TO-3 screw sizes? - Page 1
Forum section: electronics / beginners

author: py-bb
author_rank: Regular Contributor
timestamp: March 30, 2023, 12:49:19 am
content: There has to be something but I can't find it.It seems for TO3 there's a "standard size spacer" I've bought some but they've not arrived yet. What's best/good practice for fitting TO-3's and insulating them? I was going to put some good/thick vinyl tape down, put one of the spacers in the TO-3 hole with the "fat end" at the top, then put a bolt and a nut through that. That way the bolt/nut is in contact with the mounting plate, but the tape and the "spacer" keep the TO-3 device insulated.https://www.ebay.co.uk/itm/385494977741These are the spacers and they all seem to be the same.What screw/bolt and nut size do I want? Presumably as there seems to be only one "spacer" there's at least a very very common hole diameter for the spacer and I want a screw that fits in that (but not tightly)Other than screw/bolt and nut suggestions I'd also like to hear about any special tapes which are designed to conduct heat and for this situation (I am aware that I could do better heat-conduction wise if I didn't require insulation)Thoughts welcome!

author: EPAIII
author_rank: Super Contributor
timestamp: March 30, 2023, 05:59:22 am
content: The links below are for sources where I would go for the items. I show them as examples of what I am recommending. I almost never use E-Bay, Amazon, or the other internet sources that seem to be so popular today. I prefer real, professional electronic supply houses as I know the quality will be what I expect. But I know you may need to use others.Of course there are variations, but the more or less standard TO-3 insulated mounting method is to use a mica insulator, heat sink compound, plastic (nylon) washers with a step, and #6-32 screws.https://www.ti.com/lit/ml/snoa033/snoa033.pdf?ts=1680087240252&ref_url=https%253A%252F%252Fwww.google.com%252FHere are examples of the mica insulators:https://www.mouser.com/c/electromechanical/hardware/mounting-hardware/?q=TO3%20hardware&material=MicaThe TO-3 is a decades old package that was originally specified using English measure (inches). So it is easiest to understand in that light. The hole, center to center distance is nominal 1 3/16" with a +/- tolerance of 0.010". That translates to 30.16mm +/-0.25mm so 30mm may work. The mounting holes are specified as 0.151" to 0.161". The nominal OD of the #6 screw is 0.151" but all commercially produced screws will be somewhat less than that so they should fit. A 3mm screw will be loose and a 4mm one probably won't fit.Generally speaking, if it must be insulated from the chassis or heat sink, step style, nylon washers are used. The ID of the step washers sold for this should match the #6 screw and the OD is probably a fractional inch size. But with more parts being specified in metric, I suggest measuring the OD of the ones you buy and drill the holes to match. Another style of insulator that seems to be fairly common today is a molded plastic part (nylon) that has two of these step washers connected with a spacer that may or may not have an insulated fence between the two leads. These can have tapped holes, clearance holes, or holes sized for sheet metal screws. You need to consult the data sheet for the one you buy.https://www.mcmaster.com/products/washers/material~plastic/electrical-insulating-sleeve-washers-6/https://www.mouser.com/c/electromechanical/hardware/mounting-hardware/?q=TO-3%20hardwareAn alternative is nylon screws which can be used with nylon or metal nuts.https://www.mcmaster.com/products/screws/nylon-pan-head-phillips-screws-10/?s=nylon+screwsAnd there are TO-3 mounting kits with all the parts you need in one package. Keystone Electronics in one OEM for these.Any computer repair shop should have very good heat sink compounds. Any heat sink compound. used with the TO-3 mica insulators will be better than using vinyl tape which could even melt if the temperature gets high enough. The mica is tolerant of high temperatures.

author: EPAIII
author_rank: Super Contributor
timestamp: March 30, 2023, 06:05:42 am
content: Oh, if you choose to use M3 screws you may want to add metal washers under the head of the screw.Or, if you can find them, M3.5 x 0.6 screws do exist. Just not overly common. They would be a better fit.https://www.mcmaster.com/products/screws/thread-size~m3-5/metric-steel-pan-head-phillips-screws/?s=metric+screws

author: Jwillis
author_rank: Super Contributor
timestamp: March 30, 2023, 06:13:07 am
content: Most of the time you'll need 3mm machine screws and nut. Personally I like to use a metal washer as well on top to distribute the torque on top of the plastic spacer. They are used to isolate the metal heat sink from the collector. But you will need an insulator between transistor and heat sink as well. I have used silicone insulators but I prefer mica or ceramic with thermal paste. Because I know what to expect.I have used adhesive tapes but they make it hard to disassemble if needed.TO3 transistors can be mounted without isolation with self tapping screws as well if isolation isn't important. Use thermal paste . I don't really care for that method because I prefer to isolate the heat sinks entirely. Reduces risk of shorts.I also prefer to use TO3 sockets. It just makes assemble and disassembly easier and its neater. Depending on the socket you may not need nuts and no soldering directly to the transistor required.Hope that helps

author: py-bb
author_rank: Regular Contributor
timestamp: March 30, 2023, 10:46:41 pm
content: Quote from: EPAIII on March 30, 2023, 06:05:42 amThanks for the detailed answer (I've quoted your shorter one just for other viewers) - the spacers arrived today and the hole is a tight 3mm (it's at most 3.1mm and at least 2.96mm) I'd much rather use M3 screws simply because it keeps everything metric (my "imperial units tool box" is the same as my "really old WWII or older" tool box)The mica washer thing helps, I was going to use this special tape (it might be mica but why not get the cheap things)

author: py-bb
author_rank: Regular Contributor
timestamp: April 01, 2023, 02:08:24 pm
content: Hi thereJust to bump this I found a picture on wikipedia (on another computer) that showed nut, TO-3, mica gasket, mounting surface, plug, washer, soldering attachment (eg a ring terminal) screwI was going to go screw, inslating plug, washer, ring terminal, TO-3 package, mica layer, mounting, nutThat way the screw was insulatedIs there a reason not to?I hope this makes sense - I've described the situation from top to bottom. In the "wikipedia picture" the screw conducts and is insulated from the board, in what I was going to do the screw's insulated

author: Jwillis
author_rank: Super Contributor
timestamp: April 01, 2023, 09:34:46 pm
content: You mean like the picture below?The Collector on the TO-3 transistor is the Case of the transistor.If the head of the screw makes contact to case of the transistor but the shank, thread and nut are isolated by the plug from the heat sink then the transistor can be considered isolated with a mica isolating washer. The plastic plug can be on the top side of the transistor or the back of the heat sink. Both ways will work.The transistor can be said to be isolated if there is No continuity between transistor case (Collector), Emitter pin and/or Base pin with the Heat sink.

author: py-bb
author_rank: Regular Contributor
timestamp: April 01, 2023, 10:29:48 pm
content: Quote from: Jwillis on April 01, 2023, 09:34:46 pmYes both ways will still work but is there apractical reasonas to why one might be preferred. and yeah that's the image from Wikipedia.I was going to do it the way I said for easy access to the nuts and because a thick mounting surface risks coming into contact with the screw - so by isolating the screw it doesn't matter the screw and the mounting surface touch.What about any burring around the hole - maybe that will damage the mica and the plug will save the day?

author: Jwillis
author_rank: Super Contributor
timestamp: April 02, 2023, 03:04:17 am
content: Quote from: py-bb on April 01, 2023, 10:29:48 pmJust de-burr the hole. I've used an old Philips screw driver that I don't care about for de-burring small screw holes. A file or sand paper can work to take rough edges off . Whatever you have on hand. As long as your satisfied with the results it's all good.Lots of times I don't have the preferred materials on hand or the tool, and the cost and time isn't worth the effort. So I improvise. I can always improve something as the materials or tools become available later. Sometimes we need to think outside the box to get things done.