How much heat sink?

[PhredAre]

I stand by what I said ," in the first few minutes" . If that failure occurs in that period Its due to overload or defective equipment. Yes, I wouldn't use a SSR for this application with out a heatsink. Fred

[CarWiz]

Wrap your hands around a lit 40 watt light bulb for "a few minutes" then I might consider your claim. That's about what the SSR is generating at 35A load. I'm betting you won't get past 30 seconds, if that.

One thing you have to keep in mind is that the SSR's load capacity goes down as it heats up. It looses about 20% for ever 10°C over 40°C (104°F). So by the time you start feeling that light bulb warmth over your body temperature, that 40A SSR has started loosing its load capacity. By 50°C (122°F), its capacity is down to 32A. POP! goes the SSR drawing 35A.

[PhredAre]

Now thats interesting, a 40 watt light bulb, thats a lot of heat. Any way with that excepted then answer this , why would any SSR ever be built without a heat sink, and why not just build more fat into them? Thanks for the feed back, Fred

[ed_denu]

I think this is an interesting discussion, but if you really want to know how hot your ssr is getting, measure its temperature with an IR thermometer or temperature probe, or both. My ssr documentation shows an allowable operating temperature of up to 80 degrees C.

Using both an ir and temperature probe I see temperatures to around 43 degrees C(110F) pulling about 23A thru a 40A ssr . I'm using a heatsink I made out of aluminum sheet.

Carwiz, you wrote that an ssr "looses about 20% for ever 10°C over 40°C (104°F). So by the time you start feeling that light bulb warmth over your body temperature, that 40A SSR has started loosing its load capacity. By 50°C (122°F), its capacity is down to 32A."

I've read a little about ssrs and heatsinks and all the formulas to compute heat dissipation, but I've not seen this drop in capacity documented. If this were true why would ssr manufactures list acceptable operating temperatures up to 80 degrees C? Using your calculation my ssr at 80 degrees C would only support 20% of its stated load. It doesn't seem logical that manufacturers would list an acceptable temperature range to 80 degrees without a disclaimer noting that significant of a drop in switching capacity.

I also think your analogy of the 40 watt bulb is a little misguided. While I certainly agree that you could not keep your hand on a light bulb for any length of time, you certainly could contact something in the 40-50 degree C range without feeling any discomfort.

[PhredAre]

Going back to what I said," Failure in the first few minutes " . I fired up my oven and had plenty of time for," Failure in the first few minutes " before any significant heat was felt on the SSR case let alone in the built in heat sink. Granted, i'm only running at 60% of rated capacity. I'm having a little problem with the 40 watt light bulb, which has a very thin glass enclosure that provides very little insulation to the filamint, so oviously the temp. increase is felt much sooner. With this in mind, it takes much longer for the heat to migrate out of the case of a SSR an into the heat sink, so once again, if failure ocurrs in the first few minutes it could very will be caused by a "overloaded SSR or a faluty SSR'. After all,a heat sink only provides surface area to disipate BTUs and temp DT drives this action , theres really no magic involved. Fred

[CarWiz]

"It doesn't seem logical that manufacturers would list an acceptable temperature range to 80 degrees without a disclaimer noting that significant of a drop in switching capacity."

They do. You need to check the spec sheet for your SSR. Some disguise the data in charts and some just flat hide it or don't mention it. Some don't mention the fact that SSR's used to switch heaters should be de-rated to 75% either. I could care less if folks don't want to run heat sinks. I'm just stating my observations and trying to save some grief for a few. Use the data or not. Better still, do your own research.

Here's a couple of samples.

[CarWiz]

"I'm having a little problem with the 40 watt light bulb, which has a very thin glass enclosure that provides very little insulation to the filament, so obviously the temp. increase is felt much sooner."

It's just an example of the amount of heat that is generated at the junction in the SSR. Your statement tells me you know nothing of thermodynamics but I'll respond anyway. The heat is NOT felt any sooner. The best insulation is gas and glass. The element is what, maybe an inch from the inside glass surface? Imagine the temperature of the filament's surface glowing white hot! It's probably around 2500°F or more.

[ed_denu]

First I've never suggested running an ssr without a heatsink. I have one on my oven but I also only run my ssr at around 58% capacity.

I did look around a little and found a web site that provides a good explanation of heatsink design. If anyone is interested here it is:

http://www.power-io.com/library/appn...tsink-info.htm

[ed_denu]

Originally Posted by CarWiz

Here's a couple of samples.

Carwiz - I've looked up several different ssr data sheets and some show actual derating charts while others show charts similar to these. Can a derating value be calculated with these charts. I guess I don't understand how the thermal resistance value in these charts is useable?

[PhredAre]

CarWiz, In a standard light bulb , the filamint is in a vacuum isn't it? I sure wish I was smart as my critics but then I don't want to get too personal. Thanks for your interest, I'm sure that a lot of the information put out here has value to those that are new to oven building and want to do there own electrical package. Thanks, Fred