Aluminium heatsinks as conductors

[sbkenn]

Hi. Has anyone come across a chemical treatment to dissolve/convert the oxide layer so as to improve electrical conductivity between the heatsink and cooled device ? I found a product some years ago, but can't find it now.

 

[IRstuff]

?? Why do you want to do this? Most circuits would typically get increased noise, etc., with such a configuration.

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[sbkenn]

A big rectifier, where one pole is the case. Many big triacs and thyristors only have one power stud or lead too.

 

[IRstuff]

No, I guessed that, but it's generally a bad idea to run current through things like heatsinks, because of noise, but also because of potentially unintended consequences like people getting shocked, or injecting short-circuit currents into other things.

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[Comcokid]

Aluminum parts that are intended to be electrically conductive are chem film (aka alodine or iridite) treated instead of receiving an anodizing coating. Frequently done with aluminum sheet metal enclosures used in aircraft. You can even get a coating in a pen-type applicator as Henkel or Bonderite 1132, although you need to see if it has the conductivity you need.

IRstuff brings up a good point about safety. I have seen heat sinks that are electrically hot, but inside an insulated enclosure, with safety interlocks, painted red on the edges, and stenciled with high voltage warnings.

 

[waross]

For power work we usually remove the layer mechanically, a wire brush is one method, and immediately apply a coating of oxide inhibitor such as Penetrox to prevent the oxide layer from reforming.
Note, if you clean the oxide and then take a 10 minute coffee break, you must reclean the aluminum. Immediately means NOW. The oxide forms quickly.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

No, not really IRstuff. Large "hockey puck" style thyristors require double sided cooling and are clamped between heat sinks, making the heat sinks part of the current path. Look at large soft starters and DC drives that use SCRs, they are all like that.

Sbken, generally the only thing that is done is to machine the contact surface as smooth and flat as possible, and then apply some thermal conductive compound, just to prevent seizing / galling of the aluminum if you are threading in a post type device, or to allow air gaps to be squeezed out when clamping. If you have the direct contact type where the heat sinks are in contact with the current carrying component, I recommend using Penetrox Pena134. If using an isolated base device, aka "brick" where the heat sinks are not live, then use Techspray 1978 thermal only compound.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[IRstuff]

OK, a different reality.

Is Iridite or Chemfilm usable for this application?

http://en.wikipedia.org/wiki/Chromate_conversion_coating

Its conductivity is not sufficient good enough, usually, for EMI bonding, but someone makes use of it...

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[sbkenn]

I got the 3-phase rectifier in a junkyard. All the diodes were good, but resistance between diodes and h/s were all quite high. I guess this was the reason that it was retired. I was well aware that aluminium oxidised quickly, and also that the oxide is VERY hard .. hence it's use as an abrasive. In my case, I am using it in a 24v battery charger, so no shock hazard. It is gross overkill, as the diodes are probably >200A devices.
I will see if I can track down the products suggested.
Thanks for your input.

Shane

 

[IRstuff]

Your supposition may not necessarily be correct; multiple diodes on a single heat sink may not necessarily be electrically tied together. For example, a full-wave rectifier would only have any two diodes sharing one connection, so it's likely that they needed to be electrically isolated from each other. You should check to see if you see either a clear or white insulator pad between each diode and the heat sink, or, whether each diode has a separate wire to each anode and cathode. Note also, if the diodes were originally intended to make electrical contact to the heatsinks, it's unlikely that all of the diodes would have lost contact. You should check to see if the individual diode casings are electrically connected to the heat sink directly.

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[sbkenn]

No offense intended, but I do know something about electronics. I have been in the business, albeit mostly on the digital hardware side, for nearly 40 years. The 3-phase rectifier is built on 2 x electrically isolated heatsinks. 3 diodes case anode, 3, case cathode.