This style of resistor: https://www.jameco.com/z/212-7-2-Watt-6-8-Ohm-Ceramic-Wirewound-Resistor_21= 59752.html can have all sorts of mounting hardware to dissipate heat (I've seen them with fins), act as a board protecting stand off and even as a very simple fusible link that needs a soldering iron to reset On 23 Jan 2018 20:45, "Allen Mulvey" wrote: Most of you are probably aware of this but I had never seen them before. Power resistors that stand on end and have feet to keep them away from the board: http://www.newark.com/w/c/passive-components/resistors-fixed -value?range=3Dinc-in-stock&st=3Dsq&CMP=3De-email-Buyer-230118-TE& et_cid=3D29759719&et_rid=3D1338645273&cmp=3D Allen -----Original Message----- From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of Bob Blick Sent: Tuesday, January 23, 2018 12:04 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] Resistor temperature rise Hi Stephen, I was hoping to buy a little time and prevent scorching and self-desoldering by using 5W instead of 2W resistors. But when I compared the spec sheets between the two resistors, at 2 watts the temperature rise is very similar. I don't really see how that can be, the 5W resistor is larger so I'd assume it would run cooler at the same power levels. In some configurations there will be a thermal cutout on the power transistors, we'll see whether that activates before or after the resistors fail or do board damage. Thanks, Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of stephen.forrest Sent: Monday, January 22, 2018 3:02 PM To: piclist@mit.edu Subject: RE: [EE] Resistor temperature rise Damned physics! ;o) Time is the key here - you mention headroom for temporary overloads. What detects the overload? What interrupts the overload? How soon does it operate? What is the normal load? Assuming that the normal load keeps the temperature rise within acceptable bounds, the time taken to interrupt the overload will determine the temperature rise you see in the resistor during the fault. If that time is indefinite, you need to deal with the excessive temperature through heatsinking or airflow or both. It is hard to get around P =3D I^2R. The thing the 5W buys you is some time. The greater heat capacity of the body, the greater surface area, etc. etc. means the time to reach critical temperature will be longer. Given that this is for surviving fault conditions, I would be reluctant to add heatsinking just for that (COG, reliability, manufacturability etc.). Rather , add earlier detection and interruption of the fault. If critical, with some redundancy (COG ;o) ). As an aside, while it does provide extra cooling to the resistor, the another (main?) reason for mounting power resistors off the PCB is to limit scorching of the board and possible delamination. This can happen well before solder melts. Stephen -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .