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heated bed relais failure

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Hi people

I am currently dealing with the worst print failure i ever encountered.

I use a heated print bed from german http://www.2printbeta.de/ which works fine.

I am switching it off and on using a relais with a 24V and ??A rating.

Last night before leaving i started a print of a huge ~350g Object. Calibration was okay and when i checked about one hour later the first few layers were looking neat.

When I came back today at noon i was greeted by the stench of ozone, a huge pile of severly burnt ABS and coal-black (formerly blue) tape.

I am still working on getting that stuff off my aluminium bed.

I seems that at some point the relais that switches the printbed got stuck in 'on'-position which led the bed to overheat to 200°C (it is rated for 110°C, but still seems to work...).

At first i was quite shocked and checked for any damages on my UM but everything seems okay. The belts, ABS printhead, ABS fanduct... everything seems to have survived without much damage.


Have you ever had the case of a stuck relais and overheating print bed?

Are you using solid-state-relay (SSR)?

And if you are:

Are you using PID-stabilisation for the heated bed als well? Is that an advantage

To prevent another disaster like this (it cost me about 150 grams of ABS and i only have half a spool left) i choose to turn to switching the power supply on and of instead of the heatbed. My 'electronics-guy' has told me that the supply can take it because i use a very good one that he took from an old scrap server, so it should be fine.

But for the future i would use an SSR and enable PID-stabilisation if anyone here tells me that this is better than the current stabilisation.


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Wow. Please post photo!


i choose to turn to switching the power supply on and of instead of the heatbed


That's what I did. My heated bed power supply is from a computer and I just use the digital pin that turns it on and off to turn it on and off. Instead of switching the high current of the power itself. My power supply hasn't died yet.

PID stabilization makes no sense with my heated bed. A simple over temp off, under temp on is all that I need. Like a house thermostat.


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A heated bed has normally a by far larger thermal mass (and therefore thermal inertia) than a hotend and also the stability and accuracy of the temperature is far less critical, so a simple two point regulation is totally adequate, i think.

As for the switching setup, i would go for a rather simple MOSFET design (a MOSFET, a z-diode and two resistors). Actually i'm just in the process of building my own heated bed addon for my UM original. SSRs are also ok, but it's often easier and cheaper to find a good High-Power n-type MOSFET with a very low on-resistance (minimizes power- and heat-dissipation).

Best regards



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I used an auto relay. I have had it for over a year with no problems. I like it because it was simple to wire up and doesn't get hot. Some people are concerned because they click but it's hardly heard over the noise of the machine and is an indication that it is working besides. I'd go the auto relay again.


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