cupoftea
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Hi,
We are doing a trailing edge controlled heat controller for a shower.
The zero cross detector doesn't give exactly the same (symetrical) pulse timings for
positive and negative half cycles, and so the current pulse is slightly different between positive and negative
half cycles. As such, there is more than 5mA DC in the mains due to use of our equipment.
But to be honest, any 1kW+ offline SMPS is fed by a diode bridge etc, and inevitably the current draw
is more than 5mA different for positive and negative half cycles, and no one complains about that?
So can we overlook and ignore our >5mA DC drawn from the mains?
I mean, is the >5mA DC law actually enforced anywhere?....if it was then every cheap Triac lighting dimmer
in the world would have been banned by now as they all give non symmetrical
current pulses between positive and negative mains half cycles.
I mean, check the mains on a scope....the flat topping shape is not exactly the same for pos and neg half cycles,
so obviously there is DC in the mains anyway.
Problems caused:
I understand that the biggest problem (of DC in the mains) is saturation of distribution transformers?
This "corrosion of copper pipes" thing, is a myth? Theoretically it would happen if there was DC in the mains, but not in reality?
Equalising SMPS:
Supposing the ">5mA DC" thing is enforced, then is it acceptable to add in an equalising SMPS which ensures that the overall DC drawn by any converter is
overall <5mA DC?
...And if such equalising converter is allowable, then what period must it operate over?....eg supposing it just about ensures that over any 1 second interval, the DC is zero (so the DC may well have been >5mA for eg, a particular half second period).
3 phase mains:
What if you have a 130W offline SMPS that is supplied by a star_3_phase supply with a 3 diode rectifier? (ie each phase will then have DC in it, since its three single phase rectifiers, one on each phase)
We are doing a trailing edge controlled heat controller for a shower.
The zero cross detector doesn't give exactly the same (symetrical) pulse timings for
positive and negative half cycles, and so the current pulse is slightly different between positive and negative
half cycles. As such, there is more than 5mA DC in the mains due to use of our equipment.
But to be honest, any 1kW+ offline SMPS is fed by a diode bridge etc, and inevitably the current draw
is more than 5mA different for positive and negative half cycles, and no one complains about that?
So can we overlook and ignore our >5mA DC drawn from the mains?
I mean, is the >5mA DC law actually enforced anywhere?....if it was then every cheap Triac lighting dimmer
in the world would have been banned by now as they all give non symmetrical
current pulses between positive and negative mains half cycles.
I mean, check the mains on a scope....the flat topping shape is not exactly the same for pos and neg half cycles,
so obviously there is DC in the mains anyway.
Problems caused:
I understand that the biggest problem (of DC in the mains) is saturation of distribution transformers?
This "corrosion of copper pipes" thing, is a myth? Theoretically it would happen if there was DC in the mains, but not in reality?
Equalising SMPS:
Supposing the ">5mA DC" thing is enforced, then is it acceptable to add in an equalising SMPS which ensures that the overall DC drawn by any converter is
overall <5mA DC?
...And if such equalising converter is allowable, then what period must it operate over?....eg supposing it just about ensures that over any 1 second interval, the DC is zero (so the DC may well have been >5mA for eg, a particular half second period).
3 phase mains:
What if you have a 130W offline SMPS that is supplied by a star_3_phase supply with a 3 diode rectifier? (ie each phase will then have DC in it, since its three single phase rectifiers, one on each phase)
Last edited: