I use a smaller plate (1/16x3x4?) and don't have problems yet. It doesn't seem to get hot, but I'm not in your environment either Maybe holes drilled in it would help?
 
That's interesting. Our climate isn't really a factor right now. Only in the 80s. I never did rule out that the first rectifier was good to begin with. But it worked for a short time and failed in such a way that it could easily be blamed on overheating.

Well, a bit more sink can't be a bad thing.
 
I think you just got a bad rectifier. These Chinese rectifiers are mass produced by the millions, so there is bound to be a small percentage that has manufacturing defects. Quality checks in the factory..................very little if any.
The good news is that they are cheap, and the second one you buy will almost certainly be a good one.
14.2 volts is just about perfect!
I don't know if your heat sink was part of the problem, but I would not use a flat sheet that is only 1/16" thick. In fact I would not use a flat sheet at all. I know some lads do use flat sheets. If you do use a flat sheet, then at least make it 1/4" thick so that it has some mass to it. 5twins has a really good one. Lots of fins and lots of surface area gives good heat transfer to the air around it. Multiple fins are better than just flat pieces. Old computers or TVs and other electronic gear is a good place to find heat sinks. I use a heat sink that was meant to be used on a music type amplifier.
This picture is an example of the type that I use:
Typical heat sink.jpg
 
Errrrr....ummmm....cool! <get it???>

As for electrical folks - yup, toasters are great people...:D !!!! ....but how the heck do they figure this stuff out when they can't even see it move?

Pete
 
I like the way you made it fit the space. Looks to me like it should have enough heat sink to it, and I like the way you re purposed materials you already had!
IMG_4476.JPG
 
For a silicon diode the voltage drop is approximately 0.6 Volts. In a bridge configuration we deal with two diodes per phase so the voltage drop is approximately 1.2 Volts. Assuming the total current is 16 Amps then the heatsink needs to cope with about 19 Watts (1.2 x 16).

Search Ebay for "20 Watt Heatsink" to see what their typical dimensions are.

Edit: Silicon diodes have a voltage drop closer to 0.7Volts.
 
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Remember, the alternator is not always running at maximum load. Even after using the electric starter the current load drops off very quickly as the battery starts to recharge. I have read in the past that the maximum loading after use of the starter on a car is in the order of 1 - 2 minutes. A well maintained battery and short idle times will help minimise the stress on the alternator/rectifier/regulator i.e. less heat demand on the heatsink. 20 Watts is the maximum peak demand.
 
Hi Daniel / All:

Mechanical engineering has several branches and fluid/heat transfer is NOT the one in which I specialist - BUT - I think that a heat sink on a bike, it will have a pretty constant flow of air around/over it (unless you sit idling) while I think that the normal ratings are sized for "static" air conditions.

Thus, a bike-mounted component may be able to function with a somewhat smaller heat sink.

Just a thought.

Pete
 
It is also important to maximise the contact between the rectifier and the heatsink. Sheet metal, if too thin, distorts when the rectifier is bolted on and this is probably why the commercial heatsinks are quite thick at the mount point and very smooth. At some stage I would like to try soldering a Copper pad in between my rectifier and the aluminium. I realise I need the correct flux to do this but it offers certain advantages due to the high heat capacity of Copper.
 
Paul, it may not be as bad as you think. Each diode is conducting only during half of its cycle. And, Vf decreases as temp increases.

Edit: A bit of a rethink here. You're calculating about a 10% power loss, which is actually quite reasonable for a 12v system. Worst case would be around 15%, which would call for handling about 30 watts of heat...
 
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Went on a short ride, approximately 10 miles, and checked the temperature of my rectifier/regulator setup halfway and at the end. The regulator was cold and the rectifier and heatsink were both just warm to the touch. The Special has a bit of room for fitting the rectifier/regulator. My heatsink is made from two identical aluminium plates each 2mm thick and 145mm x 80mm screwed together with heat transfer compound in-between. Approximately 25 - 30mm at the end of each has been bent in to form fins:
regulator-jpg.94831


I had a close look at the original stock rectifier/regulator, which appeared to be working fine when last in use, and I noticed that the sealing epoxy has a raised dull area - I suspect overheating so perhaps a diode in the bridge section is on its way out.
 
Congrats, Paul, on a cool running rectifier!

I've seen quite a bit of info regarding the aluminum/copper choice of heatsink. The best I can figure, and it seems you do as well, a copper sink to suck up heat with aluminum fins to disperse it might be best.

So, at the start of the day yesterday, I was getting perfect readings on my charge. 14.2v at 2000rpm and above.

At the end of my 4mi commute to work the rectifier felt rather warm. On the 'how-long-can-I-touch-it scale', it measured 2-3 seconds on the epoxy right in the middle of the terminals.

Doing the same test after my ride home the rectifier was barely warm at all. Checking the charge voltage, it would only get up to 13.5v or so all the way up to 4000rpm.

Not done yet...
 
Hmmmmm. Dammit DB - I'm coming back out there to give you hand with this grumpy old gal.

Now, where did I put my passport.......?
 
I've found the charging output will drop off some after the battery has replenished itself. Test the charge output right after electric starting. You might find it back up at that 14.2 output. What I'm saying is, maybe you don't have a problem at all.
 
My unit is functional whereas 5twins is cool looking and functional. If you do get further problems then copy his design because it works well. I would like to use a heatsink like his, in fact I already have some old computer ones, but it is a bit difficult to fit that design in on a Special. Requires more thought/work with bracket construction, but I will eventually get there by the end of Summer or during next Winter. I will also change to the Fiat regulator because it is an easier shape to work with than the one I currently have - its heatsink being ground/negative is a bonus also.
 
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