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?
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?
Conrad Hoffman said:
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).
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
Peace,
Dave
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.
Question: The aluminum oxide pads mentioned, Do you use those with or without heatsink compound? I have never used those before.
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.
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.
with mica i have never had that problem. and the silpads are expensive! i will deal with the mess if i could find mica.
to-247 mica
http://www.wallcoinc.com/NTE_TO_247MICA_MICA_FOR_TO_247_CASE_p/wal7-to-247mica.htm
Might be worth a group buy
http://www.wallcoinc.com/NTE_TO_247MICA_MICA_FOR_TO_247_CASE_p/wal7-to-247mica.htm
Might be worth a group buy
Zero Cool said:
You should not be re-using the sil-pads or any of the rubber type pads anyways. They are designed to be used without heatsink compound and "form" to fill the microscopic gaps between the mating surfaces. They really are a one time tighten down in place insulator and once disturbed a new pad should be used; unlike mica where if undamaged can be cleaned and new heatsink compound applied. I have had units come in where a hack has reused the sil-pad AND applied a thin layer of heatsink compound
djk said:
Thanks i need them for TO-247's however, But the TO-218 looks like it might be real real close.
I always replace the sil pad whenever possible but i admit that there have been times where the unit uses a long strip of sli pad material that if the device comes out clean, i have replaced the device without surgically replacing the silpad but i never add grease to silpad material.
If you really like to live dangerously you can use the heatsink anodization and grease, in this case use a clip for the transistor and not a screw.
I have a 30 cm x 30 cm sheet of silicone pad, whenever I need some I just cut a bit from it. I got that from RS a few years ago, pretty cheap, can't remember the reference though.
I have a 30 cm x 30 cm sheet of silicone pad, whenever I need some I just cut a bit from it. I got that from RS a few years ago, pretty cheap, can't remember the reference though.
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