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Old 02-20-2011, 05:42 PM   #1
jfenton78
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Default SSR heatsinks needed for metal enclosure?

I'm going to mount 4 SSR's in a 16x16x4" or 6" metal enclosure and I was wondering if the SSR's still needed a heatsink? I know it's best practice, but what's reality? If I mount the SSR's directly, they will be mounted to a mounting plate in the enclosure. Each SSR will control the hot legs of a 220V 4500W or 5500W heating element for my boil kettle or HLT.

Alternatively if the SSR's still need a heatsink, can I mount to the mounting plate and use a fan to circulate cool air?

Jeff


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Old 02-20-2011, 06:28 PM   #2
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Yes, you need sinks. Fan will work as long as you set up a flow so that air is moved out of the box.


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Old 02-21-2011, 07:58 PM   #3
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I will try to explain this in laymens terms. A typical voltage drop across an SSR while in the On state is about 2 volts. If we calculate the power dissipated by the SSR at a 30 Amp draw each SSR will be dissipating about 60 watts. With four SSR's, the worst case of having all four in the on state at once, your enclosure would have to dissipate 240 watts. That isn't happening using just your enclosure. With a heatsink it's all about surface area. Thats why heatsinks have all those fins, to increase the surface area. Your panel does not have enough surface area to dissipate the heat from 4 SSR's. The fan will not help because the fan will cool only a small portion of the surface area of your panel. Your SSR's will have Thermal Derate Specification. A Crydom 30 Amp SSR for example will pass the full 30 Amps when the device's temperature is at 40*c but if the temperature is allowed to rise to 80*c because of inadequate heat sinking the device will allow only 12 Amps to flow through the device. Keeping the device cool is crucial if you want the maximum current to flow to your load.
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Old 02-21-2011, 11:25 PM   #4
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Quote:
Originally Posted by Sawdustguy View Post
I will try to explain this in laymens terms. A typical voltage drop across an SSR while in the On state is about 2 volts. If we calculate the power dissipated by the SSR at a 30 Amp draw each SSR will be dissipating about 60 watts. With four SSR's, the worst case of having all four in the on state at once, your enclosure would have to dissipate 240 watts. That isn't happening using just your enclosure. With a heatsink it's all about surface area. Thats why heatsinks have all those fins, to increase the surface area. Your panel does not have enough surface area to dissipate the heat from 4 SSR's. The fan will not help because the fan will cool only a small portion of the surface area of your panel. Your SSR's will have Thermal Derate Specification. A Crydom 30 Amp SSR for example will pass the full 30 Amps when the device's temperature is at 40*c but if the temperature is allowed to rise to 80*c because of inadequate heat sinking the device will allow only 12 Amps to flow through the device. Keeping the device cool is crucial if you want the maximum current to flow to your load.
Great explanation!
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Old 02-21-2011, 11:33 PM   #5
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Originally Posted by Sawdustguy View Post
The fan will not help because the fan will cool only a small portion of the surface area of your panel.

The fan referenced by the OP is in combination with Heat sinks inside the control panel. Many systems are done that way with success, mine being one of them.

Alternatively if the SSR's still need a heatsink, can I mount to the mounting plate and use a fan to circulate cool air?
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Old 02-22-2011, 12:56 PM   #6
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"I will try to explain this in laymens terms..."

Thanks for the explanation. I love the above "explain this in laymen terms." Reminds me of the Three Stooges episode where Moe says that he'll explain it so that even an idiot can understand it.

Not that I'm an idiot...I think...but still a great explanation.

Jeff
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Old 02-22-2011, 01:12 PM   #7
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Quote:
Originally Posted by samc View Post
The fan referenced by the OP is in combination with Heat sinks inside the control panel. Many systems are done that way with success, mine being one of them.

Alternatively if the SSR's still need a heatsink, can I mount to the mounting plate and use a fan to circulate cool air?
Sorry Sam, I missed that. You are right.
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Old 03-03-2011, 02:24 PM   #8
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You sure you need 4 SSR's? Most of the wiring diagrams I see you only need one SSR per heating element, and only put it on one of the hot legs.
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Old 03-03-2011, 02:30 PM   #9
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You sure you need 4 SSR's? Most of the wiring diagrams I see you only need one SSR per heating element, and only put it on one of the hot legs.
Yeah, that works, but is NOT a great way to do it. It's far better to totally cut power to the element. This also cuts power to the cord, which is probably where you'll see a fault if one happens some day.
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Old 03-03-2011, 03:01 PM   #10
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I added a fan to my box after believing that a heatsink mounted to a metal backplane would dissipate the heat enough. After one test run to boil I burned up my 40a SSR (and they do fail in "closed" state) I mounted a computer fan to one side and cut slots in the other side for exhausting airflow. No problem since and lesson learned.


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