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Triac # Q6040JT
- Thread starter laatsch55
- Start date
Lee,
Triacs, to use a popular term, are binary. Unlike a bipolar transistor that can operate in a linear state (such as amplifier output transistors), triacs are either turned on or turned off. They do not operate linear. A triac is, for all practical purposes, 2 SCR's operating in reverse parallel. One SCR conducts on the positive half cycele. The other SCR conducts on the negative half cycle.
One way of controlling temperature with triacs is known as phase angle control. This methods uses the triac to turn on for a portion of each half cycle. The longer the triac conducts for each 60Hz cycle, the more heat produced by the heating element. The now obsolete incandescent light bulb dimmers operate in the same manner.
Every time the triac turns on, the current rises very quickly from nothing to some value. This fast rising current can cause RF interference. Consequently AM radios can be impossible to listen to when a light dimmer is used. Some dimmers add a series inductor to cut down the noise. Another way to control heaters while minimizing RF interference is to turn on the SCRs for several complete cycles, and then leaving them off for several cycles.
Triacs tend to be limited to about 40 amps. For applications such as large industrial heaters that require thousands of amps, two reverse paralleled SCR's are used.
SCR's are very reliable and have very high surge current ratings. They can be protected from short circuits with fuses, filled with sand, so they open really fast. Bipolar transistors are far more failure prone when dealing with short circuits. High speed fuses don't protect transistors from short circuits.
While I'm on my soap box, I wish to mourn the slow death of the incandescent light bulb. Incandescent light bulbs have all kinds of uses besides making light. An LED in a Kenner Easy Bake Oven will not bake those little cakes. Another incandescent light bulb use is the dim bulb test set. Many a load bank has been made with the glass bulbs.
Triacs, to use a popular term, are binary. Unlike a bipolar transistor that can operate in a linear state (such as amplifier output transistors), triacs are either turned on or turned off. They do not operate linear. A triac is, for all practical purposes, 2 SCR's operating in reverse parallel. One SCR conducts on the positive half cycele. The other SCR conducts on the negative half cycle.
One way of controlling temperature with triacs is known as phase angle control. This methods uses the triac to turn on for a portion of each half cycle. The longer the triac conducts for each 60Hz cycle, the more heat produced by the heating element. The now obsolete incandescent light bulb dimmers operate in the same manner.
Every time the triac turns on, the current rises very quickly from nothing to some value. This fast rising current can cause RF interference. Consequently AM radios can be impossible to listen to when a light dimmer is used. Some dimmers add a series inductor to cut down the noise. Another way to control heaters while minimizing RF interference is to turn on the SCRs for several complete cycles, and then leaving them off for several cycles.
Triacs tend to be limited to about 40 amps. For applications such as large industrial heaters that require thousands of amps, two reverse paralleled SCR's are used.
SCR's are very reliable and have very high surge current ratings. They can be protected from short circuits with fuses, filled with sand, so they open really fast. Bipolar transistors are far more failure prone when dealing with short circuits. High speed fuses don't protect transistors from short circuits.
While I'm on my soap box, I wish to mourn the slow death of the incandescent light bulb. Incandescent light bulbs have all kinds of uses besides making light. An LED in a Kenner Easy Bake Oven will not bake those little cakes. Another incandescent light bulb use is the dim bulb test set. Many a load bank has been made with the glass bulbs.
Last edited:
- Joined
- Jan 14, 2011
- Messages
- 74,825
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof
- Joined
- Jan 14, 2011
- Messages
- 74,825
- Location
- Gillette, Wyo.
- Tagline
- Halfbiass...Electron Herder and Backass Woof