Dan: "Mica is somewhat flimsy in the robustness category. I wonder whether it will flake apart under the constant vibration from the motor." How would this ocurr if the insulator is coated in HS compound/thermal grease and securely held between the heatsink and transistor body? I have *never* seen a Mica insulator 'flake' apart in use - perhaps you have a scenrio where this has ocurred? Perhaps the extra-severe application on a lawn mower would contribute to this? I *have* had a Mica insulator fail before - it was due to a small machining burr in the HS to which a TO-3 power device was mounted. Over the years the burr must have worn it's way through the mica - one day on a sound peak (from the Heathkit AR1500A) the burr shorted through to the TO-3 transistor body and took out that transistor, another transistor and the PS bridge rectifier ... Granted, in a vibe prone app the mechanical securing of all fasteners has got to be a prime consideration towards longevity of product ... Mica, like concrete, would seem to thrive under compression (forces) and fail under sheer or tensile stress. Watch those burrs though ... Jim PS. All the old Motorola Motrac 2-way radios (built from 1960 - 1972) used TO-3 power transistors attached to HS's - must have used mica insulators there and I've never had any of my 30 year old plus Motracs fail due to a failed mica insulator - I've had PS switching transistors fail/short - this blows the 30 Amp 12 V mains fuse, I've had the tube PAs fail - but no mica insulators. Also - the GE Mastr Pro series of UHF transmitter strips used a large square piece of mica as C143 - this "stood off" upwards of +650 VDC plate potential. Of the several strips I've has in my GE Mastr repeater I've never had one of those fail either (other things, again, *have* failed though). ----- Original Message ----- From: "Dan Michaels" To: Sent: Tuesday, August 28, 2001 10:19 AM Subject: Re: [OT]: How to heatsink a lot of power electronics? > At 09:39 AM 8/28/01 -0500, you wrote: > >Byron: > > "I know that the mica washers are supposed > > to work, but do they really?" > > > >They have been used as far back as I can remember - on > >'gear' I was into at the time (the 70's, Delco car radios as a > >kid). > > > > > Mica is somewhat flimsy in the robustness category. I wonder > whether it will flake apart under the constant vibration > from the motor. > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body