Multi heater bedquestions
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@Tpmoses , sounds like you got things pretty well covered. As far as power is concerned, you really have limited choices - 240V or 120V. You could put heavy duty 120V circuits in that van handle 30A and run two heaters off each circuit but in my mind it doesn't make sense to run 120V for those power levels. The choice is between running 4 dedicated circuits of 120V or running two dedicated circuits of 240V. You could also run a single dedicated circuit of 240V 30 amp.
As I explained before, you do not have the option of running two pads in series because if you run the series/parallel configuration you would have 1300W total heating power.
I guess at this point it would make sense to ask how much wattage you had on the old heat pad .... that and the previous heat up time will tell you if 1300W will do the trick. -
@jens55 From what I can find the previous bed mat was a 220v 2000w, being run off of 120v, so 1000W. I can't be 100% sure because there are no markings on it.
I have been seeing that generally, 0.6W/cm^2 is the ideal when calculating what you need, so if y bed is 110cm^2 1300 should be plenty? let me know what you think.
Thanks!
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@Tpmoses, as previously noted if you apply half the voltage you get 1/4 of the power so if it was indeed a 2000W heater, it only supplied 500W if run at 120V.
If it was hooked up that way and you got sufficient heat in the bed (after how many hours of heating) then you could run two parallel strings of two series heaters on one SSR on 120V.
It is not a configuration that I would recommend.
Yes, 1300W would likely work if you are very patient. -
Just as a reference I run a single 120V pad on my 500x500 heat bed. I believe it to be rated at around 1500W plus or minus 200W. It gives me good response. I would not be happy with 1/4 of that power. I think my 300x300 printer is 700W.
If we assume 0.6W per cm^2 and your bed is 110cm x 110 cm then according to my math you should be providing 7260W for that size of a bed.
I must admit that this seems high so maybe I messed something up ..... but it does suggest that 1300W will result in verrrry long warm up times and you might not reach the bed temperature you desire based on insulation around the printer. If it is a well insulated unit you will get enough heat but with long warm-up time. If it is not well insulated you might never get what you want. -
@Tpmoses and @jens55
To mention all options: there is also the option to use star-delta conversion. (at least in Germany)
I think we all agree, that you need a 120V 3-phase outlet at least.
You can connect to eg. L1 and L2 instead of L1 and N and get 1.7 x 120V = 206V
Connect two mats in series and use L2 and L3 for the other two mats.
I wouldn't be too concerned about driving them in serial, you could read their resistance with a multimeter and find two pairs that match best. The big chunk of metal will spread the heat evenly over time.About the thermistors, you can wire them anyway you like. For RRF it doesn't make a difference as long as you specify them correctly in the config.
But if you are concerned about uneven heating, you can specify four independent thermistors. One for the heater object model, the others as your reference. -
Update, after talking to my boss, I'm switching the plan. We instead are going to go with four 1000W heaters, run them in two parallels with two separate 120V sources, and 2 SSRs with the same signal wire. That way the current draw wont be too much for the current breakers, and each of the heaters will get 120V.
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What' size breakers are you using ?
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@jens55 They are 20 amp breakers, one parallel pair of the 1000W heaters will draw a max of 16.6666667A. Found another person who made a similar bed size with the same heaters, he had run into the same issue I was having with too big heaters, and had to go to this size.
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@Tpmoses , sounds good. I asked because standard circuits are 15A and wouldn't have handled two heaters at a time. You are ok with a 20A circuit.
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Okay just to make sure, it doesn't hurt to ask. The temp sensors on the heaters are NTC 100K 3950 thermistors. From what I have seen/been told I should wire them in parallel so it reads the highest temp coming from all four, and I will have to reduce the T parameter because the resistance will be lower. Is there anything else I should keep in mind?
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If you have the option I would add a separate thermistor to the bed plate itself so that you could get a more accurate PID loop going for the bed surface temp. The thermistors from the pads themselves could still be used as you're suggesting and used as a thermal limit monitor on the heater pads.
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@Phaedrux That's the plan long term, however in the meantime (waiting on parts that are on backorder) I was going to use the built in ones as a stopgap to be able to print until then.
