Someone smarter than me…....Please Help
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So the way you have it wired looks correct.
But.. I'd ask a couple of questions:
- why use the SSR? How much power is your bed capable of, or perhaps easier measure the resistance of your bed heater, for example at 24v a heater with 1.92ohms resistance produces 300w and draws 12.5amps. This is well within the Duet's capabilities to connect the bed directly to the duet board. They can handle 18A, which means a maximum wattage at 24v of 432w.
- Consider an external mosfet board, they are cheap and offer very low voltage drop, compared to some DC-DC SSR's.
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The fact that you are using an SSR to drive a DC load caught my eye.
While there are DC SSRs, there are also AC SSR's and one cannot drive the other kind of load… Something to be mindful of. For DC I would use a MOSFET. DJ said it well^.
As for everything else, I found it difficult to read. I would recommend you provide pics of your setup and wiring, and convert your block of text into a bullet list of concerns, then we can work thru them one by one. In general, the first step would be to disconnect everything from the board and get it talking over USB and the Panel displaying proper things.
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Your schematic looks ok except that you need to swap the + and - output connections on the SSR. The way you have it connected now, the mosfet body diode will conduct, the bed heater will heat up, and the SSR will also get very hot. I hope you haven't overheated it.
The Auber SSR is a good choice.
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Thanks all, @dc42, No I only had the printer on for less than a minute twice…...So I take it the way I have it explains why the heater is constantly on at power up?
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In my work we use SCRs to control large heaters - our systems will typically have 10-20 SCRs and draw 200amps at 240V. Heating 20 ton equipment to 700F.
SCRs are GREAT, BUT they can fail "ON". Be sure you have a Thermal Limit Safety Switch to interrupt power if this happens. I am sure most any kind of contactor can fail on. So, everyone should have a safety on their bed heater unless you KNOW that your heater will not get too hot when left running wide open.
I've had one house burn down (not related to 3D printing). I am not going through that again!
I use a Bradford White Reset Thermal Switch 239-43676-01 (you can stick that in google and find it). It is meant for a water heater. I close it for easy mounting. But there are lots of options.
Mine is mounted about 50mm from the heater. For $10, it is worth the peace of mind.
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Thankyou Frank 833, I will look into that as that bed heater gets pretty hot…...pretty quick
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@Frank833 I just ordered one and question, does it go inline with one of the cables going to heat bed or does it go across both wires that go to the heatbed ? Thanks
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SCRs are good for controlling AC devices, but not very suited to controlling DC devices such as your 24V bed heater because you need to take special actions to turn them off. They also have a relatively high voltage drop. So stick with your Auber DC-DC SSR.
DC-AC SSRs generally use a triac, which is a close relative of the SCR.
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@dc42 I want to use it for safety reasons as Frank833 stated above. I ams using the Auber SSR and using the Bradford White Reset Thermal Switch to shut off the power to the Heat bed. Will this not work effectively?
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Yes a thermal switch is a good safety feature. Make sure it can carry the required current.
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@Frank833 I just ordered one and question, does it go inline with one of the cables going to heat bed or does it go across both wires that go to the heatbed ? Thanks
It goes in-line in your + or 'load' (if mains). So put it between the SSR, SCR, Relay or Duet (whatever is powering your heater) and the heater.
It is just a dumb switch - it just disconnects the circuit if it gets too hot. Press the button to reset it. They sometime 'break' if they trip (fail to reset).
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You should get something displayed on the TFT panel even with the tx and rx wires not connected, unless the firmware has been erased. You could try re-installing the firmware on the PanelDue.
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David, he is getting backlight only. I recall that if Tx and Rx lines are reversed (I did that once years ago!) that was the behavior. When you say "something displayed" what should one see?
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If the Tx and Rx lines are reversed, I would not expect to get backlight only - I would expect to get a normal screen but no status to be received. I am surprised if you saw something different.
