Beware that the Radio Shack 276-2370 portable ESD mat does not properly bleed off static charges. Appended below is a message I sent on this subject back in February.
Al N1AL On Thu, 2008-12-11 at 21:04, Dick Dievendorff wrote: > Radio Shack. > > Dick, K6KR > > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Gary Smith > Sent: Thursday, December 11, 2008 9:01 PM > To: [email protected] > Subject: [Elecraft] easy source of static dissipating work mat? > > My new K3 is on it's way. I have anti-static material that my > motherboard came in but that's not enough to work on & be static > free. > > Where can I easily & quickly get an anti-static mat and grounded > wrist strap? > > Thanks1 > > Gary > KA1J ================================================================= -----Forwarded Message----- > From: Alan Bloom <[email protected]> > To: [email protected] > Subject: [Elecraft] Radio Shack ESD mat doesn't meet spec > Date: Wed, 27 Feb 2008 09:51:25 -0800 > > Well, I've convinced myself that the Radio Shack portable ESD mat, P/N > 276-2370 doesn't work properly. > > The "ESD Association" http://www.esda.org has promulgated an > industry-standard test for ESD mats, ESD S4.1. It is the standard > specified by most commercial mats. I decided not to spring for the $70 > to buy a copy of the standard, but other information I found on the web > describes the test in general terms. It uses two circular electrodes, > each weighted with 5 pounds, spaced 10 inches apart on the mat. The > "Point to Point Resistance" is specified to be: > > At 40-60% RH: 10^6 - 10^7 ohms > At 20-40% RH: 10^7 - 10^8 ohms > At 10-20% RH: 10^8 - 10^9 ohms > > I don't know what the RH here in Santa Rosa was yesterday when I did the > test, but I don't think it was very low since it has been raining > recently and the ground is still damp. For sure the resistance > shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or > 10^7 (100 or 10 megohms). > > I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat > way out of spec. > > Test procedure: > > I didn't find a specification on the electrode size, but in the photo of > a popular tester they look to be maybe 3 or 4 inches in diameter. For > my test, the electrodes were two saucepans, each about 7 inches in > diameter and weighted with 5 pounds. They were spaced 10 inches apart > on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor > between the two pans and charged it to 15V with a power supply. > > I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If > I touch the leads across the capacitor immediately after charging, the > needle momentarily jumps to about 6 uA (1/10 full scale) as the > capacitor discharges through the meter. If I wait half an hour (1800 > seconds) for the capacitor to partially discharge through the mat > resistance, the needle jumps to about 3 uA. > > An R-C network discharges to 3/6 of original voltage in about 0.7 time > constant. So the time constant must be 1800/0.7 = 2571 seconds. That > implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. > > Al N1AL > _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
