Lights rating

[tim west]

I'm not sure what you are trying to say there Tim, but certainly running a dimmer at half mast is more stressful than turning it full on, your explanation of the phase cutting that happens with a standard dimmer is spot on, but the instantaneous currents (which matter to electronics) are going to greater,if for example you set your dimmer to loose the first third of each half cycle, your lamp is obviously going to be cooler, which means a lower resistance which means that for the part of the cycle for which the triac does conduct the currents will be greater (just ohms law)

I suppose if you just put it on say a 10th where the waveform is just about to cross zero again and the voltages involved are a long way from peak then the currents would be manageable and it'd just take the chill off the filament, like preheat does, before turning it up to full pretty quickly, but turning it up slowly and going through the range when you can actually see any visable light is going to be more stressful for it than putting it straight on (though with most dimmers going through this would be difficult to avoid!)

agree with the bolded but not with the rest as explained before. I stated that you would start off with the dimmer set low (same as preheat) so the portion of the waveform conducting through the triac would indeed be small but so would the current, the effect on the filament would be to warm it enough to stop the inrush you would get if you were to switch the dimmer full on from start, Soft start dimmers as stated in another post cater automatically for this and allow the filament load to warm before delivering the full waveform if so called for by the user.

 

[bernardgreen]

Tim

Which do you consider is the most common mode of failure in the triacs of dimmers ?

 

[tim west]

Tim

Which do you consider is the most common mode of failure in the triacs of dimmers ?

Short circuit or open circuit of one or both of the individual elements (thyristor)

 

[bernardgreen]

Tim

Which do you consider is the most common mode of failure in the triacs of dimmers ?

Short circuit or open circuit of one or both of the individual elements (thyristor)

Yes but what lead to the failure ? Going "open circuit" can be destruction of the gate with the main channel left intact but without a gate to open it.

 

[tim west]

Tim

Which do you consider is the most common mode of failure in the triacs of dimmers ?

Short circuit or open circuit of one or both of the individual elements (thyristor)

Yes but what lead to the failure ? Going "open circuit" can be destruction of the gate with the main channel left intact but without a gate to open it.

Or destruction of the thyristor output. are you asking what causes this?

 

[bernardgreen]

Or destruction of the thyristor output. are you asking what causes this?

Yes.

Assume a 60 watt dimmer running 12 volt output. This suggests the dimmer can carry a current of 5 amps.

If the lamps are being pre-heated, running very low power, what is the average current and what is the pulse current. The average may be well below the 5 amp but the pulse current may be significantly higher.

The triac suffers thermal induced failure if the average current is above 5 amps for a period of time. It suffers one of several possible failures if the pulse current is above 5 amps. Most of these are failures associated with voltages generated by the over limit pulsed current passing through the devices internal resistance. With low resistance halogens the peak current can be above the maximum rating of the device.

 

[tim west]

Or destruction of the thyristor output. are you asking what causes this?

Yes.

Assume a 60 watt dimmer running 12 volt output. This suggests the dimmer can carry a current of 5 amps.

If the lamps are being pre-heated, running very low power, what is the average current and what is the pulse current. The average may be well below the 5 amp but the pulse current may be significantly higher.

The triac suffers thermal induced failure if the average current is above 5 amps for a period of time. It suffers one of several possible failures if the pulse current is above 5 amps. Most of these are failures associated with voltages generated by the over limit pulsed current passing through the devices internal resistance. With low resistance halogens the peak current can be above the maximum rating of the device.

Sorry Bernard I don't understand your theory here, are we talking about a mains dimmer wound down to produce an output voltage of 12V?
If so then your next statement is wrong about a 60 watt load at 12V giving a current of 5A capability that is using ohms law to work out the current which doesn't apply with dimmer output as easily as that. the power rating has changed therefore your calculation is wrong yes the dimmer is capable off supplying 5A but at 230V(if calculating using a sine wave). You mention the low resistance of the halogens when cold-agreed but you seem to forget I also mentioned that with preheat those filaments would warm up sufficiently to stop any inrush current, you seem to be forgetting that the power to the circuit when the dimmer is set low is not the same as the power when the dimmer is fully wound up. Hope that clears it up for you

 

[tim west]

Or destruction of the thyristor output. are you asking what causes this?

Yes.

Assume a 60 watt dimmer running 12 volt output. This suggests the dimmer can carry a current of 5 amps.

If the lamps are being pre-heated, running very low power, what is the average current and what is the pulse current. The average may be well below the 5 amp but the pulse current may be significantly higher.

The triac suffers thermal induced failure if the average current is above 5 amps for a period of time. It suffers one of several possible failures if the pulse current is above 5 amps. Most of these are failures associated with voltages generated by the over limit pulsed current passing through the devices internal resistance. With low resistance halogens the peak current can be above the maximum rating of the device.

Sorry Bernard I don't understand your theory here, are we talking about a mains dimmer wound down to produce an output voltage of 12V?
If so then your next statement is wrong about a 60 watt load at 12V giving a current of 5A capability that is using ohms law to work out the current which doesn't apply with dimmer output as easily as that. the power rating has changed therefore your calculation is wrong yes the dimmer is capable off supplying 5A but at 230V(if calculating using a sine wave). You mention the low resistance of the halogens when cold-agreed but you seem to forget I also mentioned that with preheat those filaments would warm up sufficiently to stop any inrush current, you seem to be forgetting that the power to the circuit when the dimmer is set low is not the same as the power when the dimmer is fully wound up. Your thinking is more in line with using a Rheostat rather than a triac based dimmer.Hope that clears it up for you

 

[tim west]

apologies to the above post as i have added a line to the original and it has ended up like that?? could someone explain why when editing the original post that it created a new post apart from making it look like it was a quote all in white

 

[Softus]

could someone explain why when editing the original post that it created a new post apart from making it look like it was a quote all in white

Yeah you probably forgot that you'd just logged out after using a different user name.

 

[bernardgreen]

Tim

I have just dug out my design notes from the days when I was involved in the design of bespoke switch mode power supplies.

Using a domestic dimmer on lowest setting as keep alive for tungsten halogen lamps will be running the triac on the limits of its safe operating area. This is not the place to go into the full details.

 

[Adam_151]

Using a domestic dimmer on lowest setting as keep alive for tungsten halogen lamps will be running the triac on the limits of its safe operating area. This is not the place to go into the full details.

I did wonder if there was a point so late in the half cycle where you could trigger it where even though the lamp was cold with all the lack of resistance that brings, that the current peaks would be less than the full running current of the lamp ... though thinking about it more, I reckon it'd be much further than 90% along and not have any real warming benefit

Oh and swith mode power supplies - why does it seem that whenever consumer goods come with these that they don't ever last very long at all, I suppose bad design and cost cutting... I'd take a supply unit based on a proper full sized wound transformer any day over that kind of crap....

 

[tim west]

Sorry but i beg to differ,
Dimmers with preheat have that function for that very reason, one to stop any inrush current stress and two to make the dimmer more responsive when used in a fast chasing mode like say in a concert.

 

[bernardgreen]

To sucessfully pre-heat a bulb with very low cold resistance the current has to be controlled by the dimmer. But most domestic dimmers control the average voltage by phase angle switching and the current is determined by voltage and resistance of bulb.

A well designed but more expensive dimmer will apply a voltage to the bulb via an inductor and at the same time measure the current. The current does not go immediatley to V/R due to the impedance of the inductor but instead ramps up from 0 towards V/R. When the current reaches either the set point required by brightness control or the limit for safe operation of the dimmer the voltage is removed until the current falls below that level when the voltage is re-applied and this cycle repeats.

Even if power is applied to a cold filament at all times the current remains less than the maximum the dimmer can handle therefore protecting the dimmer. There is no high current flow in the filament thus magnetic "motor" effects in the coiled filament and resulting mechanical stresses are reduced.

 

[ricicle]

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