Not preheating par 56s...

[Ike]

Simple enough question: If a load of par 56 house lights are regularly (five or six times a week) switched on and off hard power from cold as well as being run off a dimmer is the hard switching likely to kill the lamp life? Is it worth installing some kind of soft start trickery for a couple of dozen fittings?

 

Edit: Poll added.

[David A]

No, they are designed for hard powering and there is no evidence that pre-heating has any real effect, it may even reduce life, so don't waste your money.

[Ike]

Great!

[musht]

No, they are designed for hard powering and there is no evidence that pre-heating has any real effect, it may even reduce life, so don't waste your money.

 

Soft Start being the process of ramping the lamp up over a short peiod , typically less than a second, which personally convinced has a positive effect on lamplife, lot of electronic lamp trafos have this feature built in, and

Preheating, keeping lamps on very low check for possibly extended periods, for purposes of faster response during chasing etc.

 

This is possibly the Blue-Rooms equivalent of the congo blue debate....

[Ike]

OK I think the confusion is probably my fault for not thinking what I was going to say before starting typing. If I did implement a system it would be soft start not traditional preheating. While I'm sure it does have some advantage I'm really interested in just how much and if not providing it would mean a substantial loss of lamp life.

 

As a bit of background the job I'm wondering about it an amateur venue and I'm not charging for my time so it isn't a case of just doing it and charging it to the company.

[Ekij]

I'm really interested in [knowing] if not providing [soft start] would mean a substantial loss of lamp life.

And it's the "substantial" thats the crunch.

What exactly do you consider as substantial?

How many is "a load" of lamps?

How much would your system cost?

Lets say it increases life by 20% ... is it cost effective?

(BTW 20% is just a guess).

 

Then again soft ramp on and soft ramp off are pretty effects. They also help avoid hurting someones eyes if they happen to be looking right at the lamp when it switches on. (So there are other advantages to implementing it).

 

If it's a fun project you'd like to do ... go for it!

[David A]

In real life situations there is an impedance in the wiring which will limit the 10x inrush current, while everyone knows that it is common sense that pre-heating extends life, it does not actually stand up to investigation.A par 56 is essentially an industrial lamp and would seldom not be hard powered on.I can be pretty certain that you will find no empirical evidence that soft starting has any measurable effect, but I would be fascinated to look at any evidence to the contrary.The only lamps I have seen where the manufacturer recommends soft start are short life high power medical units.

[DaveP]

Soft start is very very good !!

 

Why bulbs often burn out when you turn them on

 

Many people wonder what goes on when you turn on a light. It is often annoying that a weak, aging light bulb will not burn out until the next time you turn it on.

The answer here is with those thin spots in the filament. Since they have less mass than the less-evaporated parts of the filament, they heat up more quickly. Part of the problem is the fact that tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on.

When the thin spots have reached the temperature that they would be running at, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament's resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up.

This means that the thin spots, which run too hot anyway, get even hotter when the thicker parts of the filament have not yet fully warmed up. This is why weak, aging bulbs can't survive being turned on.

 

Why burnout is sometimes so spectacular

 

When the filament breaks, an arc sometimes forms. Since the current flowing through the arc is also flowing through the filament at this time, there is a voltage gradient across the two pieces of the filament. This voltage gradient often causes this arc to expand until it is across the entire filament.

Now, consider a slightly nasty characteristic of most electric arcs. If you increase the current going through an arc, it gets hotter, which makes it more conductive. Obviously, this could make things a bit unstable, since the more conductive arc would draw even more current. The arc easily becomes conductive enough that it draws a few hundred amps of current. At this point, the arc often melts the parts of the filament that the ends of the arc are on, and the arc glows with a very bright light blue flash. Most household light bulbs have a built-in fuse, consisting of a thin region in one of the internal wires. The extreme current drawn by a burnout arc often blows this built-in fuse. If not for this fuse, people would frequently suffer blown fuses or tripped circuit breakers from light bulbs burning out.

Although the light bulb's internal fuse will generally protect household fuses and circuit breakers, it may fail to protect the more delicate electronics often found in light dimmers and electronic switching devices from the current surges drawn by "burnout arcs".

 

How bad a current surge bulbs draw when turned on

 

It is fairly well known that a cold light bulb filament has less resistance than a hot one. Therefore, a light bulb draws excessive current until the filament warms up.

Since the filament can draw more than ten times as much current as usual when it is cold, some people are concerned about excessive energy consumption from turning on light bulbs.

The degree of this phenomenon has become a matter of urban folklore. However, the filament warms up very rapidly. The amount of energy consumed to warm up a cold filament is less than it would consume in one second of normal operation.

 

Ref: The physics of electricity as it seems to work in the northern hemisphere ;) and http://business.virgin.net/tom.baldwin/bulbguide.html

[Ike]

I guess anything over 10% would probably make it worth while. 20% defiantly, 2% not.

23 lamps in total.

Cost to the client zero, cost to me a few hours (I'd hope), cost to my bosses RS account nothing they'd notice.

Room temperature never too cold, I'd say always above 8C.

Lamps probably on for around 20 - 25 hours a week, all brand name.

Hard switching approx. 6 cycles a week.

 

Could be fun but I already have a long list of fun projects I should be getting on with not to mention the day job. ;)

[David A]

I can forecast the result of your poll. about 20/1 in favour of pre-heat, however the laws of physics don't respond to opinion polls.Now if Ike was to fit half the lights with soft start and leave half on switches that could prove something.

[Don Allen]

Having a smooth fade up and fade down for house dimmers is a more professional look so use that as your design goal and you get your soft start option included.

 

Running your house lights at 10% or higher is going to make it hard to achieve a blackout, so take that into account when you choose which option to go for.

[Tomo]

Preheat is definitely not worth bothering with for houselights, as you get less than zero benefit.

 

A Soft-Starter may be worthwhile.

Heavy industry uses these all the time for motor control, but while they are generally not used for illumination this is because most industrial illumination uses various forms of discharge lamp.

 

A quick Google didn't find any mid-range soft-start devices - just d0mestic varieties based on thermistors - so it's likely that there isn't much market for them.

[Bruff]

Simple enough question: If a load of par 56 house lights are regularly (five or six times a week) switched on and off hard power from cold as well as being run off a dimmer is the hard switching likely to kill the lamp life? Is it worth installing some kind of soft start trickery for a couple of dozen fittings?

 

Edit: Poll added.

 

A thermistor in series with the lamp will extend the lamp life (this isn't speculation, I've tested it). Personally, I wouldn't bother for PAR56's unless they were difficult to replace.

Thermistors get hot - allow for this if you're going the home-made route.

 

Steve.

[JimWebber]

I voted yes, not because it will extend lamp life - (Although it probably will, but by how much?) but from an asthetic point of view, it would be a great deal kinder to the punter's eyes!

 

Jim

[Ike]

Having a smooth fade up and fade down for house dimmers is a more professional look so use that as your design goal and you get your soft start option included.

When used as house lights they will be fed from a dimmer with the usual fade up and down. They will only be connected to hard power when being used as FOH work lights for cleaning, fit-ups etc. when the portable dimmers may not be powered up (or even there!).

 

I will of course be using a double pole break before make changeover switch to prevent back-feeding.

 

Fitting half on soft start and half not could be interesting, I might have a play when I get a bit of time. Thanks for everyones input.