# Solidstate Relay's For Muffle/heat Treating - Why?



## countryguy (Jun 5, 2015)

I notice that most everyone sells or uses Solid State relay's in their Heat Treat DIY furnace/oven projects.  I have an AB  relay / contactor w/ 4 NO contacts .   It is Heavy Duty and will take the 240Vac 24 amps.  I'd like to use it but thought I would ask first if the Solid State relay's are used for any particular reason?  Anyone know? 

Here is my rig & posts (2 of them) on the HT thread from Ray C. 
http://www.hobby-machinist.com/threads/heat-treat-oven.9929/page-4#post-304160


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## fixit (Jun 5, 2015)

countryguy said:


> I notice that most everyone sells or uses Solid State relay's in their Heat Treat DIY furnace/oven projects.  I have an AB  relay / contactor w/ 4 NO contacts .   It is Heavy Duty and will take the 240Vac 24 amps.  I'd like to use it but thought I would ask first if the Solid State relay's are used for any particular reason?  Anyone know?
> 
> Here is my rig & posts (2 of them) on the HT thread from Ray C.
> http://www.hobby-machinist.com/threads/heat-treat-oven.9929/page-4#post-304160




It will work just fine, It's mechanical & will wear out, contacts will burn. BUT you have it on hand NOW use it !
fixit


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## countryguy (Jun 5, 2015)

Sweet!! Thanks.


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## RJSakowski (Jun 5, 2015)

One reason to use solid state is that unlike motor control, a heating circuit is constantly switching on and off in maintaining a set temperature.  The number of on/off cycles are what determine the life of a relay or contactor.  Every one of them erodes the contact surface slightly.  This is particularly true for high current applications as are normally found in heating circuits.
One failure mode is to weld the contacts in an on position which could have disastrous consequences for an unattended oven.  Two commercial (low temp.) ovens that I have each have an over-temperature safety setting in the event that their mechanical contacts fail.


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## brino (Jun 5, 2015)

Hey countryguy, 

I agree with all the responses above, but just wanted to point out one more difference......

The contactor (being a heavy mechanical relay) will be more robust for power surge and lightning events.
The solid-state relays likely won't be as forgiving, due to their internal control electronics.
Just another factor to consider.

-brino


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## ARKnack (Jun 5, 2015)

Use it. Just make sure you have a high temperature trip out switch that doesn't rely on that relay. You want it to be independent in case the contacts weld shut. You should also use one if you use a solid state relay.


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## countryguy (Jun 5, 2015)

Thanks guys.  This has been another really fun project!   Learned a ton yet again.  When you note a thermal cutout, I am familiar with those on a furnace or dryer.  A couple cutout fuses would make this easy.  The site below has a bevy of options.      Would I mount this on the relay?  Inline on the AC .  Not sure where I should pick to temp check? 

As an aside:I see SS relays from automation direct are really not too badly priced!   $23 for 40a SS hockey puck 

Temp interlocks :   https://www.johnstonesupply.com/sto...s/c1027991-c1026844-c1028174-c1027315-p1.html

Automation direct AC and DC SS relay list:  http://www.automationdirect.com/adc...-_75A_(AD-SSR6_Series)#sort=price+asc&start=0


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