Proximity of SSRs

[2bluewagons]

Hi all,

Sorry if this has been asked before but search did not turn up info for my exact situation. I am building a relatively compact panel using this enclosure. The design includes two 25A SSRs, each controlling a 1500w element. For dissipation of heat, I am planning on mounting one SSR to the bottom of the nice thick enclosure and the other one to a heatsink mounted to the top of the enclosure.

I have two questions regarding this design:

Is it best if the SSR is mounted directly to the heatsink, requiring me to cut a square hole in the lid? Or is it ok (or better even?) to sandwich the lid in between the SSR and heatsink? I would use thermal grease on all contact surfaces.

How close can I locate the two SSRs? Keep in mind they will be inverted to one another (like Maverick and Goose to the MIG), so would there be a significant amount of heat that would radiate to the internal space of the enclosure with just 25A combined max passing through the two SSRs? Currently my intent is to overlap them just slightly kind like this with just over an inch vertical clearance between height of the terminals:

..heatsink
|.............|
|.............|
|.............|
|________|
........._________
.........|.............|
.........|.............|
.........|.............|
.........|.............|
...........enclosure

Thanks for any help

 

[SilverZero]

Since it's aluminum it basically is a heatsink. It's not as efficient at releasing heat but it's got a lot of thermal mass so it will spread out. I personally think it would be fine to mount the SSR to the enclosure and then have the heatsink on the opposite side of the wall. Put a PC fan on it for even better results, but thermal grease would be highly recommended. The other one on the opposite side of the enclosure will retain more heat but I still don't think it's too bad. Are you using nice SSRs or just Foteks? I'd just keep a couple of cheap ones handy to swap out if one does go bad.

 

[geoffey]

Will you be running both at once? Might make a difference. Personally I'd mount the one directly to the heatsink since the enclosure is already a heatsink for the other ssr. Looks like the enclosure is rated for only 20amps and you're loading it up with 25 amps so you might already be pushing it.

 

[BrunDog]

Don't mess around. Mount the SSR directly to a legitimate heatsink. I don't think the enclosure had the mass to pull enough heat, even if sandwiching the enclosure.

-BD

 

[2bluewagons]

Will you be running both at once? Might make a difference. Personally I'd mount the one directly to the heatsink since the enclosure is already a heatsink for the other ssr. Looks like the enclosure is rated for only 20amps and you're loading it up with 25 amps so you might already be pushing it.

Yes I will be running both at once, although a slight correction. Each SSR will be loaded with 12.5 amps (1500W/120V). I think it's a stretch to say the enclosure is rated for anything, but it does say it will handle 20A. The bulk of the enclosure is the bottom piece, so I am mounting the bottom SSR directly to the enclosure and counting on all of that heat dissipation to be handled by the bottom part of the enclosure.

The top SSR will dissipate [hopefully] the majority of its heat via the heatsink that will be in open air, with the rest of the lid surface available to help with any leftovers.

My main question relates to the heat emitted by the top/sides of the SSR. If properly "sunk" will that be minimal or is the combined contribution to the internal unvented airspace from both SSRs a real concern?

I suppose I could mount TCs on the surface of each SSR and one for the airspace in the enclosure, and a TC selector switch with display to monitor all of them...

 

[2bluewagons]

Don't mess around. Mount the SSR directly to a legitimate heatsink. I don't think the enclosure had the mass to pull enough heat, even if sandwiching the enclosure.

-BD

Are you saying that neither SSR should count on the enclosure alone for heat dissipation? The bottom part of the enclosure alone has over 4 times the surface area of a heatsink I found that is rated for 40A, and over 10 times its mass. The combination of fluid and solid thermodynamics makes my head hurt, but I think the enclosure is good for at least one of the SSRs.

For the other one, I can't help but think that the sandwiching will only help, assuming that the contact on both sides is good. The heat will dissipate more quickly from the sink, with the enclosure only helping if needed, yeah?

 

[BrunDog]

It's the mass of conductive material that matters most and its exposure to air to be able to convect off that heat. If you think the enclosure is heavier than the heatsink then go for it. I still wouldn't sandwich the heatsink. It may work just fine but anytime you purposely use an unvalidated design, you ask for unnecessary trouble, IMO. If you do it pay attention to the temps.

-BD

 

[ajdelange]

It's not the mass of the metal that is significant but its thermal impedance to ambient. The latter is inversely proportional to surface area and airflow over it. If the box is big enough and subject to enough air circulation it can sink the SSR dissipations by itself. If not, improved air flow or more surface area (such as a heat sink) is required. Going to a box of the same sized but of a heavier gauge of steel (increasing the mass but not the surface area) will lengthen the time it takes the box to reach ambient but not the temperature it will ultimately reach (there will be a small change since the thermal impedance of the thicker metal will be higher).

Conceptually one could mount the SSRs to the box interior anywhere and heat sinks anywhere to the exterior and all would be fine if the box were made of infinitely conductive material. But it isn't. This doesn't mean that it can't be made to work but in any real situation one would have to do calculations backed by measurements (or at least measurements) to insure that SSR temp doesn't rise too high.

Is it better to mount the SSR directly to a heatsink? Yes from the POV that it minimizes the thermal impedance between the junction of the switching devices and the ambient. Can it be made to work the other way? Again the answer is yes but verification is required.

 

[rudylyon57]

My box has two 40A PID-driven SSR's (MLT, HLT) and one SSVR (boil) switching three 4500W elements. The wall of the box is sandwiched between the SSR's and external heat sinks. I have about four brews under my belt with no issues but I am keeping an eye on it.

 

[geoffey]

Yes I will be running both at once, although a slight correction. Each SSR will be loaded with 12.5 amps (1500W/120V). I think it's a stretch to say the enclosure is rated for anything, but it does say it will handle 20A. The bulk of the enclosure is the bottom piece, so I am mounting the bottom SSR directly to the enclosure and counting on all of that heat dissipation to be handled by the bottom part of the enclosure.

The top SSR will dissipate [hopefully] the majority of its heat via the heatsink that will be in open air, with the rest of the lid surface available to help with any leftovers.

My main question relates to the heat emitted by the top/sides of the SSR. If properly "sunk" will that be minimal or is the combined contribution to the internal unvented airspace from both SSRs a real concern?

I suppose I could mount TCs on the surface of each SSR and one for the airspace in the enclosure, and a TC selector switch with display to monitor all of them...

If properly heat sunk then I don't see how the configuration you plan on inside the enclosure will be a problem. If I read it correctly the product description for the enclosure DID indicate That it could serve as a heat sink for up to 20amps. But if you ask me I say cut a hole for the heat sink. It is much less likely to cause a problem that way and much more likely to cause a problem if you sandwich the enclosure between heat sink and SSR.

Look at it this way: when you finally get your system finished do you think you'll want to go back and re-order SSRs and rebuild the enclosure? Or will you just want to be brewing?

 

[2bluewagons]

So like a true meat-head, I chose not to accept the majority opinion and sandwiched the top of the enclosure in between the SSR and heatsink. I posted some pictures here if you are interested.

I had intended on following the advice and cutting a hole but I ran out of time before moving and now don't have access to as many tools or a good workspace for cutting aluminum plate.

Brewed yesterday and the SSRs are still functioning. Moving forward I would like to keep an eye on at least the internal temp if not the surface temp of the top of the SSR. What is the temp range where damage starts to occur?

Hanx

 

[doug293cz]

You want to keep the heat spreader plate (bottom) of the SSR under 140°F (60°C.)

Brew on