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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I built one of these before, but that was before the days of 4HV. (for my HF TIG welder)
I built this one partly as a prototype for some larger ones I'm going to build for a rectifier, and partly to see how far I can push a 2N3055. (and other TO3 transistors)
It will have two 80 mm PC fans, one blowing and one sucking, and will be enclosed. I'll post some more photo's tomorrow.
Last time the bit that the transistor is mounted on was 13 mm thick, rather that the 10 mm this one is.
I didn't use any maths when designing it as none of the formulae I could find on the manufacturer's websites, etc. seemed to be that appropriate. I've seen some vaguely similar (H section) heatsinks, but nothing quite like this design.
I'd appreciate any constructive comments before I start on the four 120 mm square ones for the rectifier, but the basic 'terms of reference' was to design a TO3 heatsink with two 80 mm fans.
It's also designed for high airflow over the transistor itself, and designed to create some turbulence, as well as having plenty of aluminium to conduct heat away from the transistor.
It's fitted with an ST 2N3055 which I intend to blow up very soon (I also have some Motorolla 2N3055H's, which should have significantly better performance.
Changing transistors is not straightforward, though. I used nearly all of a 2ml syringe of heatsink paste when assembling it, but I'm planning on making my own in future (see post in chemistry section).
Once I get the fans fitted I'll see how many amps it can push into a flyback before it blows.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Forty wrote ...
you could stick a block of aluminum on top of the transistor to pull heat away from that side and to provide a little compression
Do you mean on the dome, or around the upper flange?
From what I've read cooling the dome doesn't achieve much.
I have some heatsinks that bolt onto the top of the flange on a TO3. Designing a 2 part heatsink may improve things. It certainly gives me something to think about, it might even improve the design of the heatsink for the stud diodes in the proposed rectifier. Thanks, Forty.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Once the heatsink gets large (low K/W) the interface between the transistor die/case and case/heatsink becomes the limiting factor. e.g. 2N3055 junction-case = 0.657 K/W case-heatsink = 0.2 K/W with high thermal conductivity pads, 0.4 K/W with silicon rubber pads 0.1 K/W with no insulation & good grease etc. Total = 0.757 to 1.057 K/W
So there is no point making a 0.1 K/W heatsink for a single 2N3055.
Cheaper and more robust would be a smaller heatsink with a few 2N3055 in parallel, possibly with a driver transistor.
For the fan(s), with axial fans there is virtually no increase in airflow when two fans are used (effectively) in series, I've tried it with six in series .. negligible airflow increase. It would be better to use one fan with a higher cfm rating.
Lastly .. why use TO3 devices? especially why use a 2N3055? TO247 etc. are easier to mount and have better thermal conductivity.
Registered Member #1526
Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216
Yep what you really need is to lower the heatsink temperature. Bulk provides `stamina` but low temperature will increase peak power which is what you`re after. How about a hollow vessel you can bolt a transistor to and fill with ice water or acetone/CO2 mix? Sounds like fun :o)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
@ Fraggle, for this project I'm only considering air cooling.
Sulaiman wrote ...
Once the heatsink gets large (low K/W) the interface between the transistor die/case and case/heatsink becomes the limiting factor. e.g. 2N3055 junction-case = 0.657 K/W case-heatsink = 0.2 K/W with high thermal conductivity pads, 0.4 K/W with silicon rubber pads 0.1 K/W with no insulation & good grease etc. Total = 0.757 to 1.057 K/W
So there is no point making a 0.1 K/W heatsink for a single 2N3055.
The only figures I have on my 2N3055 datasheet are 'thermal resistance junction-case = 1.5 degrees C per watt'
and 115 watts dissipation @ 25 C case temp.
and max junction temp =200 C
My original objective was to try to keep the case temperature to 25 C (or as close to 25 C as I can)
Sulaiman wrote ...
Cheaper and more robust would be a smaller heatsink with a few 2N3055 in parallel, possibly with a driver transistor.
I might try this, but I still have some concerns about them not all switching off together.
Sulaiman wrote ...
For the fan(s), with axial fans there is virtually no increase in airflow when two fans are used (effectively) in series, I've tried it with six in series .. negligible airflow increase. It would be better to use one fan with a higher cfm rating.
I'm aware that it certainly won't double the airflow. The main reason for using two fans is reliability issues, if one fails, the other will still keep things (hopefully) cool enough. I do expect some increase in airflow though, as air passing through the heatsink is moving from a high pressure region to a low pressure region. One fan on it's own will not blow it's rated capacity if there is an obstruction to airflow (heatsink), it will only shift it's rated capacity with no restriction. Two fans should improve things, but will probably still not shift as much air as one fan is rated for.
Sulaiman wrote ...
Lastly .. why use TO3 devices? especially why use a 2N3055? .
'Because it's there'.
I expected you to say that, I've heard your opinion of 2N3055's several times.
The 2N3055 is a good starting point. They are also cheap (~£1 each), and easy to drive. Many people here use them especially in the single transistor flyback driver.
This project should give me a good basis from which I can build on in the future.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Ash, this is the post I accidentally put in Chemistry
IF you are going to persue this ultimate heatsink design you should get a (cheap) multimeter that can use a thermocouple, make or buy a few thermocouples (you can use themocouple extension wire to make thermocouples) and measure the temperatures of - ambient air - transistor case (the top part will do for static measurements) - the heatsink as close to the transistor/heatsink interface as possible - the temperature of the heatsink far from the transistor (the last two will require drilling into the heatsink)
only then will you be able to 'see' what is important and what is not.
If you want to really see what is going on a TO3 can is easy to open so you can actually measure the die temperature
OR buy a thermal imaging camera then lend it to me when you're finished ;)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sulaiman wrote ...
Ash, this is the post I accidentally put in Chemistry
IF you are going to persue this ultimate heatsink design you should get a (cheap) multimeter that can use a thermocouple, make or buy a few thermocouples (you can use themocouple extension wire to make thermocouples) and measure the temperatures of - ambient air - transistor case (the top part will do for static measurements) - the heatsink as close to the transistor/heatsink interface as possible - the temperature of the heatsink far from the transistor (the last two will require drilling into the heatsink)
only then will you be able to 'see' what is important and what is not.
If you want to really see what is going on a TO3 can is easy to open so you can actually measure the die temperature
OR buy a thermal imaging camera then lend it to me when you're finished ;)
Yep, I had considered this. I have some thermocouples that came with a 'high end' PC case, with LED readouts. I've not checked them for accuracy yet.
The thermocouple end is inside a sticky pad so would presumably require some modification.
No instructions came with it, so I think I stuck one of the pads onto the CPU heatsink (it was some years ago now).
I don't use that PC much these days as I now have a half decent laptop, but I have data (and software) going back 20 years on the hard drives in the PC. I'll open it up later and investigate using them.
(Drilling holes in the heatsink itself sounds like an excellent idea, thanks)
I have myself gigantic heatsinks and this thread gave me an idea
rip off the cans of my TO3 devices and make brick-like devices of them by connecting directly to the die
sandwich peltiers between them and heatsink to suck heat efficiently
would it improve dissipation that much?
I'd like to build a broadband amplifier (DC to 500Khz)
sorry if this hijacks the thread but I though it was related in a matter of heat dissipation and best use of TO3 devices
I've considered peltiers but they still require cooling and apparently aren't very efficient.
Personally, I think the best alternative to air cooled aluminium heatsinks wound be refrigerated copper ones.
I had intended this thread to be about air cooled heatsinks, due to the simplicity/reliability.
I don't mind hijacking though, it often leads to interesting and diverse ideas. Some of the most interesting threads on 4HV are threads that have been hijacked several times.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I know I've double posted, but this is new content.
The thermocouples I have are these (TTF-103):
As you can see from the dimensions in the spec. sheet, the 'business end' is encased in plastic film 3.6 mm x 0.6 mm.
Sulaiman has suggested drilling into the heatsink under, or close to, the transistor.
I used to secure thermocouples into 5 mm O.D. stainless tube (3 mm I.D., I think) using a special epoxy (high temperature, I think, it was grey) in a job I used to have, but they were cylindrical in shape.
I want to be able to re-use these thermocouples, so I don't want to epoxy them in, but still need to plug the hole and transmit the heat to the thermocouple as fast as possible.
Any suggestions as to what I could use for this that would be easily removable, won't damage the thermocouple, and won't 'leak out' if it gets hot?
(I'm thinking of using aluminium anti-seize compound, but I can't find any info on melting point - this stuff might even work as heatsink paste itself, as it is aluminium suspended in grease)
The only other option I can think of is to use foam sticky pads to secure the thermocouples onto the transistor can and the heatsink, but I really want to know the temperature inside the heatsink close to the transistor/heatsink interface.
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Probably the world's most labour intensive 2N3055 air-cooled heatsink
