Req advice on heat sink (thermal) compound

[JohnA]

Greetings to all,

I am restoring several NSI DDS6000+ dimmer packs that had some bad SCRs in them. For those not familiar with the inside of these units, they have 2 SCRs per channel, each mounted to an aluminum heat sink. I would appreciate a recommendation for a thermal compound to apply to the back of each SCR to assist in maximum heat transfer to the larger aluminum sink.

Thanks--John A

[gafftaper]

I'm no expert but I would think the silver compound used on computer CPU's would work great.

[church]

you just need something to act as a thermal filler that can handle the temperature. If you go to your local auto parts store and get a small tube of RTV or silicon caulk - the colour doesn't matter at the temperatures you will get on the device in this application. Put a thin smear on the back of the device. If it is a device with an insulated case then just put straight onto the heat sink and bolt in place. If the device has a non-insulated case then place the mica washer onto the device case and smear another layer of RTV onto the mica washer and then bolt the device to the heat sink. Don't get the RTV onto the device leads as it makes them impossible to solder without very thorough cleaning. RTV can withstand almost 500 degrees F - your device will fail long before it sees these temperaatures.

[JD]

Woo, hold the phone jack! You need heat sink compound (zinc oxide!) In most cases electrical isolation is required and you should not use any compound that conducts! If there was a mica washer under the SCR, then this is critical!
http://www.mcminone.com/product.asp?...5Fid=20%2D1975

If the SCR was isolated, replace the washer and insulator as well. You will then need to check for any leakage to the frame. If there was no washer and the body of the part was mounted directly to the heat sink, then either the SCR is self isolated, or the heat sink itself was isolated. Regardless, use the proper compound as you want the heat to move freely out of the SCR, this is what the zinc compound was developed for.

[Van]

It also greatly depends on whether the heat sink is held in place vis a screw / retaining bracket, or if the heatsink is held in place by an adhesive thermal compound. It's been a while since I have opened an NSI dim, but I believe there are serveral scrs that are all screwed to two seperate heatsinks. In that case all you need is a heatsink compound, < also called thermal grease> standard zinc oxide type would do. Remember these are only SCRs and not nearly as sensitive as say , a socket 939 pentium overclocked 75%. It would be a lot more expensive but you could use the Artic Silver, or standard CPU heatsink comound. Remember you only want enough to fill in all the voids on the SCR's faces. Having a bunch spilling out past the edges is wastefull and actually can increase the heat rentention of the component. Put a BB sized amounton the SCR, press it against the heatsink. Pull it off to check for coverage across the entire component. Add more, if necesarry. Push it back on, wiggle it a little replace the bolt or screw and you're in business.

Thermal grease, standard grade heatsink compund should be redily availible at Radio Shack, or a really good hardware store. If you don't have a handy electronics components store,< Frys, etc> then try a hobby shop that does a lot of model trains and airplanes as often these places cater to guys that build a lot of custom components for their rigs. Oh yeah RC Car guys use a good compound on the mounts for their electric motors too. That stuff would be more than adequate for SCRs.
Happy hunting.

[church]

I suggested RTV or silicone because it is easier to get hold of and actually works better than the zinc oxide past you are refering to. My reccomendation is based on many years of engineering experience where we use it for exactly this purpose and it gets to fly in space. So we have done countless hours of testing under conditions your dimmers will never see, vibration, temperature, shock, humidity, vacuum etc. By the way it also has to operate guaranteed for up to twenty years- routinely lasts much longer, without ever being repaired..

All the advice about keeping quantities to a minimum is excellent - it is true it is to fill the cavities on the heatsink and the device which you will only see at X7 magnification or better. here in canada it is actually quite hard to get zinc oxide grease except from specialit suppliers that only sell to account holders. radio Shack has become Source One and is the source for naff all except tat that you can get in every other shop.

[JohnA]

Thanks to all who replied.

To clarify things, the SCRs have a metal heat sink on them. This metal is attached directly to the larger aluminum "chassis heat sink" by means of a screw. There is no washer or insulator--just metal to (aluminum) metal contact.

Some of the newer SCRs I installed seemed to be running "hot", so I thought an application of a good thermally conductive paste would help to move the heat away from the SCRs .
--John

[SHARYNF]

the thermally conductive paste like artic silver for instance works by eliminating the surface imperfections that leave air space, and also the addition of the silver inproves the conductivity of the paste.

The better the surface to surface conductive connection for heat the better, but as others have said, you are not looking at a high temp cpu or gpu so your options are more flexible

Sharyn

[JD]

For calculating about how much heat a device should produce, here's the Rule of Thumb for thermal loading of SCRs and triacs: 1 watt for every amp conducted. In other words, if the dimmer is handling 20 amps, the triac needs to dump 20 watts of heat. In the case of back to back SCRs, each would be dumping 10 watts as they are only running 50% of the total waveform. For those who do not know, triacs and SCRs are either on of off. There is no in-between state. Dimming is achieved by turning the device on at a set time after the waveform crosses 0 volts. The earlier the turn on, the more waveform passed and the brighter the light. The sudden chop in the waveform is why the output needs to be filtered by a choke. It is also the cause of "Lamp Sing" and audio buzz. When selecting a replacement triac or SCR, make sure the device is case isolated if the original was. (OEM part is best) These usually have 3 tabs on them as compared to two. Some of the SCR dimmers use a two device package with 4 tabs. (EDI comes to mind.) As you can see, heat should be consistent from channel to channel if the load is the same.

[JohnA]

Thanks for the additional advice, esp. the per watt dissipation calculation.
Per NSI, the SCR is a S6025L. I am replacing them with identical pieces. I have found Arctic Silver 5 for $7 including shipping.

--John