heat sink question for SSR

[garbear]

I recently purchased a JLD612 PID temperature controller and a 25amp SSR to control a meat smoking oven. I have autotuned the unit and it is performing perfectly. In my haste to get everything going, I forgot to install a heatsink on the SSR. The SSR is working fine as I only let the unit run long enough to overshoot and complete the autotune. I am going to install a heat sink similar those listed on the lightobject web site. My question is whether or not the heat sink also needs a small cooling fan. I am running a 1550 watt element on 120volt (~13 AMPS), so the relay is operating at roughly 50% of its rated capacity. Should the heat sink be alone to adequately protect my SSR? It is located in a 8" x 8" x 4" indoor electrical panel. Or do I need to provide some type of air flow through the panel. The unit will be operated outside in ambient temps ranging from 35 to 90 degrees.

[richiem]

Most definitely need a heat sink and it should be on the outside of the 8x8x4 enclosure. I would also recommend an SSR rated for at least 60 Amps, especially when your ambient temp is 90 deg. There should be a datasheet for the SSR showing its derating curve for current vs. ambient temp, and that will answer many questions. A small fan blowing over the heatsink would definitely help.

I use an 8x8x4 enclosure too, and I cut a slot in the side wall and slipped the fins of the heatsink nearest the SSR into the slot, so that the SSR is inside the enclosure but the fins are outside. This has worked well for me.

[garbear]

Thanks for the comments richiem! I think I will do the same as far as the heat sink/relay orientation. I'll relocate it to the rear of my control panel and let the fins protrude outside. I will go with a higher reated SSR and wire in a small A/C cooling fan to push air across the heat sink. I have attached a few pics of my smoker. It also has a reostat for controlling a small fan inside to circulate heat and smoke. I made this from scratch, except for the elctrical enclosure. Thanks again for the input/advice

[greatdaneroco]

YOUR SMOKER LOOKS GREAT I HAVE BEEN DOING THESE UNITS FOR ABOUT 7 YEARS NOW BUILDING THEM AND USEING THEM ON A LOT OF DEER AND SO MUCH OTHER USES I JUST NOW STARTED USING THE PID CONTROLLERS AND SETTING THEM UP ON THE ONE I AM USING NOW BUT I AM STILL WAITING FOR MY SSR RELAY TO GET HERE I AM NEEDING THE AC IN AND OUT RELAY USING A 40 AMP SSR I AM ONLY HEATING ONE ELEMENT UP WITH RELAY AND IT ONLY COMES ON A FEW TIMES ON TEMPERATURE IN A WHILE THE BOX HOLDS TEMP FOR A WHILE SO I DONT KNOW IF IT WILL NEED THE HEAT SINK CAUSE IT IS ON SUCH A SHORT TIME.

[richiem]

A heat sink is pretty cheap insurance.

[simpsonrl]

I will chime in for my 2 cents worth. You most likely will not need a fan. I built a heat treat furnace that operates on 240V and 4500 watts and gets to 2000 F with ease. I used an Opto22 240D45 SSR. If you go to their site they have nice data sheets which show the operating envelope of their devices. The 45 amp SSR which I am using only dissapates 0.9 watts of heat per amp. So at the full load 19 amps my oven draws, that is only 17 watts of heat. (think 17 watt light bulb). At the 90F ambient you are talking about, this SSR could run about 10 amps continuously with out a heat sink in still air. With a 6" X 6" flat plate as a heat sink in still air, it is about 32 amps. With a 12"X12" flat plate in still air it is 45 amps. Note that these are not heat sinks with lots of fins and surface area to dissapate the heat, just a flat plate. Also note that these are continouous loads. Even with my furnace at 2000 F, the SSR is only on about 50% of the time. So the average current is only 1/2 of the full load amps. My guess is that your smoker is the same, the SSR is running at a realatively low on time.

So to make a long story shorter I mounted my SSR on a small scrap of angle iron and have never had any problem. But you could mount it to the inside of the metal enclosure that you use for a control panel. You do want a few air holes for circulation. I have often taken a drill and drilled a few holes to create a screen.

If you are using the Chinese SSR's you probably can't get a datasheet with all of the heat info on it, but I would guess it would be very similar. In any case, there is no way you will need a fan, and heat sink needs will be very minimal.

If in doubt, put your finger on the back of the SSR and feel for the heat it develops. (don't get shocked by touching the main terminals) I am sure you will be surprised at how little there is.

[tward]

Thanks to everyone on this thread. I now better understand the Amp rating on SSRs.

simpsonrl pointed me to Opto22. I found the datasheet at http://www.opto22.com/site/documents/do ... x?aid=1677 and I've attached it to this post. Page 5 was the key learning point for me. It shows for SSRs with nominal ratings (amps) over a few amps that the nominal rating (amps) is only available if there is an attached heat sink, even at room temperature. It also shows how quicly the rating drops with increased temperature.

Without that information I was puzzled by richiem's reply that garbear should use "an SSR rated for at least 60 Amps, especially when your ambient temp is 90 deg" even though garbear seemed to be operating "at roughly 50%" of his 25A rating. Looking at the chart it seems garbear was really running above the rating for his conditions (no heat sink and 90F (32C)).

I've got a "Chinese SSR" - a FOTEK - and as simpsonrl notes, I can't find a datasheet for these. I guess you get what you pay for and I'll have to assume similar derate curves to the Opto22s.

Again, thanks to everyone for sharing the knowledge. I'll follow up with some questions.

[tward]

I'm trying to understand how to not kill an SSR. If anyone can help with these questions, I'd appreciate it.

1) If an SSR runs above its Thermal Rating (i.e. the amps are above the curve given the heat sink configuration and ambient temperature) what happens? Does the SSR just switch off due to overheating or is it damaged? Does it depend on how long it runs above the curve?

2) Related to the above, does the life of an SSR depend on how close to capacity it runs? In other words, would running amps at 80% of capacity "kill" the SSR faster than running it at 50%?

3) Or... does SSR life relate more to the on/off cycles?

Thanks for any help.

[richiem]

1) If an SSR runs above its Thermal Rating (i.e. the amps are above the curve given the heat sink configuration and ambient temperature) what happens? Does the SSR just switch off due to overheating or is it damaged? Does it depend on how long it runs above the curve?

SSRs typically either short or burn open -- the latter is better, since your heater won't stay on 100% but will shut off.

2) Related to the above, does the life of an SSR depend on how close to capacity it runs? In other words, would running amps at 80% of capacity "kill" the SSR faster than running it at 50%?

Generally it only depends on the temperature rise -- you can runn at 100% if you can get the heat out of the SSR. This is why I recommend using an SSR rated for at least 5X the current you expect to deliver, regardless of the duty cycle. And use a heat sink.

3) Or... does SSR life relate more to the on/off cycles?

Almost entirely heat related, although big voltage spikes on the AC line can kill any solid state device that isn't protected by some kind of surge limiter. Typical causes of big spikes are lightning strikes, wind shoving transmission lines together, etc. SSRs are fairly forgiving of current pulses, in the same way that diodes are, but everything has a limit.

[tward]

Thanks much for the helpful info.