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Registered Member #579
Joined: Mon Mar 12 2007, 09:45AM
Location: Burntwood, Staffs, England
Posts: 46
I have recently seen an a rticle that says MICA insulators and insulated fixing screws/bolts are essensial if one is to prevent transistors shorting out when used to drive HV circuits ... Ignition coils or Flybacks.
What is the consensus?
I have blownlots of 2n3055 in Ignition coil and LOPT drivers
I have not used any mica insulators or insulating fixings
SHOULD I HAVE DONE My reasoning is: if the fixing bolts need to be insulated why does the manufactureres always make can or tag the collector
I can never find any one , locally, that stocks mica insulators and I am not going to order some by mail and pay £4.00 postage and some times £20 minimum order charge
I could make the insulator pads from something else : rubber or polyethylene but that would then thermally insulate the transistor from the heat sink
Registered Member #505
Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329
JamesH wrote ... I have recently seen an a rticle that says MICA insulators and insulated fixing screws/bolts are essensial if one is to prevent transistors shorting out when used to drive HV circuits ... Ignition coils or Flybacks.
You only need to insulate the tab of a transistor if something else is connected to the same heatsink that is not at the same voltage.
This could be if you connected another transistor of a half bridge to the same heatsink for example, or if the heatsink was bolted to an earthed chassis.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Where possible I prefer to not use insulators between transistors and heatsinks, just thermally conductive paste. The cooling of the transistor is much better, at the cost of insulating the heatsink.
Registered Member #32
Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549
JamesH wrote ...
I have blownlots of 2n3055 in Ignition coil and LOPT drivers
I have not used any mica insulators or insulating fixings
But you have used lots of 2n3055s. If you use the 2n3055 for real experimentation and push it hard enough, you're bound to blow a few. It's beefy enough when it comes to power but just isn't designed for HV work. (It's still popular for hobbyists because of cost and availability.)
If you can hold of an MJ10012, it's worth it. It's available in Australia at Jaycar but I understand you're probably limited to RadioShack for local electronics shops...
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
at one point I had to bite the bulit and bought 500 mica insulatos for TO-247. They have a higher voltage hold off and thermal caristics. You can get them at Concord electronics and Keystone electronics. That may not be your problem if you are using silicone insulators and voltages bellow 600V. If every collector is insulated properly try grounding the heat sync. If you paralelling collectors you could get rid of them all together
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Insulators and heat sinks for power transistors - are they essensial ?
