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Old 03-15-2011, 08:58 PM   #1
sambogi76
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Default heat sink question

I am getting ready to build my enclosure for my control panel on my HERMS. The way my heat sink is made im going to have to mount this thing inside, so I think I'm going to have to put a cooling fan on my enclosure. My question is how hot does your ssr heat sinks get? My element is a 5500W. And I have a little 12V fan.

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Old 03-15-2011, 09:41 PM   #2
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You are going to want an inlet and outlet fan for the box. Just blowing air in probably will not create enough air movement to keep the heat sync cool.

Here is what I did for my portable RIMS project.

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Old 03-15-2011, 11:09 PM   #3
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Quote:
Originally Posted by sambogi76 View Post
I am getting ready to build my enclosure for my control panel on my HERMS. The way my heat sink is made im going to have to mount this thing inside, so I think I'm going to have to put a cooling fan on my enclosure. My question is how hot does your ssr heat sinks get? My element is a 5500W. And I have a little 12V fan.
I am in the same boat. Here's what I will do. I will keep a backup plan in mind, but I will not install a fan nor a vent hole in the enclosure. I will measure the temperature just below the heat sink to see what its ambient temperature is during use. Then, if necessary, I will install a fan inside the enclosure. My feeling is that the need for cooling the heat sink has been exaggerated. Did you see Ed's post? He used a really small enclosure, still no significant heat buildup. As he stated, the SSR can operate up to 70°C so it is tougher than one may think.

The heat sink will operate at optimum effectiveness if you install it with the fins vertical. As the air is heated between the fins, it can then rise straight up out of the heat sink without obstruction. This sets up a natural convection flow that helps keep the sink cool. Also keeping the heat sink towards the lower end of the enclosure improves the available cool air supply.

Unfortunately, the thermal resistance of the common SSR heat sinks is not available. If it was, it would be a simple matter to compute the temperature increase with a given heat sink. If anyone has found this data I'd appreciate it if you could pass it along. I am thinking in particular of the two SSR heat sinks offered by Auber Instruments, or similar.
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Old 03-15-2011, 11:09 PM   #4
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Wow....that's the Knight Rider of toolbox controllers!

It depends on your build. I have a very dense but low profile heat sink on my box, and it barely gets warm, even after 100% power for 30 mins....

It might be cheaper to get a nice heatsink for $25 than worry about buying fans and DC power supplies to make them work. You don't need a MASSIVE heatsink, just something better than the little guys you see supplied with SSR's, with enough surface area to mount 4 or so relays. Fans can be distracting and compromise the "splash proofness" of the box, GOD FORBID something happen. I keep my box 4 feet away from the rig, just in case

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Old 03-15-2011, 11:25 PM   #5
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Why not just mount on the heatsink on the outside. If you are using a metal toolbox but sandwhich the heatsink/ssr. Make sure to use a high quality thermal paste. I used an old Pentium II heatsink for mine.

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Old 03-15-2011, 11:48 PM   #6
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Wow....that's the Knight Rider of toolbox controllers!

It depends on your build. I have a very dense but low profile heat sink on my box, and it barely gets warm, even after 100% power for 30 mins....

It might be cheaper to get a nice heatsink for $25 than worry about buying fans and DC power supplies to make them work. You don't need a MASSIVE heatsink, just something better than the little guys you see supplied with SSR's, with enough surface area to mount 4 or so relays. Fans can be distracting and compromise the "splash proofness" of the box, GOD FORBID something happen. I keep my box 4 feet away from the rig, just in case
I have a nice big heatsink, I just wasn't sure about stuffing it in a closed box without any air flow. I'm sure if I had it on the outside it would be fine. What size element are you driving with your ssr?

Squeeky: Good idea but mine isn't made of metal, the material I'm using would insulate the heat.

Tasq: Very nice control box! Maybe I will try just drilling a 3/4 inch hole on side near the bottom (where my ssr will be) and another on the oposite side near the top to allow for natural air flow if the temp gets too hot after I run my first test.
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Old 03-16-2011, 01:36 AM   #7
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Here's a little more data. If we use one SSR to control a 5500W/240V element, then 27.5W of heat will be dissipated by the SSR. That's when the PID is running the heater at 100% power. If at 50% then the heat dissipation drops accordingly, to 13.75W. This is not a whole lot. If your control box is very crowded and there is no room for air flow, then an external heat sink may be necessary, it depends.

For those that prefer two SSRs per load then you will have twice the heat dissipation to deal with.

I based this calculation on a 1.2V effective voltage drop across the SSR, as suggested by this paper on SSR Thermal Considerations.

If we now had the thermal resistance of the heat sinks in question we could calculate what the maximum temperature inside the panel is allowed to be. My prediction is that it will be quite high, say 50°C, depending on the heat sink. Of course we do not want to run it at the limit, but knowing the limit allows us to apply a safety margin.

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Old 03-16-2011, 01:43 AM   #8
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Tasq: Very nice control box! Maybe I will try just drilling a 3/4 inch hole on side near the bottom (where my ssr will be) and another on the oposite side near the top to allow for natural air flow if the temp gets too hot after I run my first test.
I think that will work. I overkill everything :P
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Old 03-16-2011, 03:51 AM   #9
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Here's a little more data. If we use one SSR to control a 5500W/240V element, then 27.5W of heat will be dissipated by the SSR. That's when the PID is running the heater at 100% power. If at 50% then the heat dissipation drops accordingly, to 13.75W. This is not a whole lot. If your control box is very crowded and there is no room for air flow, then an external heat sink may be necessary, it depends.

For those that prefer two SSRs per load then you will have twice the heat dissipation to deal with.

I based this calculation on a 1.2V effective voltage drop across the SSR, as suggested by this paper on SSR Thermal Considerations.

If we now had the thermal resistance of the heat sinks in question we could calculate what the maximum temperature inside the panel is allowed to be. My prediction is that it will be quite high, say 50°C, depending on the heat sink. Of course we do not want to run it at the limit, but knowing the limit allows us to apply a safety margin.
Thanks this helps. My control box isn't going to be crowded at all. I will just wait and see how hot it gets inside when I run my first test.
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