Anti-Surge Thick Film Resistors? - Page 1
Forum section: electronics / general

author: iampoor
author_rank: Frequent Contributor
timestamp: January 15, 2017, 01:31:58 pm
content: HiI am currently working on a project that was going to use 1/2w standard through hole resistors (Xicon 273 series). I am currently hoping to use SMD resistors instead. I need 1/2w resistors with a 350v voltage rating and came across the Panasonic ERJ-P06 series. These are listed as "Anti-Surge" resistors and can dissipate a 1/2w in an 0805 package. I have read the datasheet, and there does not appear to be a catch....but considering how I am used to 0805 resistors being rated at an 1/8th of a Watt its hard to break out of that assumption. Is an Anti-Surge resistor different from a general purpose resistor in "normal" applications? I am using these on the plates and cathodes in some vacuum tube circuits. Thanks!Part Linkhttp://www.mouser.com/ProductDetail/Panasonic/ERJ-P06F1001V/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 15, 2017, 04:58:42 pm
content: Using SMTs on vacuum tube circuits?1/8W 0805s at that?Even if they're rated for it, I don't trust 0805s much over 100V. Beyond 70V, I'll use two or more in series.1/2W in the size of an 0805 would push it over 200C. It (or the PCB!) won't last long like that. You can get more by adding copper pours, as long as the resistor is fairly thermally conductive. (Alumina isn't as conductive as copper, so you can only do so much.) Ultimately, you're limited by the thermal diffusion out of the chip. Which varies linearly with size, since they're all pretty much the same shape. (Wide style chips can dissipate more because there's more pad length to dissipate into, and big chips because they have enough area.)I don't know that they do anything special or different with the surge rated resistors. I don't recommend taking a "1/2W" rating, in 0805 size, at face value. At most, 1/4W, and that only with lots of copper pour around it.Tim

author: tszaboo
author_rank: Super Contributor
timestamp: January 15, 2017, 05:38:36 pm
content: Use 0207 MELF ressitors. They usually are better for surges than thick film. Better tempco (than regular thick film), power dissipation, and higher reliability usually goes for MELF. Since they are not made by the billions, they might be little bit more pricey, but on a tube amp, who cares, right?

author: iampoor
author_rank: Frequent Contributor
timestamp: January 15, 2017, 06:15:10 pm
content: Quote from: T3sl4co1l on January 15, 2017, 04:58:42 pmThe 1/8w 0805's Im using in the design are for the solid state input circuitry. I am using a few opamps in this circuit too. I was trying to convey that I have only used 1/8w 0805's, and only in solid state circuitry, so seeing an entire line of "400v rated 1/2w 0805's" was quite...shocking to say the least!Okay, I think you answered my question...let me get this straight. The 0805 could potentially dissipate a 1/2w but since the surface area of the chip is so small the limited thermal dissipation will cause the temperature to rise faster than it would in a standard 10mm 1/2w through hole resistor. Correct? I think thats the obvious piece of information I was missing when I found these on mouser and thought "Something must be wrong".I think I may just stick with standard through hole parts, as I was hoping to reduce the size of the PCB, adding copper pours sort of defeats that purpose. Plus the cost difference is not massive.I am beginning to think that the 400v resistor rating is for surge voltages, or that the resistor CAN dissipate a 1/2w without blowing up, but not continuously.Thank you Tim, I always enjoy reading your replies and have learned alot from them!Quote from: NANDBlog on January 15, 2017, 05:38:36 pmThank you, I will look into those. Exactly, cost is not very important in this application. I was just hoping to reduce my PCB size.

author: daqq
author_rank: Super Contributor
timestamp: January 15, 2017, 07:55:08 pm
content: QuoteThe values for voltage ratings with pulse resistors are tricky. Seehttp://www.farnell.com/datasheets/2140397.pdf, page 6.I've used this series of resistors, not to its limit mind you, but they have yet to blow up on me.