Suntan Blog

In AC power control applications—such as lighting dimmers, motor drives, or heater circuits—the use of TRIACs (Bidirectional Thyristors) remains a common and cost-effective solution. However, inconsistent triggering remains a frequent challenge that can compromise system stability, especially under variable load or low gate current conditions.

Understanding the root causes of this instability—and how to choose the right TRIAC accordingly—is key to reliable control.

Why Triggering Instability Happens

Triggering instability typically occurs when a TRIAC does not receive sufficient gate current to turn on reliably, or when the holding current is not sustained due to mismatched load conditions. Common signs include:

• Flickering in lighting applications
• Unresponsive or erratic motor control
• Failed switching under low-load or inductive environments

These symptoms often originate not from faulty components, but from improper TRIAC selection, especially in regard to gate trigger current (IGT), latching current (IL), and package thermal limitations.

The Role of Gate Trigger Current and Latching Current

A key consideration in TRIAC selection is the gate trigger current (IGT)—the minimum current required at the gate to initiate conduction.

• In control circuits with limited drive current (e.g., opto-isolators, microcontrollers), selecting a TRIAC with a low IGT, such as 3–10 mA, is critical.
• Once triggered, the latching current (IL) must be sufficient to sustain conduction. If the load current drops below this threshold during startup, the TRIAC may turn off unexpectedly.

For example, the Suntan BT134-600E (TO-126 package) offers an IGT as low as 5 mA, making it suitable for compact, low-power AC control scenarios.

Thermal Considerations and Package Selection

Apart from trigger characteristics, package type and current capacity significantly affect performance and reliability:

When controlling larger loads—such as industrial motors or heating elements—the use of TO-220 packages with adequate heatsinking is essential. The BTA16-800B, while rated for 16A, requires a higher gate current (35mA), making it more suitable for circuits with robust driver stages.

Choosing the Right TRIAC: A Design-First Approach

Here’s a simplified checklist for selection:

1. Start with your load: What’s the peak current and voltage rating required?
2. Assess your gate drive: Can your controller or driver provide sufficient IGT?
3. Consider thermal performance: Is a heatsink feasible in your design?
4. Check for certifications: Industrial or appliance-grade requirements?

Choosing a TRIAC that meets these requirements ensures clean turn-on, stable conduction, and minimal false triggering—especially in noisy or inductive environments.

Wrap-up: Not All TRIACs Are the Same

Selecting a TRIAC is more than matching voltage and current ratings. For engineers dealing with precision AC control, understanding gate behavior and thermal profiles is essential. A mismatch in trigger characteristics can easily undermine system stability—even with otherwise high-quality components.

Need More Information?

Feel free to reach out:
📧 info@suntan.com.hk | sales@suntan.com.hk

Related Links

• BT134-600E – TO-126, 600V / 4A (Low IGT)
• BT138-800E – TO-220, 800V / 12A
• BT139-800T – TO-220, 800V / 16A
• BTA16-800B – TO-220, 800V / 16A (High Power)

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