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reprap:anet:a8:improvements:understanding_my_heatbed [2018/12/30 09:42] – [Investigating other effects] cj_elec_tech | reprap:anet:a8:improvements:understanding_my_heatbed [2018/12/30 10:01] – [Simulating the heat bed] cj_elec_tech |
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==== Investigating other effects ==== | ==== Investigating other effects ==== |
A good hint from a Facebook group member was to check the cables. There is a significant voltage drop from the power supply to the heat-bed. Note that the power dissipated at the heat bed is proportional to the square of the heat-bed voltage. | A good hint from a Facebook group member was to check the cables. There is a significant voltage drop from the power supply to the heat-bed. Note that the power dissipated at the heat bed is proportional to the square of the heat-bed voltage. |
Replacing the power supply to heat-bed wires with thicker, shorter wires meant I could increase the power a little bit - doing that does not affect the heat-bed loss or contribute any insulating effect though. I also investigated if the problem was caused by forced convection due of the extruder fans. I could achieve slight improvements, but nothing spectacular. Independently, Dan Rogers posted results for his heat-bed (only 2 points) - the results suggested my bed was operating similarly. | Replacing the power supply to heat-bed wires with thicker, shorter wires meant I could increase the power a little bit - doing that does not affect the heat-bed loss or contribute any insulating effect though. I also investigated if the loss was caused by forced convection from the extruder fans. I could achieve slight improvements, but nothing spectacular. Independently, Dan Rogers posted results for his heat-bed (only 2 points) - the results suggested my bed was operating similarly. |
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==== The power supllied to the heatbed ==== | ==== Heat-bed Power ==== |
As the curves where very similar I investigated if the controller was ajusting the PWM duty cycle and therefore the curves where a controler effect and not a thermodynamic. The voltage was a flat line on the osziloscope, therefore this theory was discarded. | As the curves were very similar, I investigated if the effect was caused by the controller rather than being a thermodynamic effect (the controller adjusts the PWM duty cycle - and thus the curve). The voltage was a flat line on the oscilloscope, therefore this theory was discarded. |
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Than I measured the current and observed that it drops with increasing temperature. | Then I measured the current and observed that it drops with increasing temperature. |
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{{ :reprap:anet:a8:improvements:mypicture2.gif |}} | {{ :reprap:anet:a8:improvements:mypicture2.gif |}} |
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If the resistance is calculated by divinding the voltage through the current this leads to the following resistance{{ :reprap:anet:a8:improvements:mypicture3.gif?direct&400 |}} | Resistance is calculated by dividing voltage by current - this lead to the following resistance{{ :reprap:anet:a8:improvements:mypicture3.gif?direct&400 |}} |
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And this gradinet fits nearly to the gradient expected for cupper with an temperature coefficient of appximatly 0.0039 1/K | This gradient nearly fits the gradient expected for copper with an temperature coefficient of approximately 0.0039 1/K |
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This is probably also the reason why there are values of 1.2 to 1.6 Ohms mentioned in some forums and sites for this heat bed. | This is probably the reason why there are values of 1.2 to 1.6 Ohms mentioned in some forums and sites for this heat-bed. |
==== Simulating the heat bed ==== | ==== Simulating the heat bed ==== |
In order to undertstand the heat bed behavior in more datail I made a {{ :reprap:anet:a8:improvements:warm_up_2.xlsx |heat bed simulator}} in excel (dispite I don't like to this software for simulations). It fits quite well to the measured data, therefore it seems to be not so bad. But keep in mind that it is a model and not the reality. | In order to understand the heat bed behavior in more detail I made a {{ :reprap:anet:a8:improvements:warm_up_2.xlsx |heat bed simulator}} in Excel (despite not liking this software for simulations). It fits quite well to the measured data, therefore it seems to be not so bad - but keep in mind that it is a model and not reality. |
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{{ :reprap:anet:a8:improvements:mypicture4.gif |}} | {{ :reprap:anet:a8:improvements:mypicture4.gif |}} |
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The cool thing about simulations is that you can evaluate heat fluxes which are difficult to measure and to make a serios of variations which would require months of testing in a few minutes. | The cool thing about simulations is that you can evaluate heat fluxes which are difficult to measure and to make a series of variations which would require months of testing in a few minutes. |
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{{ :reprap:anet:a8:improvements:mypicture7.gif |}} | {{ :reprap:anet:a8:improvements:mypicture7.gif |}} |
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When the steady state is nearly reached the initial 110W of electric power are reduced to 0 by the following contributions (for a heat bed without insulation). It can be seen that a large portion of the heat is irradieted by infrared radiation. This should possible to easily reduced by a factor of 8 to 9 by applying a aluminium foil(emissivity 0.11) beneath the heatbed or insulation. | When the steady state is nearly reached the initial 110W of electric power are reduced to 0 by the following contributions (for a heat bed without insulation). It can be seen that a large portion of the heat is radiated by infrared radiation. This should possible to easily reduced by a factor of 8 to 9 by applying a aluminium foil (emissivity 0.11) beneath the heat-bed or insulation. |
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{{ :reprap:anet:a8:improvements:mypicture8.gif |}} | {{ :reprap:anet:a8:improvements:mypicture8.gif |}} |