Demux 24 Preheat

[KevinE]

Im with musht on this one.

 

The amount a dimmer leaks will depend on the load, and the design of the dimmer. I suggest if a dimmer leaks so much that it causes natural filament preheat then it's either faulty or a very large dimmer on a very small load. Triacs and thyristors aren't naturally leaky, any leakage comes from the snubber & RFI circuits which are becoming less leaky as triac design improves.

 

A 12 volt car headlamp will have the resistance of the wiring in circuit to help prevent surges, after all it's only working on 12V and the impedance of the supply is not infinitely low. A 240V halogen lamp will still have a low cold resistance yet the mains will have a proportionately lower impedance and will have a much higher pro-rata surge. Whilst many theatre lamps undoubtedly are designed to be switched on from cold, some lamps used in luminaires used in the theatre (which is not the same thing) do have short lives without preheat.

 

The usual way to set the preheat is to advance the control (where fitted) until the filament is just seen to glow like the element on an electric fire. (Quoting from an NJD DP10000X dimmer manual). If a lamp fails, its usually at switch-on, isn't it!?! Due to the switch-on surge. It's more relevant to mains-driven lamps, certain types supplied from transformers don't need it at all because of the inherent impedance of the transformer.

 

If you're going to add a preheat value from a DMX desk then of course it's all academic so leave the pack preheat off.

 

I would say that 30% of full level through a dimmer is not preheat at all but 30% on!

[David A]

OK then grab a meter and check your dimmer, if your dimmer is not outputting 2.5 volts or much more it will be a very unusual unit.I work on 50 - 100 dimmers a year and have yet to see one that gives out less than 5 volts and often around 10.

[KevinE]

But this low voltage is surely unlikely to give any kind of preheat effect?

[musht]

But this low voltage is surely unlikely to give any kind of preheat effect?

 

Mebbe in the cable from the dimmer to the lamp ;-)

[J Pearce]

Remember that thyristor/triac dimmers don't directly reduce the voltage, just the amount of time the mains sine wave is outputted for.

 

Therefore, depending on the dimmer (leading or falling edge) the lantern may well be receiving 230V, but for a very short period of time.

[David A]

Wuddy produced manufacturers data which stated that "less than 1%" will extend life and dimmers provide that anyway, therefore preheat is a waste of time, money and lamplife.

[KevinE]

1% of input power will depend on the lamp though, wont it, for example 1% of a 1000W lamp is 10W, as the filament gets hotter the increase of voltage required to increase the power through it will change since a filament is not an ohmic load and does not follow ohm's law..and so 1% of dimmer level is not necessarily the same thing. It's why modern dimmers have selectable dimmer law for different types of lamp characteristics!

 

Whether a dimmer is leading or trailing edge, high frequency switch-mode or burst fire is neither here nor there since the power out is a function of 'area under curve' of voltage vs time.

[musht]

Wuddy produced manufacturers data which stated that "less than 1%" will extend life and dimmers provide that anyway, therefore preheat is a waste of time, money and lamplife.

 

Preheat for an extended period, minutes, dosent do a lot for lamps really would agree, unless its prior or during a chase for example.

 

Soft starting lamps, which any lamp on a dimmer does to an extent and some electronic supplies do, better LV trafos, does in my humble experience extend lamp life by acting as a limit on inrush current.

[David A]

1% of input power will depend on the lamp though, wont it, for example 1% of a 1000W lamp is 10W, as the filament gets hotter the increase of voltage required to increase the power through it will change since a filament is not an ohmic load and does not follow ohm's law..and so 1% of dimmer level is not necessarily the same thing. It's why modern dimmers have selectable dimmer law for different types of lamp characteristics!

 

Whether a dimmer is leading or trailing edge, high frequency switch-mode or burst fire is neither here nor there since the power out is a function of 'area under curve' of voltage vs time.

A filament is a resistive load and Ohms law applies, in this universe at least.Different curves have nothing to do with it.

[mar]

A filament is a resistive load and Ohms law applies, in this universe at least.Different curves have nothing to do with it.

 

Except that Ohm's Law in the form R = V / I only applies for constant temperature. As soon as the temperature increases, the resistance will increase (in most cases anyway). As I'm sure you realise, filaments do get rather hot as the power increases (in fact they are designed to!) and as such their resitances will be anythying but constant and Ohm's Law does not apply.

[DavidLee]

A filament is a resistive load and Ohms law applies, in this universe at least.Different curves have nothing to do with it.

Ohm's - ie the current passing through a conductor between two points is directly proportional to the potential difference between them - is only followed where you can define an unique constant of proportionality - the resistance. For a filament this is only true if you hold its temperature constant, which rather defeats its purpose in a lamp!

 

David

 

Edit: Beaten to it - obviously typing speed must deteriotate with advancing age!

[Tomo]

Ohm's Law does apply, but in the case of a filament lamp connected to a dimmer ALL THREE VARIABLES change value throughout the dimming cycle.

 

0%, Cold - resistance very low, no volts, no current.

1%, Cold - resistance increased slightly, 12VAC, some current.

1%, Warm - resistance higher, 12VAC, less current

 

50%, Hot - resistance high, 115VAC, half rated current.

100%, HOT - Resistance high, 230VAC, rated current.

 

That fade is called a "Soft Start", and it is extremely effective in limiting the inrush current.

 

Soft Starts are used in many industries, not just the lighting industry, and are widely acknowledged to provide extreme cost savings in maintenance.

 

Edit: Must type faster

 

Anyway - this has all got extremely off-topic.

 

Zulu:

Best bet is to turn preheat OFF at the demux and dimmers.

 

If you have a sequence where certain lamps benefit from a momentary preheat before flashing hard - program it into the lighting console. It's much easier to adjust that way.

[David A]

Thanks Tomo Ohms law always applies, everything else may vary, the only incorrect part of your thread is that 1/2 current occurs at around 25%-30%, depending on the curve of your dimmer.In fact most theatre cues are "soft start"anyway.