SSR heatsinks needed for metal enclosure?

Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

jfenton78

Well-Known Member
Joined
Aug 11, 2008
Messages
89
Reaction score
10
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
 
Yes, you need sinks. Fan will work as long as you set up a flow so that air is moved out of the box.
 
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.
 
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!
 
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?
 
"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
 
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.
 
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.
 
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.
 
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.
 
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.

I'll speak for the other camp here. I wouldn't rely on two SSRs to completely cut power to the element.

I use one SSR for my element for control purposes. If I want to turn the power off, I do manually do that with a BAS (big ass switch).
 
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.

So far, I've not had any problems with one SSR and no ventilation (5500W element w/ 40A SSR and a headsink, connected to the backpanel). The SSR gets warm, but I don't think the temp inside the panel is too bad.

But, just because I can, I am going to add a couple tiny fans to the box. One blowing air in and one blowing air out.
 
Walker I thought the same thing when I was designing mine and poof the second test I ran the element was running uncontrollably "on" Checked the SSR with a lamp a sure enough no worky. Could have been a bad SSR out of the box but I had a pc fan laying around and used the guts of a walwort for DC power I also wired in a 120v panel mount bulb to the output so that I can see when the element is on and off that way if it fails again I can quickly look at the PID and SSR lamp to compare output.
 
I'll speak for the other camp here. I wouldn't rely on two SSRs to completely cut power to the element.

I use one SSR for my element for control purposes. If I want to turn the power off, I do manually do that with a BAS (big ass switch).

Can't hurt to have two. I look at it this way - if one SSR fails I can still use the system with the other one.
 
I'll speak for the other camp here. I wouldn't rely on two SSRs to completely cut power to the element.

I use one SSR for my element for control purposes. If I want to turn the power off, I do manually do that with a BAS (big ass switch).

Well, the camp I actually represent is to use
  • 2 SSR's to interrupt power to the element, then a
  • 240V medium switch on my control panel (MAS) to cut just the element power, then a
  • big switch on the wall (BAS) to cut power to the the whole panel, including the 110 (from one leg of 240)
 
Walker I thought the same thing when I was designing mine and poof the second test I ran the element was running uncontrollably "on" Checked the SSR with a lamp a sure enough no worky.

LOL. I put a little 120v lamp on my panel, connected between the SSR output and the neutral to let me know when the SSR was allowing current to flow through.

Unfortunately, the SSR leaks enough current even when OFF than the little bulb is constantly illuminated. Confused the **** out of me when I first put the bulb in the control panel. :D
 
Back
Top