# Thoughts on using PWM to drive incandescent bulbs, LEDs, smoke units etc.



## krs (Feb 29, 2008)

There are still DCC decoder manufacturers who somehow don't realize that an effective 5 volt PWM signal to drive incandescent bulbs drastically reduces the life span of the bulbs.

With LEDs this is not an issue, they are designed to be pulsed at a much higher than the rated steady-state current as longa s one doesn't exceed their power rating.
Smoke unit elements?
Never really thought about it or investigated it. I would think they have similar problems to incandescent bulbs since their filaments are also very fine and the whole concept dpnds on the filamnt evaporating the smoke fluid.
With electric motors it is again the overall power dissipation that matters.

And all of this depends to some degree on the actual PWM frequency as well which is typically not specified - the higher the frequency, the less of a problem with power dissipation.
That's why core-less motors need a PWM frequency of 16 KHz or higher.


Knut


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## Greg Elmassian (Jan 3, 2008)

Red Herring alert 

You brought this up before... why not continue your old thread? 

NO, the smoke units have thicker "filaments", although that does not matter at all. 

Basically you are not running them at a temperature where they would burn up unless they were in a vacuum... 

No problems at all... 

Greg


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## krs (Feb 29, 2008)

Posted By Greg Elmassian on 30 Aug 2012 03:23 PM 

Red Herring alert 

You brought this up before... why not continue your old thread? 

NO, the smoke units have thicker "filaments", although that does not matter at all. 

Basically you are not running them at a temperature where they would burn up unless they were in a vacuum... 

No problems at all... 

Greg 
Greg,

I never ever brought this issue up before regarding smoke units here or anywhere else because I just thought of that potential issue as I was typing that post.
And I don't know where you get that idea that smoke units can only burn up in a vacuum. Lots of smoke units burn up when running dry - no vacuum required.
You yourself posted a comment just recently that the circuit used by one manufacturer to shut off the smoke unit when it runs dry, didn't actually work.
Do you think manufacturers would add the extra expense of such a circuit if a smoke unit would only burn up in a vacuum?

Take a 5-volt smoke unit - it uses a heating that gets hot enough when 5 volts are applied to evaporate the smoke fluid to generate the smoke.
Now with PWM you are hitting it with four times the voltage - this has to have some effect unless the unit is specificaly designed to handle that.
Exactly what effect I don't know, that's why a included the question mark.

I think I'll write Seuthe to see what they have to say about it.

Knut


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## Greg Elmassian (Jan 3, 2008)

You don't get any idea? 

Do you understand the failure mode of an incandescent bulb? 

You are completely dismissing my explanation to you? 

You equate a filiment in a vacuum glowing white hot to a resistance wire running at a much lower temperature, not in a vacuum, etc.? 

Well then you don't deserve a reply, and you're just rabble rousing. 

I guess you have a mission... 

Greg


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## astrayelmgod (Jan 2, 2008)

"Now with PWM you are hitting it with four times the voltage " 

No, you aren't. The applied voltage has nothing to do with whether is DC or PWM.


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## Cougar Rock Rail (Jan 2, 2008)

Knut, I don't think it would be an issue at all for smoke makers, because from the ones I've seen it is just a big resistor that heats up and the wicking material just surrounds that resistor. So unless the resistor itself can be damaged somehow, which shouldn't happen assuming the PWM applied is adjusted so it doesn't result in a higher temperature than normal, it should be fine. No doubt in my mind that PWM will shorten incandescent bulbs, and no effect on LEDs of course. One thing to consider on PWM dimming is the effect on photography. I made a RhB signal light with LED's, and used the function decoder to dim them way down to a nice soft glow. To the naked eye it was no problem, but in videos you could see the flickering because I had dimmed them down a little too much. 

Keith


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## Greg Elmassian (Jan 3, 2008)

Yes Knut, 4 times the voltage is correct, if you assume 24 volts supply... although it's dropped a few volts by the full wave bridge in the decoder... 

But the effect is just like Keith said, we are not dealing with a semiconductor and avalanche breakdown by exceeding the voltage... 

We are not dealing with a white hot filament in a vacuum just a hair's breadth from burning up on it's own. 

We are dealing with a chunk of metal, and if you have ever seen a nichrome heater that glows read, or looked inside a toaster, you know it can last a long time glowing red hot. 

So running the same metallic element only warm enough to vaporize oil, way less than red hot, is SAFE, and not going to shorten it's life. 

The problems to be encountered is running the unit hotter than normal and burning up boards, melting plastic, clogging up the wick using elevated temperatures. 

Greg


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