Trouble with SSR @ D8: Heatbed never turning off

Hi there,

I've connected a 24V PSU controlled by RAMPS 1.4 (D8) via SSR to heat the printbed.
While heating up works (kinda) it seems that the Marlin firmware doesn't turn off D8 after reaching target temperature - nor does it if the target temperature has been decreased (for later layer).
The SSR keeps permanently routing power to the heatbed which temperature gets higher and higher, causing warping and so on.

I took the heatbed off (Pronterface) and measured still around 4V on D8.. this may be a side effect by the powered SSR.. and may be this causes the heatbed to 'overheat'?
Its a chinese SSR, input: 3-32V , output: 5-110V single phase DC/DC - well cooled by a CPU cooler

Any help?
The uncontrollable heatbed causes warping and makes it impossible to print larger parts :/

thanks!

Sounds odd - the output on D8 should be either on or off - 12v or 0v. and 4v would indeed keep power gated to the bed. Have you checked that voltage with the SSR removed from D8? The left pin on the Mosfet should read 5v when its enabled (outputting 12v) and 0v when its not enabled, i.e switched off.

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Waitaki 3D Printer

I have a similar problem. Did you find out what it was? and is it possible that a faulty/burned mosfet can be the cause of that?

I will have a look this saturday and report. I expect it to be a problem with the SSR or the kind Marlins PID is working..

Well, it was a wiring-problem. I swapped the output connectors at the SSR (PSU and Heatbed) and now it works.

I remember that it didn't make sense to me connecting the 24V + line to the negative SSR connector. Because that resulted in only 20V at the heatbed, i swapped it which gave me 24V