Electronics > Projects, Designs, and Technical Stuff
IGBT Dimmer - PWM AC Power Control for an Immersion Heater
Richard Crowley:
--- Quote from: Hero999 on August 25, 2018, 04:06:12 pm ---I don't understand the question.
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Look at the ENTIRE system (from solar energy to the temperature of the water). The water is a THERMAL "capacitor". It doesn't care whether you are powering the heater from a varying DC current, or from a PWM AC current, or from a slow on-off power (whether AC or DC). The "slow on-off" is the method used by the overwhelming majority of temperature control systems. Because of the massive integration of the energy sink (whether the water in your hot water tank, or the air temp in your house.)
drussell:
--- Quote from: Hero999 on August 25, 2018, 04:06:12 pm ---The resistance of a water heater doesn't change much over its working temperature range and given the same duty cycle, the power dissipation should be the same whether the 230VAC is chopped up and PWMed at several kHz or PWMed with zero crossing, at a much lower frequency than the mains.
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It is true that it doesn't change the total amount of power drawn, but it does affect how the power is drawn from his inverter.
My understanding is that he is trying to make this load be a more consistent load to his inverter, so that it looks more like a varying resistive load instead of hacked-up pulse-here, pulse-there load. This makes perfect sense to me and seems that it could well be desirable in some cases like this so that the inverter is isn't having to supply random spikes of 3kW extra. Not that it shouldn't be able to handle it but I think I understand the logic....
Richard Crowley:
--- Quote from: drussell on August 25, 2018, 04:16:55 pm ---Well, from the controlling the heating side of things, yes, of course it is over-complicated to have such "fine-grained control", but that isn't what the OP trying to accomplish. He is trying to (as precisely as possible) use up his extra solar capacity by heating the water. The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.
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Point taken. From that perspective, then PWM seems justified.
But then what difference does the efficiency of the switching methodology make when the ultimate point to the exercise is to simply waste power as heat energy? What difference does it make whether the energy is dissipated as heat in the switch vs in the load?
willz1200:
--- Quote ---A TRIAC is probably the most efficient option.
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Thanks for clearing that up, doesn't the unsymmetric nature of triac phase cutting, generate more noise than PWM?
--- Quote ---The MOSFET needs to have an on resistance of under 0.75/13 = 58mOhms
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That is very low, I think I'd struggle to even find that.
--- Quote ---The additional complexity can be avoided by using an opto-coupler TRIAC with a built-in zero crossing circuit, such as the NTE3097 or MOC3042.
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Aren't these drivers only suitable for burst mode control? Which would give me a small dimming window? Meaning less dimming levels.
willz1200:
--- Quote ---The fine-grained control is needed for the supply side, not the load, to be able to smoothly use only the excess generated power.
--- End quote ---
Yes that's my main aim, slowly increase power using a pid controller until the grid tie is on the edge of import/export.
--- Quote ---But then what difference does the efficiency of the switching methodology make when the ultimate point to the exercise is to simply waste power as heat energy?
--- End quote ---
Point taken maybe I should bolt the FETs to the hot water tank :-DD I was just wondering how the use of IGBTs and MOSFETs compared and each PWM topology. But yes the main idea was to put a more uniform load on my inverter. The triac method causes some odd audible noises from my inverter, compared to boiling the kettle which doesn't produce the same noises.
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