# Power Factor and the need for correction

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#### AndrewT

Here is an extract from the Wiki on Power Factor limits for Classes A to D.
https://en.wikipedia.org/wiki/IEC_EN_61000-3-2
I'll mention only ClassA which is contained in the first two columns.
Limit lines
Harmonic order Current limit
. . Class A . . . Class B . . . Class C . . . .Class D
n . . .[A] . . . . . . . [A] . Expressed as percentage of the input current [%] Permissible current per watt [mA/W] PermissibleHarmonicCurrent A
2 . . . 1.08 . . . . . 1,62 . . . . 2 . . . – . . . . –
3 . . . 2.3 . . . . . .3,45 . . . .3*λ . 3.4 . . . 2.3
4 . . . 0.43 . . . . . 0,64 . . . . – . . . – . . . . .–
This table of limits extends to the 39th harmonic
According to the Wiki, PF is required for loads that draw more than 75W and for this table have a phase current of <16A.

For n = 2 the ClassA harmonic current limit is 1.08A
For n = 3 the ClassA harmonic current limit is 2.3A
For n = 4 the ClassA harmonic current limit is 0.43A

Does this mean that the current drawn for each harmonic must be below those limits?
And if your PSU draws less than 16Arms then it would only need PFC if any one or more of those harmonic current limits were exceeded?

To me those limits seem pretty lax?
Am I understanding this correctly?

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#### DF96

I think it means n=2 is less than 1.08% of total current etc. These limits are quite tight. My guess is that a cap input supply would not get near them, and even a choke input might struggle. Fortunately it (probably) only applies to items placed on the market, not DIY.

Interesting that specifically excluded are light dimmers, which can put out lots of hash!

#### AndrewT

The header in columns 2 & 3 are [A] (for Amperes), the header for column (classC) is % of the current.
Expressed as percentage of the input current [%]

It looks to me that classC uses the %, but that classes A & B use absolute Ampere limits for each harmonic.

I was reading this and came across the EN reference