A couple of years ago, I spent time making a 'resistor wrecker" system to answer a question similar to this. I had noticed that if my scope probe shorted out a capacitor that was charged to 3.3V, the 1 Ohm series resistor would often fail (open circuit). I figured that the resistors had a threshold similar to the I^2t (current squared times time) curves for fuses. My system connected my DUT (resistor) to a capacitor (with a programmable voltage charge) using a FET switch. The system ramped the voltage in small steps, discharged the cap into the resistor, then recorded the resistance of the DUT. The test ended when the resistor had changed value by 50%, but usually the resistor blew (open circuit) the next step after a 5% to 10% change in value. After recording the average, max, min, and std-dev of my resistors (ranging from 1 Ohm to 20 Ohm), I plotted the results. The data clusters were "all over the map", giving one bit of useful information to the question of "how much current?". Answer: "It depends." And it's worse than that. Chips from different vendors (and probably from different manufacturing lots from the same vendor) are different. I think it is because the parts are made from a film (thin or thick) that is "some thickness" and is LASER trimmed to value, typically with an "L cut" or "S cut" . The amount of trimming needed depends on the thickness of the film and the desired resistance. A thicker film requires deeper notch in the conductor to make the correct resistance value. A range of values can be made from the same thickness of film, limited by how deep they can trim the notch for the highest value in that range. The conductor cross sectional area at the notch is where the part fails when the pulse current density exceeds the short term thermal capabilities of the notch. Conclusions: Short duration high current surges are limited by the narrowest point in the resistor, with the part failing by vaporizing the conductive film from the substrate, creating an open circuit. Lower levels of over-power faults cause the whole part to heat up, with the failure caused by over-temperature. The heat causes a temperature rise which causes a permanent shift in the resistor value. The power level required for this effect depends on the ability of the part to conduct heat to the environment. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg Sent: Thursday, December 02, 2010 12:39 PM To: [log in to unmask] Subject: [TN] [Somewhat NTC] When does a resistor become a fuse? This is a little off the normal Technet topic genre, but I was wondering if anyone out there may have some data on how long a resistor can withstand overcurrent before it changes resistance / fuses open? I'm taling about a standard 0805 carbon film resistor, for example 100 ohms: at 1 mA it drops 100 mV and dissipates 100 uW, no big deal for a 125 mW device at 10 mA it drops 1 V and dissipates 10 mW, still no big deal at 100 mA it drops 10 V and dissipates 1 W, until the resistance increases dramatically How much of a time duration can the device withstand at 100 mA? Is there a correlation between overload and time? The graph in http://www.koaspeer.com/pdfs/Thick_Film_Pulse_Limitation.pdf seems to indicate that an 0805 (2A on the graph) can dissipate 1.5 W for 1 second. Does this make sense? 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