Heatsink question

**Fireman73** · 10-01-2009, 12:55 AM

Anyone know where to buy a good heatsink for a 60a SSR?

**SCOTTRODS** · 10-01-2009, 07:29 AM

Originally Posted by Fireman73

Anyone know where to buy a good heatsink for a 60a SSR?

Someone posted a reply to somehing related to this recently. They were saying to look on Craigslist for one of those Water coolers foir the home/office, that have been put up for free. They have a nice big heatsink in them that can be used as-is, or cut down to size. The cooler is a curbside donor after that...... If it has the heatsink, and you can find one,..... Free is good. (FIG)

There may be other items, such as over sized computer CPU heat sinks too,..... local computer wiz ought to have some great ideas if you carry an SSR to him and show him what you need.

**ed_denu** · 10-01-2009, 09:12 AM

Computer heat sinks with fan are probably the most cost effective. Only downside is you need to add a small DC transformer to power the fan.

**CarWiz** · 10-01-2009, 05:45 PM

HE-54 Crydom Relay Sockets & Hardware - $12.59
CRYDOM|HE-54|Relay Heat Sink | Newark.com - $13.89

Lots more. Google "Crydom HE-54" but Mouser is the cheapest I've found. They show a silver heat sink but the ones I received were yellow chromated aluminum. The HE-54 is listed for up to 50A but the spec sheet shows it will handle a lot more. If your box is vented, it should work well. A fan will aid in cooling.

**FigureLLC** · 10-02-2009, 04:28 PM

is there a distinct advantage to an ssr? i was going to use one in last oven i built, but after getting to the point where you are now i didn't understand why i should spend more money on an ssr, have to track down a heatsink, and worry about space and airflow, etc. in my control box when i could just use an inexpensive contactor. plus, i like that it click-clacks, because i know the oven's up to temp when it starts clacking without ever looking at it.

**philipd** · 10-02-2009, 07:09 PM

Yes, there is an advantage in using an SSR if you are using a PID to control the temperature. When a regular thermostat is used to control the temperature the heating element will start to heat when the thermostat tells it to. When the temperature reaches the set point, say 450�, the thermostat, via a contactor, turns the element off. However, even though the element is off the temperature will still rise a couple of degrees due to thermal holdover. Then, depending on the temperature differential of the thermostat, the element will come back on when the temperature of the oven falls to another set point. That may be 445�, 440�, 435� or even lower. Now let's say it take about 10 minutes for the temperature to fall from the high shut off point to the low turn on point. This means the coil of the contactor will energize about six times an hour or 48 times in an 8 hour period.

Now, a PID does pretty much the same thing. However, it attempts to hit the 450� (or whatever temperature you have it set to) mark and stay as close to it as it can without going over. Sorta like driving your car. You get up to the speed limit but you don't want to get a ticket so you stay as close as you can without speeding. The PID does this by cycling the heating element MUCH more frequently than a thermostat. I'm not sure how many times it may cycle each minute but it is quite a bit more than a thermostat. It could be several hundred in an hour. And this is where a SSR holds an advantage. Because it is 100% electronic and has no moving parts it will last MUCH longer in this situation.

So a contactor will cost less but you will have more down time for replacement; though I admit it will not take much time to change out a contactor.

**ed_denu** · 10-02-2009, 08:22 PM

There are lot's of people using both. An SSR has the advantages Philipd pointed out. That is they are by design able to switch multiple times a second and can switch multi thousands of cycles without failure. One disadvantage is without a proper heatsink their life expectancy will be short. Another downside is they are usually more expensive than a contactor.

A contactor on the other hand is affordable and doesn't require a heatsink. The downside is that they cannot switch as frequently as an ssr without failure, but this can be overcome somewhat by setting the PID parameter "OT" to a higher value thus slowing down the switching cycle. The constant click-clack of the mechanical relay is either annoying or comforting, depending on your viewpoint.

Either will work, I view it more as a personal preference than anything. With the proper pid parameter settings either should be capable of holding an oven set temperature. BTW I use an SSR.

**SCOTTRODS** · 10-02-2009, 09:30 PM

I also believe this is personal preference. There's a couple more items to consider, but I'll stick to the one.

Typically when a Contactor fails, it fails in the "open" position, disabling whatever it is controlling, or supplying.

The SSR is as likely to fail "closed" as it is open, this putting continuous power to whatever it is controlling, or supplying, if it fails in theat "closed" position.

Some like the "failsafe" factor a Contactor would more likely provide, over an SSR's propensity to fail as described.

Some like the SSR anyway, and will usually put an alarm on the temp controller of an oven, to warn them of the over temp that would be likely in this case.