Foolish question re heatsinking

[DamonHD]

Hi,

This is surely a good place to ask foolish questions, or at least I hope it is.  Because I have the question all typed up below...  B^>

I have now built my emergency crowbar wind turbine shunt load (to trigger at 14.8V upstream of a diode to my 12V SLA).

What I have not yet done is fit a heatsink to the SCR.

I have bought some chunky heatsinks that fit.

I have bought some "extremely toxic to aquatic life" heat sink compound.  (Well, I don't want any fish in my circuit boards, so that must be good, yes?)

I've bought thermally-conductive insulating heat-sink washers to match the TO220 profile of my SCR.

BUT... What am I meant to fasten the SCR to the heatsink with?  A normal metal bolt and nut (in which case presumably I need a small insulating washer, but I could not see one in the catalogue I usually use)?  A nylon bolt and nut (but will it melt/burn)?

I don't actually expect this crowbar to be triggered any time soon, and I was operating without any such protection for a while, and manually triggering the crowbar a few times in decent wind just now made no perceptible difference to the SCR temperature by touch as I'd expect (but did slow the turbines right down), so I don't need answers this instant, but...

Rgds

Damon

[Flux]

Not really a foolish question. I think the weak link is going to be the to22O thyristor, I wouldn't touch it, but I won't force my ideas on you.

I don't think you need worry much about heatsinking in your case, the thing will shut down in such a short time that heatsinking may not even be needed. ( if it fails to shut down then you have other worries).

The best thermal conductivity is achieved if you can mount the thyristor directly to the heat sink, but if you have bought the washers you probably intend to isolate it.

The mounting kit you have should supply you with an insulating bush to prevent the screw shorting to the heat sink. This will let you use a steel screw, I wouldn't consider nylon, I doubt that you will get enough clamping pressure and it will relax with time.

If the kit uses mica washers then use the heat sink compound ( to hell with the fish). If it comes with a flexible grey "silpad" then there is no need for the compound and if you do use it don't use more than a thin film.

Your big heat sink will be wasted for this but being over cautious doesn't hurt.

Flux

[DamonHD]

Hi,

It is indeed I a silpad that I have (though I don't know what a silpad is!), and I would like to isolate the tab since it's otherwise asking for trouble floating around at 12V+ IMHO, being connected to the SCR's anode.

The SCR I have is a 2N6507G and by happy coincidence (I noticed after I had bought both) is explicitly recommended for use with the MC3423 that I'm using to drive it as a crowbar.

I'm using this as much to learn about heatsinking (in case I need to upgrade to GHurd's dump controller at some point) as anything else.

There wasn't a mounting kit and thus no insulating bush (there has to be a US climate change joke there somewhere), but I will go out and buy a nylon or other insulating washer and a steel nut and bolt now that I'm armed with your advice, thanks.

Rgds

Damon

[Flux]

You may struggle if you don't have the complete mounting kit, those little insulating bushes are not easy to replace at random. Not only do you need the insulation to the tab, you need to keep the screw centred in the hole in the sink so that nothing shorts out. Life is a little easier at 12v and if you get it isolated as shown on a multimeter then it will be good enough. If you want to play with dump regulators then it may be worth buying a pack of mounting kits. The silpads are silicone impregnated glass fibre or something and have a reasonably good thermal conductivity as well as decent electrical insulating properties, it is the silicone impregnation that makes them work without the thermal grease.

Flux

[DamonHD]

Thanks again for the info.

I will look for some proper mounting kits...

Rgds

Damon

[Capt Slog]

Damon

If you can't find a bush/mica/screw for the To220, please email your address and I'll send you a set.

.

 

[DamonHD]

Hi,

I haven't really looked hard yet, but now I have a better idea of what to look for.

However, if I fail, I'll be back to you.  Thanks!

Rgds

Damon