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previous: Glenn Thanks again Don!Mo... -- 1104821372 View Thread

Re: Toroid inrush current???

1/4/2005 10:55 AM
Re: Toroid inrush current???
IIRC, the inrush from the toroid itself can add significantly to the otherwise *normal* inrush of the rectification & filtration circuit.  
Using the same size caps and any series resistance which may be present (even variable resistance such as having Inrush Current Limiters (ICL's)) in series between the secondaries and filters, the inrush from this part of the circuit would be the same whether the PT was a toroid or an EI.  
However, the toroid magnetically stores a *moment* of charge itself which is much more signiicant than that stored and retained by an EI core. This depends on which part of the AC cycle was present in the toroid when the power was disconnected. Then when it is repowered later, if the AC cycle to the primary is not at the same point in the 360degree cycle, there can be enough differential to cause a much larger than expected spark in the power switch. You can demonstrate this effect using an AC adapter from a laptop computer or other fairly high-current device with the miniaturized high efficiency external charger which depend on a toroid for their small size.  
Sometimes you plug the thing into the wall and there is no spark, other times it really jumps from the wall to the line cord or vice versa. This behavior is independent of whether there is a computer plugged in to the DC output of the power supply.  
The Negative Temperature Coefficient (NTC) inrush current limiting resistors can be used on the primary as well as the secondary, with a little experimentation you could select the optimum value of components for either the primary, the secondary, or both if desired, there are a lot of different temperature-dependent resistance profiles available.  

Glenn How bad is this sparking issue? I ... -- 1/4/2005 11:24 AM
Don Symes [QUOTE]the toroid magnetically stor... -- 1/4/2005 7:32 PM