{"product_id":"25w-4-ohm-aluminium-wirewound-resistor","title":"25W 4 Ohm Aluminium Housed Wirewound Resistor - RX24","description":"\u003cdiv class=\"nznpd\"\u003e\n\u003cdiv class=\"nznpd__lead\"\u003e\n\u003cp class=\"nznpd__intro\"\u003eThis is a 4 ohm power resistor in a finned aluminium case, rated 25W when it is bolted to a heatsink. The metal case pulls heat away from the wire element inside, so it can turn far more power into heat than an ordinary resistor of the same size.\u003c\/p\u003e\n\u003cp class=\"nznpd__intro\"\u003eYou can use it as a 4 ohm dummy load when testing a small amplifier, as a 5V test load for a USB charger or power supply, or as a precharge resistor for a capacitor bank. Two in series make an 8 ohm load that takes 50W on a heatsink. The 25W rating only holds on a heatsink, so plan on about 5W if it sits in free air. It is not a 12V part: across 12V it takes 36W, well over its rating, so a 12V load needs 8 ohms or more.\u003c\/p\u003e\n\u003c\/div\u003e\n\n\u003csection class=\"nznpd__block\"\u003e\n\u003ch3 class=\"nznpd__h\"\u003eSpecifications\u003c\/h3\u003e\n\u003cdiv class=\"nznpd__specs\"\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eResistance\u003c\/span\u003e\u003cstrong\u003e4 ohm ±5% (marked 25W 4ΩJ)\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003ePower on a heatsink\u003c\/span\u003e\u003cstrong\u003e25W, with thermal paste\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003ePower in free air\u003c\/span\u003e\u003cstrong\u003eAbout 5W (our suggested limit)\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eMost voltage at 25W\u003c\/span\u003e\u003cstrong\u003e10V DC, 2.5A\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eAt 5V\u003c\/span\u003e\u003cstrong\u003e6.25W, 1.25A\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eElement\u003c\/span\u003e\u003cstrong\u003eWirewound, for DC and audio, not radio frequencies\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eSize\u003c\/span\u003e\u003cstrong\u003e~51 x 28 x 15mm including tags\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eMounting\u003c\/span\u003e\u003cstrong\u003e2 x ~3.2mm holes for M3 screws, ~18.3 x 19.8mm apart\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003cdiv class=\"nznpd__spec\"\u003e\n\u003cspan\u003eTerminals\u003c\/span\u003e\u003cstrong\u003eSolder tags with a wire hole, one each end\u003c\/strong\u003e\n\u003c\/div\u003e\n\u003c\/div\u003e\n\u003c\/section\u003e\n\n\u003csection class=\"nznpd__block\"\u003e\n\u003ch3 class=\"nznpd__h\"\u003eDatasheets\u003c\/h3\u003e\n\u003cdiv class=\"nznpd__ds\"\u003e\n\u003ca class=\"nznpd__dsbar\" href=\"https:\/\/cdn.shopify.com\/s\/files\/1\/0664\/6127\/0112\/files\/NZE-RES-AL-25W-4R_Datasheet.pdf\" target=\"_blank\" rel=\"noopener\" aria-label=\"Download the 25W 4 ohm resistor datasheet (PDF)\"\u003e\n\u003csvg width=\"20\" height=\"20\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"\u003e\u003cpath d=\"M14 2H6a2 2 0 0 0-2 2v16a2 2 0 0 0 2 2h12a2 2 0 0 0 2-2V8z\"\u003e\u003c\/path\u003e\u003cpolyline points=\"14 2 14 8 20 8\"\u003e\u003c\/polyline\u003e\u003c\/svg\u003e\n\u003cspan\u003eFull technical datasheet\u003c\/span\u003e\n\u003cspan class=\"nznpd__dsmeta\"\u003ePDF\u003csvg width=\"17\" height=\"17\" viewbox=\"0 0 24 24\" fill=\"none\" stroke=\"currentColor\" stroke-width=\"2\" stroke-linecap=\"round\" stroke-linejoin=\"round\"\u003e\u003cpath d=\"M21 15v4a2 2 0 0 1-2 2H5a2 2 0 0 1-2-2v-4\"\u003e\u003c\/path\u003e\u003cpolyline points=\"7 10 12 15 17 10\"\u003e\u003c\/polyline\u003e\u003cline x1=\"12\" y1=\"15\" x2=\"12\" y2=\"3\"\u003e\u003c\/line\u003e\u003c\/svg\u003e\u003c\/span\u003e\n\u003c\/a\u003e\n\u003c\/div\u003e\n\u003c\/section\u003e\n\n\u003csection class=\"nznpd__block\"\u003e\n\u003cdiv class=\"nznpd__qs\"\u003e\n\u003ch3\u003eWiring \u0026amp; getting started\u003c\/h3\u003e\n\u003col class=\"nznpd__steps\"\u003e\n\u003cli\u003e\u003cdiv\u003e\n\u003cstrong\u003eCheck the power first\u003c\/strong\u003e\u003cp\u003eWork out the watts before you connect anything: volts x volts ÷ 4. At 5V that is 6.25W, at 9V about 20W and at 12V 36W. Anything over 5W needs a heatsink, and anything over 25W needs a higher-value resistor or several of these sharing the load.\u003c\/p\u003e\n\u003c\/div\u003e\u003c\/li\u003e\n\u003cli\u003e\u003cdiv\u003e\n\u003cstrong\u003eBolt it to a heatsink\u003c\/strong\u003e\u003cp\u003eSpread a thin layer of thermal paste on the flat base, then screw both tabs down to a flat aluminium plate or heatsink with M3 screws. For the full 25W the plate needs to be about 23 x 23cm and 1mm thick, or a finned heatsink that does the same job.\u003c\/p\u003e\n\u003c\/div\u003e\u003c\/li\u003e\n\u003cli\u003e\u003cdiv\u003e\n\u003cstrong\u003eSolder the wires\u003c\/strong\u003e\u003cp\u003ePass each wire through the hole in a solder tag, bend it over and solder it. Use wire that suits the current: at 10V the resistor draws 2.5A, so 0.5mm² hookup wire or thicker.\u003c\/p\u003e\n\u003c\/div\u003e\u003c\/li\u003e\n\u003cli\u003e\u003cdiv\u003e\n\u003cstrong\u003eCheck it works\u003c\/strong\u003e\u003cp\u003eBefore powering it, a multimeter across the two tags should read about 3.8 to 4.2 ohms, plus a little for the meter leads. Once powered, the case warms up within a minute. If the heatsink gets too hot to keep your hand on for a few seconds, lower the voltage or add a bigger heatsink.\u003c\/p\u003e\n\u003c\/div\u003e\u003c\/li\u003e\n\u003c\/ol\u003e\n\u003c\/div\u003e\n\u003c\/section\u003e\n\n\u003cp class=\"nznpd__note\"\u003e\u003cstrong\u003eNote:\u003c\/strong\u003e The case runs well over 100°C near full power and will burn skin. Keep it clear of plastic, cables and anything that can melt, and let it cool before you handle it.\u003c\/p\u003e\n\u003c\/div\u003e\n","brand":"NZN Electronics","offers":[{"title":"Default Title","offer_id":67736062296160,"sku":"NZE-RES-AL-25W-4R","price":4.99,"currency_code":"NZD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0664\/6127\/0112\/files\/25w-4-ohm-aluminium-resistor-hero.png?v=1793827511","url":"https:\/\/www.nznelectronics.co.nz\/products\/25w-4-ohm-aluminium-wirewound-resistor","provider":"NZN Electronics","version":"1.0","type":"link"}