Heat management for Bridge rectifiers.

[McBryce]

Hi all,
    I'm currently repairing a power distribution PCB from an Arcade machine (one of these: http://bigdaddy-enterprises.com/images/BPS018.jpg - this is a picture of a remake, but very similar to the original). The three massive rectifiers on the original didn't have any heatsinking and were know to get so hot, that they even managed to desolder themselves and the top fuseholders on occassions.
I'm replacing the 3 rectifiers with KBPC1502 parts, which can easily manage the current (max current is 10A), but I'd like to add a decent heatsink too. I have a suitable single heatsink which I could bolt across all three rectifiers. According to the datasheet the metal plate on the rectifier is isolated, so electrically using one big sink shouldn't be a problem, however, the current flowing through each is completely different. Two of them tend to get extremely hot, whereas the other barely gets hot at all. Would I be better using 3 seperate heatsinks, or will the conducted heat from the two heavy loaders have negligible effect on the third rectifier?

McBryce.

[T3sl4co1l]

In theory, the heat will make the last one very slightly more efficient (lower Vf -- negative tempco).  Seems fine to me.

Tim

[McBryce]

Thanks for the confirmation, then one heatsink it is!

McBryce.

[mikerj]

The rectifiers are very unlikely to be level and parallel, so purely from a mechanical stress and thermal contact point of view separate heatsinks would be a good idea. 

The "pin fin" heatsinks used on some PC chipsets and older graphics card GPUs might be useful here, lots of surface area for a small footprint.  You'd need to remove some pins in the middle for a fix screw though.

[mariush]

I would first check the rectifiers first, maybe a diode inside those rectifiers shorted or something like that, causing the rectifier packages to overheat and desolder themselves.  Alternatively, maybe something shorted after the fuses but the current was not high enough to blow them.

If everything's OK, then yes you should consider adding heatsink to those rectifiers.

Look in the datasheet : http://www.comchiptech.com/cms/UserFiles/KBPC10005-G%20Thru224770.%20KBPC5010-G%20Series%20RevA.pdf

Your new rectifier is rated for 15A as long as the temperature stays below about 55c . Above 55c, with each degree, the rectifier will handle less current. I don't think those rectifiers are used for such high currents but you never know...  Ideally, you would remove one fuse and  put a multimeter there in series to measure how much current is going through the rectifier, just in case some part of your arcade draws too much power.

The tops of those rectifiers should be isolated (but check with continuity mode of your multimeter to be sure) so you should be able to just apply some thermal paste/adhesive and put some heatsinks on them.. Here's some suggestions:

http://www.digikey.com/product-detail/en/658-60AB/345-1072-ND/340365
http://www.digikey.com/product-detail/en/658-60ABT1E/345-1108-ND/2643561

It shouldn't be an issue to connect all three at one large heatsink. With higher temperature, usually the forward voltage of the diodes inside the rectifier goes down a bit which in turn would cause higher peak dc voltage out of the rectifiers but I doubt the designers of the arcade machine had temperature in mind when designing that, the output of that colder rectifier is probably regulated on some other board in the arcade machine.

[T3sl4co1l]

The rectifiers are very unlikely to be level and parallel, so purely from a mechanical stress and thermal contact point of view separate heatsinks would be a good idea.

Good point -- make sure that, when soldering the new ones in, you insert them into the holes (loosely), bolt them to the heatsink, and finally, solder them in place.  Minimizing mechanical strain on leads is a priority when assembling circuits with heatsinks.

Tim

[McBryce]

My plan was to bolt them to the heatsink first and then solder them in. I intend leaving the leads above the PCB quite long anyway so that they have some room for expansion when things do get hot.

@mariush: The rectifiers haven't failed, this is a known "feature" of this board.

Just found this picture on the web of what a typical one of these boards looks like after many years of use. They usually still work, they just don't look great: http://www.yellowstitch.com/klovp4.JPG (In this case both power resistors seem to have physical damage, so I doubt this one still works).

The rectifiers were originally under the PCB, but I will be mounting them on top to allow for the heatsink.

McBryce.

[SeanB]

Warranty board, fails and you need a service call just out of warranty.......

[McBryce]

Nah, these boards ran for years 24/7 in Bally arcade machines around the world. In most cases the machines would have been leased or on a service contract anyway.

McBryce.