{"id":16494,"date":"2026-02-10T18:10:01","date_gmt":"2026-02-10T12:40:01","guid":{"rendered":"https:\/\/johnsonfrancis.org\/techworld\/?p=16494"},"modified":"2026-02-10T18:10:03","modified_gmt":"2026-02-10T12:40:03","slug":"how-to-plan-essential-protection-circuitry-overdrive-swr-overheat-for-hf-linear-amplifier","status":"publish","type":"post","link":"https:\/\/johnsonfrancis.org\/techworld\/how-to-plan-essential-protection-circuitry-overdrive-swr-overheat-for-hf-linear-amplifier\/","title":{"rendered":"How to Plan Essential Protection Circuitry (Overdrive, SWR, Overheat) for HF Linear Amplifier?"},"content":{"rendered":"<iframe loading=\"lazy\" width=\"560\" height=\"315\" src=\"https:\/\/www.youtube.com\/embed\/3E1zBB7PBlA?si=bUir6S_NYEnaDhBQ\" title=\"YouTube video player\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe>\n\n<p class=\"wp-block-paragraph\">Building a reliable HF linear amplifier, especially one utilizing robust components like the IRFP150, requires more than just a solid RF deck. The protection circuitry acts as the &#8220;brain&#8221; that prevents expensive silicon from turning into expensive smoke. For a 100W+ build, you need to monitor three critical failure points: <strong>Overdrive<\/strong>, <strong>High SWR<\/strong>, and <strong>Overheat<\/strong>. As I have already faced an overdrive problem and luckily escaped without &#8216;expensive smoke&#8217;, I am wondering how I can implement the following possible protections!<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">1. Input Overdrive Protection<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The gates of MOSFETs are incredibly sensitive to voltage spikes. If your transceiver accidentally spikes or you forget to dial back the power, the IRFP150 can fail instantly.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>The Mechanism:<\/strong> Use a <strong>sampling coupler<\/strong> or a simple resistive tap at the input. This RF is rectified into a DC voltage.<\/li>\n\n\n\n<li><strong>The Circuit:<\/strong> A fast-acting comparator (like the LM311 or LM339) compares this sampled voltage against a preset reference (set by a multi-turn trimpot). LM311 is a single, high-speed precision comparator while LM339 is a &#8220;quad&#8221; comparator.<\/li>\n\n\n\n<li><strong>The Action:<\/strong> The comparator output triggers a high-speed relay or an electronic attenuator to disconnect the input or shunt it to ground.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">2. SWR (Reflected Power) Protection<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">If a wrong antenna is accidentally connected, high reflected power creates high voltage peaks at the MOSFET drains, leading to breakdown.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>The Mechanism:<\/strong> A <strong>Bruene Bridge<\/strong> is an option which can be used at the amplifier output. Bruene Bridge is a classic and highly reliable circuit used in amateur radio to measure Forward and Reflected power. Unlike a simple SWR bridge, the Bruene design uses a combination of a capacitive voltage divider and a current transformer to provide accurate readings across a wide frequency range.<\/li>\n\n\n\n<li><strong>The Logic:<\/strong> * Forward power and Reflected power are converted to DC.\n<ul class=\"wp-block-list\">\n<li>If the reflected DC voltage exceeds a threshold (typically representing an SWR > 2.0:1 or 3.0:1), the protection kicks in.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li><strong>Integration:<\/strong> This should trigger a &#8220;Latching&#8221; fault. Once SWR protection trips, the amplifier should stay off until you manually reset it, preventing it from &#8220;chattering&#8221; under high SWR.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">3. Thermal Protection (Overheat)<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Since I am working with an aluminum heat sink for IRFP150s, managing the thermal mass is vital. MOSFETs exhibit &#8220;thermal runaway&#8221; if they get too hot, where they draw more current, leading to even more heat.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>The Sensor:<\/strong> Mount a <strong>10k NTC (Negative Temperature Coefficient)<\/strong> <strong>Thermistor<\/strong> or a dedicated IC like the <strong>LM35<\/strong> directly onto the heat sink, as close to the MOSFET casing as possible. LM35 outputs exactly 10 mV per degree Celsius<\/li>\n\n\n\n<li><strong>Two-Stage Logic:<\/strong>\n<ul class=\"wp-block-list\">\n<li><strong>Stage 1 (Fan Control):<\/strong> At 45\u00b0C, trigger the cooling fans to high speed.<\/li>\n\n\n\n<li><strong>Stage 2 (Emergency Cutoff):<\/strong> At 75\u00b0C\u201380\u00b0C, the comparator should disable the PTT line or cut the Bias voltage to the MOSFETs to stop amplification immediately.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">VU3GZR has already implemented a thermal sensor cutoff in his build of VU2EVQ linear amplifier.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">Essential Components for the Protection Board<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">To build this into a single &#8220;Supervisory Board,&#8221; you will need:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table class=\"has-fixed-layout\"><thead><tr><td><strong>Component<\/strong><\/td><td><strong>Function<\/strong><\/td><\/tr><\/thead><tbody><tr><td><strong>LM339\/LM324<\/strong><\/td><td>Quad Op-Amp\/Comparator to handle all three inputs.<\/td><\/tr><tr><td><strong>2N2222 or equivalent<\/strong><\/td><td>Transistors to drive the protection relays.<\/td><\/tr><tr><td><strong>12V DPDT Relay<\/strong><\/td><td>To interrupt the PTT line and\/or the VCC supply.<\/td><\/tr><tr><td><strong>LED Indicators<\/strong><\/td><td>Red for &#8220;Fault,&#8221; Green for &#8220;Ready,&#8221; and Yellow for &#8220;High Temp.&#8221;<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\">\n<p class=\"wp-block-paragraph\">Another option is to ensure the bias circuit is also tied to the protection. If any fault is detected, dropping the gate bias to 0V is the fastest way to &#8220;shut down&#8221; the MOSFETs while the slower mechanical relays are still moving.<\/p>\n<\/blockquote>\n","protected":false},"excerpt":{"rendered":"<p>Building a reliable HF linear amplifier, especially one utilizing robust components like the IRFP150, requires more than just a solid RF deck. The protection circuitry acts as the &#8220;brain&#8221; that prevents expensive silicon from turning into expensive smoke. For a 100W+ build, you need to monitor three critical failure points: Overdrive, High SWR, and Overheat. [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":16495,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-16494","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-amateur-radio-ham-radio"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.8 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>How to Plan Essential Protection Circuitry (Overdrive, SWR, Overheat) for HF Linear Amplifier? - Johnson&#039;s Techworld<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/johnsonfrancis.org\/techworld\/how-to-plan-essential-protection-circuitry-overdrive-swr-overheat-for-hf-linear-amplifier\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"How to Plan Essential Protection Circuitry (Overdrive, SWR, Overheat) for HF Linear Amplifier? - Johnson&#039;s Techworld\" \/>\n<meta property=\"og:description\" content=\"Building a reliable HF linear amplifier, especially one utilizing robust components like the IRFP150, requires more than just a solid RF deck. 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