Controlling my heated bed and SSR from DuetWifi
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since it has just affected me I would make a warning that if the SSD relay fails and short circuits then the heated bed supply is switched on 100% and not controllable by the Duet. Yesterday my SSD failed and I found the 300mm diameter hotbed heated by my 240v supply to "240 degrees" (maybe it was more than that depending on the thermistor accuracy ). The Duet recognised it as a heating error (temp on the PanelDue was in purple) and no voltage was present on the SSD input. A new SSD fixed the problem, but I am going to put a further safety switch off system into place.
It's unusual for DC-AC SSRs to fail when driving resistive loads, but when they do they fail short circuit as you say. What make/model was it?
I recommend using one of the following two approaches to protect against failure in the heating control system:
1. Passive protection. Choose the bed heater power such that if the bed heater is left full on, the temperature doesn't rise above a safe limit, e.g. 150C. Most silicone heaters can withstand 200C; although the glue holding them to the bed plate might soften first, so the cork os other insulation below the heater should be capable of supporting it.
2. Mount a thermal cutout on the bed plate and connect it in series with the heater.
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It measures 24.1v at the bed so that’s good. The resistance is 1.6ohm. The pad itself is 300x300mm. The supplier tells me it’s natural to get a wattage loss, I agree but it’s surely shouldn’t be so huge.
Could it also be a PID setting? I can’t get the auto tune to work. It says it can’t reach specified temperature, which I set at 90deg. It can as I have used it at 95 but maybe it’s a time issue?
Run M307 H0 with no other parameters, and check that it isn't using a maximum PWM below 1.
Ar 24.1V your bed heater should produce 363W power when it is cold. If it is a PCB bed heater then the power will reduce as it heats up. If it's a nichrome/silicone one then the reduction in power when hot won't be significant.
How large is the bed? I recommend a heating power of around 0.4W per square cm, although you can get away with 0.25. Insulation below the bed will help, if you don't have that already.
The actual bed size is around 340x 320 so it works out around 0.3w? If my math is correct?
It is indeed rated at 360w so it seems to be matching up with what you’ve said I just can’t get the temp out of it. When the current print finished I will do what you’ve said and run the tuning with no parameters.
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It's not tuning you need to run, it's M307 H0 which just reports the heater parameters:
10:53:41
M307 H0
Heater 0 model: gain 90.0, time constant 700.0, dead time 10.0, max PWM 1.00, calibration voltage 0.0, mode bang-bang, inverted no, frequency defaultSo it's safe to do while printing.
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Gain 141 time constant 620.4, dead time 0.7, max PWM1.00 mode PID
So I ran the bed tuning at 75deg. Put a little tin foil beneath the bed. Now I just managed to reach 130deg!
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SSD that failed was bought from ebay:
Solid State Relay Module SSR-25DA 25A /250V 3-32V DC Input 24-380VAC Output GT
"futek" brand
( 182110092170 goodgadgetsa )
Planning to put Thermal cutout under the heatbed, in series with the 240V supply to the bed.
The cork insulation worked well and worthwhile. Limiting the maximal temperature by choosing appropriate heater wattage sounds wise to me but is only known after it is fitted. -
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If anyone here is interested, I have a pile of AC switching SSRs that were pulled from retired aerospace repair equipment, that I would be more than happy to share for the cost of postage.
Omron, Crydom, and I think one other brand, all rated for 30A or better.
No intent to profit on this, just thought to offer as I've seen the discussion about fake Fotek SSRs a couple of times now.
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Hey_Allen.
Iwas just fixing to order one.I would love to get one from you. I am sure they are probably better than what I could buy.
Tell me how you would like to arrange this. -
I was on here looking for an answer as to why my T0 heater will not cut off when it reaches the set temperature. I have ran the Auto Tuning and put in the PID's. Played around with them for a couple of hours and I still get voltage to the heater after it reaches set desired set point.
Any Ideas?
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When using PID control, the heater power will not be cut off when it reaches the set temperature. Instead it will reduce the PWM to a suitable value to maintain that temperature. You may get a small amount of temperature overshoot after it reaches the set temperature, but it should be less than 5C if the heater is well-tuned.
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I had it set to 180 and everything I try keeps it going till it faults. Its like its heating up faster than the sensor reads
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Still fighting with this heater. Either I get it to heat up right and then it cools down to fast when extruding or it heats up way to fast and goes into fault for being too hot. Tried the Auto Calibration again and it went to 303 degrees. Then it failed. Got this message:
Warning: Auto tune of heater 1 failed due to bad curve fit (G=2140.6, tc=141.1, td=5.9)
M301 H1 P.4 I.006 D.25 T0 S0.3
M305 P0 T100000 B4138 C0 R4700 ; Set thermistor + ADC parameters for heater 0
M143 H0 S120 ; Set temperature limit for heater 0 to 120C
M305 P1 T100000 B4138 C0 R4700 ; Set thermistor + ADC parameters for heater 1
M143 H1 S280 ; Set temperature limit for heater 1 to 280C
M305 P2 T100000 B4138 C0 R4700 ; Set thermistor + ADC parameters for heater 2
M143 H2 S28024 volt heater in a flexion hot end. Sensor is working correctly. Just to make sure, I tried a spare.
Any advice would be appreciated.
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I suspect you are using a 12V heater cartridge with 24V power. The gain is far too high.
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I have two that are supposed to be 24v. I will try the other one.
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If you have a multimeter, I suggest you measure the resistance of the heater cartridge. See step 9 at https://e3d-online.dozuki.com/Guide/V6+Assembly/6?lang=en.
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I measured the resistance and sure enough, 4.4 Ohms on one and 2.9 on the other
