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Old 27th April 2008, 03:20 AM   #1
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Default Transistor Insulator Sources - US

Well its that time once again that i need to go on the hunt for some TO-247 style Insulators and while i am at is i am always looking for sources for MT-200 type insulators.

I ordered some from Digi-Key part BER131-ND, But i am not happy with them. Checking continuity after installing them i find resistance less then 100 ohms between the transistor and the heatsink. In an amp with 10 transistors per side the problem is compounded and was down to 4 ohms on one side and I am not comfortable with that. loosening each transistor raised the resistance incrementally so these are too thin. switching to mica and silicone solved the problem but i have now depleted my stock of mica.

Searching Digi, Allied, MCM etc doesnt turn up much, NTE and Keystone brands but i am not thrilled with those.


Anyone have a good source for some high quality Silpad type or mica insulators ???

I would like to find a source to buy continuous silpad material in rolls if i could find it.


Advice?
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Old 27th April 2008, 04:12 AM   #2
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Advice? Well, RTFDS? You ordered a *non-insulating* type pad, made for thermal performance, not electrical insulation. I.e., it's a grease substutute only. Just order the standard type of pad and you'll have far better luck.
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Old 27th April 2008, 04:32 AM   #3
syn08 is offline syn08  Canada
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Quote:
Originally posted by Conrad Hoffman
Advice? Well, RTFDS? You ordered a *non-insulating* type pad, made for thermal performance, not electrical insulation. I.e., it's a grease substutute only. Just order the standard type of pad and you'll have far better luck.
BER178 - pinkish, dielectric and very good quality. Expensive, though, at 75c a pop, in tenths quantities. For the same money I would recommend the Aavid 4180G aluminum oxide pads from Mouser. Excellent thermal properties and of great help in reducing the drain/collector to heatsink (ground) capacitance (like in a CFB output stage).
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Old 27th April 2008, 05:22 AM   #4
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*non-insulating* Well NO WONDER! GRRRR!
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Old 27th April 2008, 05:30 AM   #5
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Hello ZeroCool,if you are interested in purchasing sheets of silpad and cutting out the size insulator you need yourself, I would suggest contacting the manufacturer and asking them who the rep is for your area for this type of supply. These reps don't normally deal with individuals, mostly businesses that buy a lot. But if you are interested in buying a small bulk quantity like that the regional rep may be able to help you. If I remember correctly the Sil-pad brand is a Bergman product, but don't hold me to that.

Peace,

Dave
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Old 27th April 2008, 11:54 AM   #6
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Bergquist
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Old 27th April 2008, 02:02 PM   #7
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I found Berquist is located right here in MN! I will give them a call and see if they can help with small quantities. The Pink Berququist insulators quoted will work for now. But i still would like to find a source for good old fashioned Mica.


Question: The aluminum oxide pads mentioned, Do you use those with or without heatsink compound? I have never used those before.
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Old 27th April 2008, 04:05 PM   #8
syn08 is offline syn08  Canada
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Quote:
Originally posted by Zero Cool
Question: The aluminum oxide pads mentioned, Do you use those with or without heatsink compound? I have never used those before.
With aluminum oxide pads, I use no compound on the tranny side, and a very little on the heatsink side.
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Old 27th April 2008, 05:05 PM   #9
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I've used aluminum oxide (alumina) insulators under TO-220 devices with good success. They definitely need grease on both sides. I doubt the thermal conductivity is as good as something thinner, but they have one really wonderful attribute- they can lower the capacitance between the device and the heat sink by a huge amount. I've had circuits where this was the difference between success and failure. You can also live dangerously and use beryllium oxide, but don't break or crush it. Conductivity is better than just about anything else. Thermalloy used to sell heavily anodized aluminum insulators. The performance was super, but you have to be insanely careful about burrs. IMO, people sometimes don't pay enough attention to surface flatness on their heatsinks. I make a quick pass on the mill with any surfaces that were left as-extruded. The key is not having to fill air space with grease, which isn't as good a thermal conductor as you might hope. I still haven't met a silicone pad as good as grease and mica, but I won't put up with the mess anymore.
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Old 27th April 2008, 08:10 PM   #10
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the biggest problem i have with the sil-pads is that it can be hard to remove a device after it has been in use for sometime with out damaging the pads. In this amp all the transistors stuck to the pad and the pad stuck to the sink and was destroyed trying to remove the devices and pad from the sink.

with mica i have never had that problem. and the silpads are expensive! i will deal with the mess if i could find mica.
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