Having issues sourcing  1% or better Metal Film 1/4Watt resistors  - Page 1
Forum section: electronics / general

author: Topband RF
author_rank: Contributor
timestamp: September 03, 2012, 10:11:29 am
content: Looking @ the usuall places, Mouser-Digikey-Newark I have sourced some of the parts I am looking for but not all. Mainly having issues finding a few 1/4 Metal Film Resistors. SMD or Thru-hole Radial.. I have a circuit that I want to build from scratch, and it had extremely specific requirements. 2% parts will not cut the mustard.Any Advice? Other distributors? Quantity purchasing is not an option.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 03, 2012, 11:42:42 am
content: If you really need a 1/4 watt rating and 1% accuracy you should probably be using multiple resistors in series to spread the heat out.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: September 03, 2012, 12:14:18 pm
content: Most resistors are much more accurate than their advertized tolerance.So good chance you'll find the desired accuracy among your standard 2% parts.Dave has done a couple of videos on that subject, #215 and #216.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 03, 2012, 01:13:15 pm
content: I'm surprised you're having a problem... I went to Farnell (same group as Newark) and found:http://uk.farnell.com/jsp/search/browse.jsp?N=203968+110091680+110062552+110003321+110156669&No=0&getResults=true&appliedparametrics=true&locale=en_UK&divisionLocale=en_UK&catalogId=&skipManufacturer=false&skipParametricAttributeId=&prevNValues=203968+110091680+110062552+110003321&mm=1000001||,1000635|110062552|110003321,1001789||,&filtersHidden=false&appliedHidden=false&autoApply=false&originalQueryURL=%2Fjsp%2Fsearch%2Fbrowse.jsp%3FN%3D203968%26No%3D0%26getResults%3Dtrue%26appliedparametrics%3Dtrue%26locale%3Den_UK%26divisionLocale%3Den_UK%26catalogId%3D%26skipManufacturer%3Dfalse%26skipParametricAttributeId%3D%26prevNValues%3D203968

author: free_electron
author_rank: Super Contributor
timestamp: September 03, 2012, 02:27:23 pm
content: If you are really going to burn off 1/4 watt in a 1/4 watt resistor.. Check the tempco too !

author: grenert
author_rank: Frequent Contributor
timestamp: September 03, 2012, 02:37:44 pm
content: You either aren't trying hard enough, or you're trying in the wrong way. Mouser carries the full range of Vishay Dale CMF55 1/4W 1% metal film resistors (and I think 0.1% as well).

author: Topband RF
author_rank: Contributor
timestamp: September 03, 2012, 04:45:26 pm
content: I'm going to take a closer look today. Thank you everyone for the advice.

author: T4P
author_rank: Super Contributor
timestamp: September 03, 2012, 05:07:42 pm
content: Just a tip : resistors are rated at 70C

author: Topband RF
author_rank: Contributor
timestamp: September 03, 2012, 09:14:21 pm
content: Update.. I've found all the parts I need most @ Mouser (through hole parts.. I may look @ SMD and see if there is a savings).. now for a LAUGHABLE problem & One question. I plan on using a 2N5109 @ abt .75 mA +/- 5mA & it needs to be cooled, TO-5/TO-39 Heatsinks are in many cases double the cost of the transistor. or at least $ 1.50 US. I may just bite the 'laughable bullet' and buy a pack of 5.Now for my question, most of all the info I find suggests that simply a pressure fitted heatsink will suffice.. would I gain any benefit by using thermal grease before applying it. The project at this point in time includes no forced air or air movement. In fact it may get daym hot in a small enclosure just from that transistor alone. (it is the only 'active' device in the circuit & at this point of time in 'design' will be for indoor use only.. that may change once I build one up, test and take temp readings ect ect ect)Thanks much!

author: richcj10
author_rank: Unknown
timestamp: September 03, 2012, 09:25:38 pm
content: Adding thermal grease always helps.. Never hurtsThis gives a good way of lowering the thermal resistance between two devices.

author: EEVblog
author_rank: Unknown
timestamp: September 03, 2012, 09:28:21 pm
content: Quote from: T4P on September 03, 2012, 05:07:42 pmSome are rated at higher than so, so beware. e.g. those physically smaller "1/4W" resistors.Dave.

author: AndyC_772
author_rank: Super Contributor
timestamp: September 03, 2012, 09:30:38 pm
content: Can you not use a bigger surface mount transistor and use a copper pad on the PCB as a heat sink?

author: Topband RF
author_rank: Contributor
timestamp: September 03, 2012, 10:17:48 pm
content: Quote from: AndyC_772 on September 03, 2012, 09:30:38 pmAndy, that is a possibility, one that I will absolutely look into after I build one with threw hole parts. The person that designed this circuit suggests that the DCP68 with a 3/8 Square pad on the collector when using .062 thick 1oz double sided board material is sufficient.I want to price everything out with both Thru-hole and SMD parts and see if there is a considerable savings. I know ease of personal manufacture would probably be with SMD, considering size of board needed, not having to drill holes ect ect.This project will incorporate many firsts for me. I have built a few kits.. using SMD devices and small chips and oscilators such as the SI750, but I have never made my own boards. Having boards made is not an option at this time. And I would imagine that one board made even in 25 or 50qty would cost more than the cost of all the parts not including the enclosure. As it is seeming at the point in time the enclosure will be the biggest chunk of cost, (must be metal for shielding ) however untill I build one up.. test it.. burn it in..I dont know what enclosure I want to go with. One of the Hammond boxes I looked at was about 17$ .. owch.