| ??? 11/24/04 16:36 Read: times Msg Score: +1 +1 Informative |
#81862 - Mains looks seldom like pure sinus Responding to: ???'s previous message |
It's time to give you a positive Karma point, Russell! Your posts in this thread are excellent!
Russell said:
This signal is so strong and not synchronised with the mains that when it passes through zero crossing it can cause a few mS of zero cross shift in the zero cross detector - this causes flickering...
...as little as 10uS of phase change can cause the brightness to visibly change. Well, I tend to agree with Russell. Maybe nothing is wrong with your application at all? It's only due to imperfections of actual signal shape of mains voltage and sensitivity of human eye? It's worth for everyone to have a look with scope at mains voltage from time to time. You will be surprised how much signal shape differs from pure sinus!!! If mains voltage is not clean, then it seems to be impossible to detect this ideal moment of 'zero crossing', because it just don't exist. The smaller the phase angle is set, the more the imperfections of mains voltage signal shape make your lamp flicker. You will notice that flicker decreases the more you increase phase angle, right? One idea is to introduce low pass filtering before zero cross detection. By the way, these simple DIAC/TRIAC dimmers do also provide some filtering, by the phase shifter in front of DIAC. A disadvantage of low pass filtering only for the zero crossing detector signal path is, that these filters tend to provide a wrong zero crossing after dips and synchronization imperfections of mains. This can also lead to unwanted flickering... A better way is to filter mains voltage by suited mains filter. This filter can also help to filter out the interference resulting from your triac switching. Kai |
