Isolation of 2n3055 transistors - Page 1
Forum section: electronics / beginners

author: glinjik
author_rank: Regular Contributor
timestamp: March 08, 2022, 06:05:04 am
content: Hi I'm using 2 2n3055 transistors I want to put the giant heatsink on the outside of the case but I can't find a way to isolate them using the rubber strips is OK but the bolts holding the transistors still conduct any ideas?

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2022, 06:21:57 am
content: Search online for: TO-3 mounting kits

author: retiredfeline
author_rank: Frequent Contributor
timestamp: March 08, 2022, 06:25:59 am
content: Typically these things are used: mica or silicone insulators, nylon bushing/washers for screws, and sometimes a cap over the case.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2022, 06:50:23 am
content: You needhttps://www.keyelco.com/product.cfm/product_id/2430or similar, which are available from many suppliers + some non-conductive heat transfer compound (aka: thermal grease). The plastic shoulder washers provide the essential isolation between the screw connected to the transistor case, and the grounded chassis. Note that all holes must be clean and free from burrs, otherwise the mica insulator can be damaged and the transistor can short to the heatsink. Also the additional thermal resistance of the insulator and the additional joint surfaces reduces the power-handling capability of the transistor.Modern kits are likely to replace the mica washer with a glassfibre reinforced silicone heat transfer pad, A silicone pad is preferable for production line assembly as no heat transfer compound is needed the first time you assemble it. If disturbed during the course of repair work, replacement or a light smear of heat transfer compound is advisable. The Mica insulator is more durable if good quality and properly assembled.Special mounting kits are needed for high voltage use, with shoulder washers that are a lot thicker and extend right through the heatsink, fitting in a much larger hole diameter, to allow adequate creepage distance.

author: wraper
author_rank: Unknown
timestamp: March 08, 2022, 10:16:11 am
content: IMHO just use a similar transistor in a more modern package and do not bother with this. Sure you can isolate the screws, but metal package still will be exposed and must not be touched from the outside. 2N3055 which are sold these days are not even close to the parts made in the past, and usually have the same die inside as some another transistor in plastic package.

author: Karel
author_rank: Super Contributor
timestamp: March 08, 2022, 12:17:57 pm
content: Like the TIP3055:https://www.digikey.com/en/products/detail/stmicroelectronics/TIP3055/603611Max. power is a bit lower though (90 Watt).

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 08, 2022, 12:28:35 pm
content: Mica Insulators are better than Silicone, Ithinkthey transmit heat better. Use goop to make the physical thermal contact better.You can make silicone insulators from a sheet of silicon. Some kitchen things are made of this.You can make the little screw insulating washers out of the clear hard wire insulation tubes usually called "spaghetti", not heat shrink.If you use the right length the top of the small insulating tube will splay out when you tighten the screw and insulate the transistor hole and the top of the transistor.However it is better to get the insulating kit with the correct washers and insulators.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 08, 2022, 05:48:42 pm
content: I'm with you there. Mica is best.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 08, 2022, 06:15:24 pm
content: If used with proper and the right amount of thermal compound, mica is better than the silicone pads. There is howver more to go wrong. Good heat conduction pads are more than just the "kitichen silicone" and better than conductivity.For the srews there are isolating washers and also caps to cover the top.For a new design a more modern TO247 or similar case is usually preferred.The metal can still usually has a higher permissible temperature and may be needed in a rather hot environment.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2022, 06:44:03 pm
content: There's also the issue of the exposed can/collector of a TO-3 package transistor on an external heatsink. If there's any chance whatsoever of it getting pushed against any metal object, you need an insulating cover over the TO-3 case or a mesh cage over the whole heatsink.When it comes to heatsink compound, less is more effective. The thinnest film possible that still fully wets both surfaces is what you are aiming for. Then put a tiny extra dab in the center of one surface and assemble, and you should get a tiny reasonably even line of squeeze-out all the way round, showing there are no voids or bubbles, but not so much that it needs any clean-up. Unless you are doing a lot of assembly or repair work, a 30g tube should last a hobbyist for many years.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 08, 2022, 06:54:23 pm
content: There are also nylon screws

author: Gyro
author_rank: Super Contributor
timestamp: March 08, 2022, 07:40:59 pm
content: Although Nylon screws won't allow you to achieve and maintain a reasonable clamping pressure, and tend to stretch or become brittle with time and temperature. Also, you would then need to find some other means of making the case connection (I wouldn't trust a solder tag held to the transistor body by a Nylon screw).

author: glinjik
author_rank: Regular Contributor
timestamp: March 08, 2022, 08:03:18 pm
content: Thank you everyone for your advice so option 1 mounting kit at the moments very difficult to get imported things (Russia) option 2 use different transistors so after rummaging in my junk bins i found transistors d4515 to i think it is the way to go there a bit heavy on the volts but can handle 15a each they are originals not copies and i have like 9 of them so if i get smoked out of the house i have a few spares

author: themadhippy
author_rank: Super Contributor
timestamp: March 08, 2022, 08:49:40 pm
content: cant you bodge something ,a bit of plastic tube for example,or use nylon nuts and bolts

author: glinjik
author_rank: Regular Contributor
timestamp: March 08, 2022, 09:03:54 pm
content: LOL bodging is my middle name but tbh it's easier to use different transistors, and I've got plenty of them at hand

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2022, 09:04:56 pm
content: At a pinch you might get away with using TO-220 bushes however for power applications certainly not nylon nuts and bolts.Bushes and washers are cheap and most any electronics supplier will stock them.https://www.wiltronics.com.au/product-category/transistor-mounting-hardware/

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2022, 12:49:33 pm
content: For isolating the screws there are several makeshift options, like plastic / ceramic washers and some isolaton sleve around a screw. Nylon screws don't get enough force and they get soft when warm.AFAIK the TO3 case was also used for USSR time transistors.The D4515 may stand the current, but it could have lower gain. So it depends on the circuit of this works as a substitute.

author: grumpydoc
author_rank: Super Contributor
timestamp: March 09, 2022, 04:38:38 pm
content: Quote from: glinjik on March 08, 2022, 06:05:04 amHow much power do you expect to be able to dissipate per 2N3055?

author: glinjik
author_rank: Regular Contributor
timestamp: March 09, 2022, 07:02:48 pm
content: about 5A max but i would expect to use as little as 1A

author: grumpydoc
author_rank: Super Contributor
timestamp: March 09, 2022, 07:05:59 pm
content: Quote from: glinjik on March 09, 2022, 07:02:48 pmPower, not just current.

author: wraper
author_rank: Unknown
timestamp: March 09, 2022, 07:11:03 pm
content: Yes, current without voltage is meaningless. For example there could be 2V voltage drop, At 5A it would be just 10W heat dissipation and completely manageable with a quite small heatsink. On other hand if there is a 50V voltage drop, it will result in a quick transistor destruction regardless of how large heatsink is used.