I am building a Pass F5 clone and had problems finding mica insulators from "normal" parts distributers. Today I received a shipment of TO-3P mica insulators from China purchased thru EBay. Here is the EBay listing: TO-3P Transistor Mica Insulator,Insulation sheet, x 50
TO-3P Transistor Mica Insulator,Insulation sheet, x 50 - eBay (item 260559144568 end time Jan-20-11 20:19:03 PST)
They measure .864" x .715" and have a single hole of .137" diameter. (These measures are accurate to about .002"). Most appear to be about 4 mils thick and they are easily split into 1 mil thick pieces.
I am willing to share some of this supply with those in need. Send your mailing address and I send you some.
TO-3P Transistor Mica Insulator,Insulation sheet, x 50 - eBay (item 260559144568 end time Jan-20-11 20:19:03 PST)
They measure .864" x .715" and have a single hole of .137" diameter. (These measures are accurate to about .002"). Most appear to be about 4 mils thick and they are easily split into 1 mil thick pieces.
I am willing to share some of this supply with those in need. Send your mailing address and I send you some.
I used an sharp, pointed Xacto knife to split mine. A razor blade also works. See the "mica" section in:
ESP - Heatsink design and transistor mounting
ESP - Heatsink design and transistor mounting
Mica insulator thickness
I do not (yet) have practical experience with limits of mica insulator thickness.
Here is an extract from ESP - Heatsink design and transistor mounting regarding mica insulators:
8 - Mica What is Mica? Mica is transparent, and can be split into a very thin film along its cleavage. Electrically, it has the unique combination of high dielectric strength, endurance, uniform dielectric constant, low dielectric loss (or high Q), and extremely good insulating properties. Mica is moisture-proof and has low heat conductivity. It is infusible and may be exposed to high temperatures (in excess of 700°C) without any noticeable effect. If you want more info - try a web search
So much for the descriptions, but why would mica get a section all of its own? Simply because the quality control is virtually non-existent (from what I have seen lately). The overall shape is fine, but the thickness is generally too great - commonly by a huge margin. I have used mica washers that were so thick that I was able to separate them with a (very) sharp knife, and obtained four washers for the price of one (plus some scrap from the splitting operation).
Splitting a mica washer is not hard, but requires a steady hand, a good magnifier, and the sharpest knife you can find (or a new razor blade). The thickness should ideally be in the order of 0.025 to 0.05 mm for normal use, but you will probably find that up to 0.1 mm is quite acceptable. It is possible to make it much thinner than this, which decreases thermal resistance but makes the washer fragile and easily damaged.
The electrical characteristics of mica are such that even the thinnest possible washer will be more than adequate for typical amplifier voltages. The dielectric strength allows mica to withstand 1,000 - 1,500 volts per mil (1/1000 inch), or about 0.025 mm (25 micrometres) of thickness without puncturing or arcing.
This being the case, I am unsure why commercially available transistor washers are anything up to 0.25mm thick (and no, I am not kidding - I have even found some thicker than that!). This is capable of withstanding over 10kV in theory, let alone a measly 200V or so, and will introduce considerable thermal resistance. If one had a steady enough hand, just one of these would yield 10 washers of acceptable thickness (thinness?), with each still capable of at least 1kV insulation breakdown.
I am even tempted to make a mica splitter, that can be adjusted to a suitable thickness. One of these days ...
I do not (yet) have practical experience with limits of mica insulator thickness.
Here is an extract from ESP - Heatsink design and transistor mounting regarding mica insulators:
8 - Mica What is Mica? Mica is transparent, and can be split into a very thin film along its cleavage. Electrically, it has the unique combination of high dielectric strength, endurance, uniform dielectric constant, low dielectric loss (or high Q), and extremely good insulating properties. Mica is moisture-proof and has low heat conductivity. It is infusible and may be exposed to high temperatures (in excess of 700°C) without any noticeable effect. If you want more info - try a web search
So much for the descriptions, but why would mica get a section all of its own? Simply because the quality control is virtually non-existent (from what I have seen lately). The overall shape is fine, but the thickness is generally too great - commonly by a huge margin. I have used mica washers that were so thick that I was able to separate them with a (very) sharp knife, and obtained four washers for the price of one (plus some scrap from the splitting operation).
Splitting a mica washer is not hard, but requires a steady hand, a good magnifier, and the sharpest knife you can find (or a new razor blade). The thickness should ideally be in the order of 0.025 to 0.05 mm for normal use, but you will probably find that up to 0.1 mm is quite acceptable. It is possible to make it much thinner than this, which decreases thermal resistance but makes the washer fragile and easily damaged.
The electrical characteristics of mica are such that even the thinnest possible washer will be more than adequate for typical amplifier voltages. The dielectric strength allows mica to withstand 1,000 - 1,500 volts per mil (1/1000 inch), or about 0.025 mm (25 micrometres) of thickness without puncturing or arcing.
This being the case, I am unsure why commercially available transistor washers are anything up to 0.25mm thick (and no, I am not kidding - I have even found some thicker than that!). This is capable of withstanding over 10kV in theory, let alone a measly 200V or so, and will introduce considerable thermal resistance. If one had a steady enough hand, just one of these would yield 10 washers of acceptable thickness (thinness?), with each still capable of at least 1kV insulation breakdown.
I am even tempted to make a mica splitter, that can be adjusted to a suitable thickness. One of these days ...
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