At 12:53 PM 5/11/98 -0400, you wrote:
>On Mon, 11 May 1998 11:43:56 +0200 Caisson <caisson@TELEBYTE.NL> writes:
>>> Van: FScalini <FScalini@AOL.COM>
>
>>> I've seen zero crossing detection of AC currents come up frequently
>>on
>>this
>>> list lately, especially with respect to dimming applications.  What
>>is
>>the
>>> practicality of zero crossing detection?
>
>>The AC current is mostly switched by a Triac.  This device will, when
>>switched, stay that way until there is no voltage left over the
>>Kathode &
>>Anode. then it switches off and waits for a new (trigger) signal (on
>>the
>>Gate).
>

<SNIP>

>Aside from the big disadvantage of needing some factor in the output
>circuit to turn them off, thyristors have lots of advantages.  Only a
>small turn-on drive signal is required, and only for a short time until
>the device turns on.  Since the internal feedback will always turn it on
>fully, it is hard to create circumstances where the thyristor burns out
>from not being turned on completely.  Devices with large current and
>voltage capacities are inexpensive to make.
>
>In an AC circuit, the voltage and current go to zero twice in each cycle
>(though not necessarily at the same time).  Thus a thyristor switch will
>be able to turn itself off at certain times.  Often the object is to make
>a thyristor conduct for only part of the time to reduce the power
>delivered to the load (such as a lamp).  The driving circuit can only
>control the turn-on time.  The turn-off time is determined by the phase
>of the power supply voltage.  So the driving circuit needs to synchronize
>itself to the power supply in order to turn the thyristor on at the
>proper times so the on-time is as desired.  A "zero crossing detector"
>function is a necessary part of the driving circuit to achieve this
>synchronization.
>

<SNIP>

Speaking of triac based light dimmers, when designing one of these, does
one need to take into accout the heating and cooling of the lightbulbs?
When the triac switches off, the lightbulb will cool somewhat,and its
conductivity will be higher when it gets switched on, which I would
immagine would cause LOTS of current spikes at low duty cycles. Is this
ONLY an EMI concern, or might it decrease the lifetime of the bulb, as well?

Sean

+--------------------------------+
| Sean Breheny                   |
| Amateur Radio Callsign: KA3YXM |
| Electrical Engineering Student |
+--------------------------------+
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