Description
This 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.
You 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.
Specifications
Datasheets
Wiring & getting started
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Check the power first
Work 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.
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Bolt it to a heatsink
Spread 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.
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Solder the wires
Pass 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.
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Check it works
Before 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.
Note: 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.
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