are silicon bilateral switches be used as diacs?

[mikem]

I had a circuit that worked with a 5 amp triac (using a diac for a trigger in the gate)and when I used a 40 amp triac, it doesn't dim smoothly. It snaps on at about 3/4 of the turn of the pot. I have some silicon bilateral switches and wonder if they are an equiv. substitute for a diac? Anybody have any ideas why I am having this problem? I am making a dimmer to control the voltage supplied to a ceramic kiln.

[Chris Smith]

A lot of the cheaper dimmers were like this. You changed the Triac and my guess is that the "Gate" values are quite different. You could experiment with the trigger circuitry if it has caps and resistors other than just the pot, and the pot may not be sensitive also to do the adjusting in a sensitive manner. Your start value for the pot may be 50k and then up while 48 k to 55 k might need a very sensitive increase.

[russlk]

Triacs are classified as "standard" or "sensitive gate". Some are designed to switch on with logic levels. You may have replaced a standard type with a sensitive gate type.

[L. Daniel Rosa]

For a ceramic kiln, could you stand to have the heat cycled on and off every few seconds instead of at power line frequency? If so there are electric range controls good to 20 amps or so. If you have two or more heating elements, you could split the load between two controls.

[Chris Smith]

If the element is a standard quartz or metal element, you could cycle it after it gets warm some what, [cold is bad and fatigues the element] and perhaps you might shorten the life span but I would say not too much? I wouldn't have the dwell between the on and off too long because this change in temperature if great, would fatigue the heating element. On one second, off one second and then slow this down this time frame [up or down] for more time to adjust the temperature properly. You would have to experiment to get your temperature. Faster would lower the temperature while slower should warm it up. You can adjust both the on, or off time separately to adjust. I would back off the on time to cool it down rather than extend the off time. Perhaps as high as a 5:1 ratio might work best. One on, five off? One fifth of a second ON? One second OFF? Work from there?

[mikem]

The heating element is a Kapton wire wound kiln heater. I cut a 220V one into two and want to run it on 110V. The element temp is a function of the length of the element( actually the resistance) and the voltage. Since the length is fixed I need to be able to control the voltage. When I melt aluminum, I need to raise the temp over the melting point by enough to keep it from solidifying before it flows into the mold. A range element would work but they are expensive and I think the lights would blink everytime it turned on and off. I appreciate all the ideas and I will report back when I get things worked out. Thanks--Mike.