phil

20. Phil AllisonGuest

"Phil Hobbs"
** Really ?

What is that supposed to prove ? ?

Is that not a 120V quartz-halogen lamp ???

A 24V, 150W, 3300K QI ( 35mm slide) projector lamp has a rated life of 50
hours.

Such lamps have been in use common since the late 1960s.

** ROTFL.

It is NOT in the SAME places and huge gaps open up.

See the pics on Wiki.

..... Phil