Fun trick with epoxy drying resistance  - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: January 16, 2014, 07:08:34 am
content: Ok so a bit of history im working on a 5A 32v adjustable powersupply and it uses an LM338 and a mosfet (for current regulation) so i had to glue 2 heatsincs together using epoxy and i used my multiplier to make sure they were really not touching but close enough to exchange a wee bit of heat and have the fan blow in to them both without much losswhen i glued them together (between each fin about 4 inches long and 5 fins) the meter showed 5 Mohm and went up slowly and when the 5 minute set time was reached it was at 20 Mohm after the 30 minute harden time it was 55Mohm and it maxed out my 75Mohm meter at the 1 hour complete timenot EXACTLY useful but if your making a rig to set epoxy, want to monitor your epoxy, want to have fun showing friends, making the most useless resistor, or whathaveyoui think its atleast worth sharing and the resistance depends fully on the surface area and other factors but it might come to use some time in your life!EDIT: they were about a mm apartEDIT2: if it helps ANYONE it was "devcon (damn i wish i thought of that name first ...) home 5 minute clear binary epoxy"... it also says "5 minute" is a registered trademark ... huh?EDIT3: its been like 2 hours and i put the 36v of my supply across it and its not passing a single uA so i think its totally isolated now

author: Rerouter
author_rank: Super Contributor
timestamp: January 16, 2014, 07:11:04 am
content: still interesting that the resistance increased by 15 times, which should be relatively constant no matter the thickness of the epoxy or the metal

author: BiOzZ
author_rank: Regular Contributor
timestamp: January 16, 2014, 07:26:18 am
content: Quote from: Rerouter on January 16, 2014, 07:11:04 amyeah it increases apparently when the epoxy hardens to be a fairly good insulator from a fairly poor one, found it a real interesting reaction ... might be worth some looking in to or it might be a "no shit" moment to someone who knows how binary epoxy works ...

author: hiddensoul
author_rank: Regular Contributor
timestamp: January 16, 2014, 07:39:55 am
content: Quote from: BiOzZ on January 16, 2014, 07:26:18 amI am by no means a binary epoxy adhesive expert but it really makes perfect sense, while in a liquid/semi liquid state it has more free moving electrons as it hardens the movement of the electrons and in turn the flow of electrons decreases. Pretty much normal thermodynamics behaviour but I could just be blowing out my arse as I dont know if there is some underlying chemistry at play

author: robrenz
author_rank: Super Contributor
timestamp: January 16, 2014, 01:12:15 pm
content: Some filled epoxies like JB weld are "non conductive" but in reality are when used in averythin bond line they do conduct well.

author: SeanB
author_rank: Super Contributor
timestamp: January 16, 2014, 07:01:23 pm
content: I often use a quickset steel epoxy to hold heatsinks to devices, like very hot chips. Thin layer on clean smooth surfaces and spread it out and leave to cure. I have checked them for insulation resistance and still will not allow high voltage across them, though they are fine under 50v in many cases.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 16, 2014, 09:36:42 pm
content: 3 thoughts:a) Did you do some damage by flowing a current through the liquid, electrolytically separating stuff that should be together to bond ? Is a epoxi exposed to a current during curing (it rhymes!) different to one that doesnt see current ?b) how compares the temperature to resistance behaviour to the temperature-to-curing speed behaviour ?c) homemade electret !

author: calexanian
author_rank: Super Contributor
timestamp: January 17, 2014, 01:13:44 am
content: cool observation.