Re: Average Current

From: John Popelish (jpopelish_at_rica.net)
Date: 10/17/04 

Date: Sun, 17 Oct 2004 12:12:08 -0400

Jack// ani wrote:
>
> John Popelish <jpopelish@rica.net> wrote in message news:<41712004.CFDCEC48@rica.net>...
>
> > Almost certainly. But I never use a component at its absolute maximum
> > rating, voltage, current, or power.
>
> Thanks John
>
> I probably knew that, just wanted to get assured of it.
>
> Ok now it will be wise to ask, suppose i want to get 3amp dc out of it
> using bridge rectifier, what should be the transformer amps rating.
>
> By calculation it should be 3.33amps,
>
> Iav=2.82*Irms/pi

The diodes have an average current rating because they drop a nearly
constant voltage over a wide range of current. So the heat produced
is fairly closely proportional to the average current, as long as you
don't get into extreme cases (of very large pulses widely spaced).
Transformers are heated by the square of current, like resistors,
because their windings are resistive. So they have an RMS current
rating. RMS stands for square root of the mean (average over a
representative time period) of the square of the instantaneous
current. The proportion of the RMS current from the transformer to
the average DC rectifier output current depends a lot on what follows
the rectifier. If the load is a resistor, then the RMS load current
is also the RMS transformer current. In this case, the RMS load
current is 1.11 times the rectifier average current.

If the load is primarily capacitive, all the rectifier current is
confined to the brief periods when the transformer voltage is higher
than what is stored in the capacitor, so the RMS current can get quite
a bit higher than the average current. This affects the current
rating required in the transformer.

> so, Irms=3.33amps
>
> It that correct? And will it depend upon full/half wave rectifier?

It is correct for a resistive load and a full wave rectifier (load
current proportional to absolute value of instantaneous transformer
voltage). Half wave rectifiers pose additional problems for
transformers that involve core saturation that don't occur with full
wave rectification. And the ratio of RMS to average current is also
different (higher).

What load will your rectifier be driving? 

-- 
John Popelish

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