Re: 1 KVA 240 v transformer output 6v,8v,12 v AC how to test if it can supply 83 amps?

From: Fritz Schlunder (me_at_privacy.net)
Date: 03/05/05 

Date: Sat, 5 Mar 2005 00:52:44 -0700

"rob" <sasman@sasman.com> wrote in message
news:c3fg21ln394nvkaapunea212lnaus6kanv@4ax.com...
> Hi
>
> I am a total novice in electrics and am not sure were to post this
> question?.. i hope some one can help me...I need a transformer that
> can supply aprrox 6 Volts Dc at 75 amps..
> I have just purchased a 240 v ac input 1 KVA rated transformer its AC
> output is 6v ,8v and 12 V..Now i understand that i need to rectify
> the current and have just got off ebay a dozen MBRP40030CTL 400A 30V
> schottky diodes with low Vf.. i assume that these will be able to
> supply a 75 amp current.. at 6 v?..I got them because there rated at
> 400 amps..and were very cheap!
> http://www.onsemi.com/pub/Collateral/MBRP40030CTL-D.PDF

These diodes are well suited to the task at hand.

> Is there any way i can test to see if it can supply 75amps ?

If you don't need any kind of filtering and are perfectly content with
really lumpy DC power direct from the bridge rectifier, then yes this
transformer should be adequately capable of producing your 6V(ish) at 75A
DC. Naturally you would have to remove or replace the fuse with something
greater than 75A. Fusing on the primary side is somewhat easier and cheaper
since 250V fuses at a few amps are much cheaper and more common than giant
75A+ fuses (although at your low voltages 30V automotive blade fuses should
suffice). On the other hand a fuse on the primary side would need to have
greater overrating to avoid nuisance fusing since it must contend with
transformer saturation surge currents at initial power up.

If you need 6V DC at 75A DC well filtered, then the situation changes
significantly. The transformer outputs vaguely 6V AC, but when filtered
yields a peak voltage of roughly 8.5V less two schottky diode drops. When
well filtered with a large capacitor, the power factor will be substantially
reduced. Capacitive input filters fed from full wave rectified sinusoidal
sources can have typical power factors of around 0.6. In other words, if
your load still draws 75A DC (at around or a bit less than 8V DC with the 6V
AC tap) with the capacitive filter, the effective heating in the transformer
secondary will be more than what a 6V 75A RMS AC load would produce.

Suppose your load draws 75A DC at 8V but with a powerfactor of 0.6 (due to
input filter capacitor). This means your load uses 75*8 = 600W, but
600W/0.6 = 1000VA which is the maximum rating of your transformer. It isn't
totally clear if your transformer is rated for 1000VA load from any of the
taps, or if it is rated for 1000VA from the 12V tap only, but somewhat less
on the lower voltage taps. The 6V tap has half the turns of the 12V tap, so
if one assumes the length and size of the wire used for the 6V tap is half
that of the 12V tap, then 1000VA at half the voltage is twice the current.
However, P=I^2*R, so for half the resistance (half the length of wire for
the full 12V tap), but twice the current, the power dissipated is double.
On the other hand the 6V winding is probably located closer to the center of
the transformer where the diameter of each loop of wire is smaller, so the
6V tap probably has somewhat less than half the resistance of the 12V tap,
so the heating situation isn't quite so bad.

All and all, it is hard to say for certain if 75A DC is overdoing it a
little bit if you use a capacitive input filter. Without the filter a load
current of 75A DC is fine, but with the filter 75A might be pushing it
slightly over its limits. Long term reliability may be somewhat
compromised, so for peace of mind improved cooling using forced air and/or
reducing the load current to something a bit less than 75A might be
advisable.

What exactly is your application?

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