Bed Overheating – SSR Relay Issue on Tractus T1250?

JJJJ

Hi everyone,

I was calibrating my Tractus T1250 and was just about to start a print when I noticed the bed was heating above the set temperature.

Issue Description:
I turned off the heater, but it kept heating.
It reached 90°C, so I powered everything off.
After cooling down, I turned it back on — same issue: bed kept heating uncontrollably.
I then unplugged the bed power terminals and powered it on again — the temperature started rising again.

Setup Details:
The bed is powered by a SSR-100 DD Relay.

The power supply plug splits into two:
One pair goes to the 24V input on the board.
The other pair powers the bed.

Bed Wiring:
Positive from PSU → directly to the bed.
Negative from bed → terminal 2 on SSR.
Terminal 1 on SSR → back to PSU negative.
Terminal 3 → positive bed terminal on the board.
Terminal 4 → negative bed terminal on the board.

Troubleshooting:
Continuity between terminals 1 and 2, but no continuity between 3 and 4.
I suspect the relay has failed and plan to order a new one.

Questions:
Has the relay failed?
Is this wiring correct? This was the stock setup, but it doesn’t feel safe.
Is there a failsafe I can add to prevent overheating and potential fire risk if the relay fails again?
What could cause the relay to fail?
Could it be due to lack of cooling or overheating during normal operation?

Thanks in advance for your help!

JJJJ

@JJJJ Just to add, the bed heats up the second its connected to power, even when the printer is turned off

achrn

Failure so the SSR is switched on is the most common SSR failure. See e.g. https://omronfs.omron.com/en_US/ecb/products/pdf/precautions_ssr.pdf "Short-circuit failures represent the main failure mode and can result in an inability to shut OFF the load" (but you probably don't need all the protection circuitry that note suggests, though presumably it does all serve a purpose).

When replacing it beware that most of the SSRs on ebay or aliexpress are fake. Buy your SSR from a reputable supplier.

Notepad

@JJJJ Hi there,

No matter what relay you use you will keep encountering this problem. The reason is this is a DC relay. and they get HOT very quickly.
SSR relays fail deadly when they get too hot, and they fail in a way unable to shut off the relay.
Have a look at W70 heatsinks. These are designed to be used with the relays passively, but I personally always add an active cooling fan as well.

Sometimes when a relay overheats it can permanently damage itself. If it resets it might be fine to still use. but personally I would replace the entire thing.
If you bed is also powerful enough to cause a fire hazard if the relay fails, you can always put two relays in series, so it would require both relays to fail before the bed becomes uncontrolled.
alternatively, you can add a thermal fuse onto the bed -which is what I do

Additionally, these relays CAN BE (but not always) directionally limited. Try plugging the PSU positive into the relay terminal 2, then the relay into the bed positive into Relay terminal 1.

JJJJ

@Notepad There is no cooling where the relay was, so sounds like a heat failure then.

I am going to order a new one but I can only find them on ebay, I found a heat sink and will install a fan on that as well.

I am also going to install a 128 degree c fuse. What's the best method to attach this to the bed ?

Would it be possible to use something like This?

or would this be worse?

Thank you for your help

jay_s_uk

@JJJJ you could also try an external mosfet
something like this if 30A is enough https://www.amazon.co.uk/Qiilu-Printer-Heating-Controller-Extruder/dp/B07CQ1FMQ3

Notepad

@JJJJ agreed, gut feel is death via overheating.

install a 128 degree c fuse

Feel free if its easier to install a 150C one. then you dont have to worry about it blowing early if you want to print 110C on the bed for PC-GF or other higher end materials.

That linked relay should be fine, however I personally use SSRs like you found, just make sure they are rated about 2x higher, and have a heatsink, then itll be fine.

dc42

@JJJJ said in Bed Overheating – SSR Relay Issue on Tractus T1250?:

I am going to order a new one but I can only find them on ebay, I found a heat sink and will install a fan on that as well.

I highly recommend that you avoid all DC-DC SSRs on eBay and buy an Auber Instruments one instead. See https://www.auberins.com/index.php?main_page=product_info&products_id=860.

Notepad

@dc42 I also agree with this choice.

@JJJJ Most of the ones on ebay / amazon are fake.
Either buy from RS-components. Direct from factory. Or the link dc42. I use GOLDSSR brand, but its very hard to get them in the country. Most good relays cost over £15.