On Mon, 11 May 1998 11:43:56 +0200 Caisson writes: >> Van: FScalini >> 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). Triacs and SCRs belong to a class of semiconductors called "thyristors" The structure of these devices contains internal feedback so once switched on, they will switch to full saturation and stay on even if the gate input signal is removed. Only stopping the current flow through the device (for most devices, for a rather "long time" of several hundred us) will allow it to turn off. 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. > If you switch your current when it is Zero, it will go up >(or >down) gradualy (along the sine-wave) and thus _not_ generate to much >problems with coils and the like. Now we move to an advanced topic. Switching a transformer is not that simple. You do want to switch off at a zero current point. A triac will do that inherently. However, switching on at the zero voltage point is not optimal. The reason is that commercial transformers are built with as little iron as possible. The core comes very close to saturation during each half-cycle during normal operation. During continuous operation, at the zero voltage point there is still some "magnetic charge" (somewhat related to current) in the core in the opposite direction. Starting the half-cycle with this negative magnetism prevents the core from saturating. However, at switch-on the core is not magnetized. The first half-cycle will saturate it. This causes a large audible "thump" from the transformer and a big surge of current from the line (and through the triac). The effect can be noticed using a manual switch. The time when a manual switch closes is random relative to the power line. Sometimes the transformer will "thump" and the lights dim, sometimes not. Using a solid-state relay with zero-voltage turn-on, the effect will happen every time. I talked to the relay manufacturer about this. They said that transformers were problematic. The optimum turn-on time is about 1/4 to 1/2 through a half-cycle, but it varies. They didn't recommend switching large transformers with SSR's, unless the current surge is acceptable and a much higher rated SSR used to prevent damage. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]