Re: Power Supplies / Transformers

John Ross wrote:

> From: John Ross <johnr@...>
>
> I need one of you electronics experts to set me strait.
> I've had someone offer me a 110 Pri 24V Sec Transformer at 100 Volt-amps.
> Isn't Amps at 24V allot different than Va? I always thought Va was Source
> voltage + Amperage. Wouldn't a 110 - 24v 1A transformer be 111Va? This guy
> is trying to convince me this or a 20va transformer is what I need. If I
> want 15 amps shouldn't it be like 110+15 or 125Va?

Seer clear of this idiot! 100 VA is roughly 100 Watts (if the power factor is
close to 1.0). You won't run much of a stepper drive on that. We know that the power
factor on a capacitor input filter is bad, somewhere between .3 and .5 at best.
So, you need to derate the transformer by that factor. So, if you need a 5 Amp
DC supply, you'd be wise to use one rated for at least 10 Amps secondary
current. A 100 VA transformer with a 24 V secondary would give about 4 Amps
to a resistive load. (24 V * 4 A = 96 VA.)

For 15 Amps DC, at 24 V ac (RMS) * 1.414 (peak of RMS Volts) ~= 34 V,
you need either a transformer rated for a capacitor input filter at 15 Amps, or
a transformer rated for about 30 Amps resistive. Either way, the RMS current
from the transformer secondary will be well over 15 Amps due to the non-linear
power factor. So, either way, assuming a .5 PF, you need a transformer rated
at 30 Amps at 24 V (RMS), or 720 VA. So, when the guy shows you a 20 VA
wall-wart power transformer, you have my permission to laugh!

You won't laugh, however, when you see the size, weight and price of a new
720 VA transformer. Hopefully, you can find one surplus.

Jon