Mica Insulator Sheet for BU508A (TO-218 / TO-247)

Description

The Mica Insulator Sheet for BU508A is a precision-cut thermal management and electrical isolation pad designed specifically for large, high-power transistors. Because heavy-duty components like the famous BU508A (used heavily in CRT television horizontal deflection circuits) operate at extreme voltages and generate massive heat, they must be bolted to large metal heatsinks. However, the metal mounting tab on the back of the transistor is electrically live.

Available at Tomson Electronics, this pre-cut mica pad solves that problem. Placed directly between the transistor and the heatsink, it acts as an impenetrable electrical wall (preventing a catastrophic short circuit) while still allowing the heat to pass through efficiently. Perfectly sized for TO-218, TO-247, and TO-3P transistor packages, it comes with the mounting screw hole already cleanly punched out to save you time on the repair bench.

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Specifications

• Material: Natural Muscovite Mica
• Compatible Packages: BU508A, TO-218, TO-247, TO-3P
• Pre-Punched Hole: Yes (Standard 3mm / M3 screw clearance)
• Operating Temperature: Up to 500°C
• Dielectric Strength: > 20 kV/mm
• Function: High electrical isolation with excellent thermal conductivity

Applications

• CRT TV & Monitor Repair: Safely mounting high-voltage horizontal deflection transistors (like the BU508A or BU2520A) to their chassis heatsinks.
• Audio Amplifiers: Isolating large TO-247 power MOSFETs and bipolar transistors in high-fidelity audio output stages.
• Power Supplies: Mounting high-current voltage regulators and switching transistors in switch-mode power supplies (SMPS) or welding inverters.

Frequently Asked Questions (FAQ)

Q: Do I need to use thermal paste with this mica pad?
A: Yes! Mica is naturally stiff and has microscopic surface imperfections. To ensure maximum heat transfer, you must apply a very thin, even layer of silicone thermal grease (heatsink compound) to both sides of the mica pad before bolting the transistor down.

Q: Why use a mica pad instead of a modern grey silicone thermal pad?
A: Modern silicone pads are convenient because they don't require thermal paste, but they generally have a lower dielectric breakdown voltage and cannot survive the extreme heat transients that natural mica can. For a high-voltage part like the BU508A (which can see 1500V spikes), traditional mica remains the safest and most reliable industry standard.

Q: Will I need a plastic washer for the screw as well?
A: Yes. Because the metal mounting screw passes through the live transistor tab and into the grounded heatsink, you must use a small plastic insulating "shoulder washer" (sometimes called a bush) on the screw head to completely isolate the screw from the transistor.

Package Includes

• 1 × Mica Insulator Sheet for BU508A (TO-218 / TO-247 / TO-3P)