2
\$\begingroup\$

In numerous sources I've heard that it's "easier" for a heater if you don't switch fast current transients on it. How come? Why does the wire degrade?

\$\endgroup\$
2
\$\begingroup\$

There is going to be some stress every time the temperature changes. Small fillaments might be able to change temperature significatly enough over half a line cycle so that phase-dimming might matter, although in most cases it is probably not a large effect. If you are asking about a "heater", then presumably the thermal time constant is much much longer than a power line cycle, so there would be no harm to them due to phase control dimming.

The issue with phase control dimming is the nasty power factor is presents back to the power source and interference it can radiate and conduct back onto the power line.

plot of phase control dimming waveform

For example, look at this waveform (in red) of a phase-control dimmer set to 60V. Notice how rapidly it rises- indicating that the waveform contains very high-frequency harmonics which will radiate out from the power lines, potentially interfering with sensitive circuits.

Both these are real issues, and reasons why you don't see phase control dimming much anymore. Good riddance.

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ and what is the "state of the art" dimming now? \$\endgroup\$ – miceuz Apr 26 '13 at 20:51
  • \$\begingroup\$ @mice: It depends on the device being dimmed, but usually some sort of PWM running at many times the power line frequency. That adds high frequency noise, which can be filtered out much more easily, thereby still presenting a good power factor back to the power source. \$\endgroup\$ – Olin Lathrop Apr 26 '13 at 20:55
  • \$\begingroup\$ @OlinLathrop: At least in the US I think triac-based dimmers are still the norm. Are they not? \$\endgroup\$ – supercat Apr 26 '13 at 23:58
0
\$\begingroup\$

It's also too do with the "Halogen Cycle"

Basically the tungsten that flys off the filament in a normal lamp in normal use in a halogen re-attaches (but in a random place) for this to happen the lamp need to be at its maximum temprature.

By dimming it you are denying it the heat it needs and so the life span advantages of halogen are void.

https://en.wikipedia.org/wiki/Halogen_lamp#Halogen_cycle

| improve this answer | |
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.