From: michthom@bnr.ca (Michael Thomson)
Subject: Re: How do I create a 2A load to test a power supply?
Organization: BNR Europe Ltd, Maidenhead, UK
X-Newsreader: TIN [version 1.1 PL8]
Distribution: sci.electronics
Date: Sun, 24 Jan 1993 13:09:15 GMT

Russell J. Pagenkopf (cs000rjp@selway.umt.edu) wrote:
..
: The power supply uses 120VAC 0.40A grounded and outputs (supposedly) 20V,
: 2.0A, 40VA Class 2  with four pins ( I have no idea what pins do what yet).
..
: what does the 40va mean? 40VAC? I guess what this comes down to is, how
: should I test this thing to make sure it is giving me what it should?

Hokay - Here's my $0.02 worth...

1) A quick lesson on power delivery...

If the load you are driving into is *purely* resistive (no capacitive /
inductive elements) then the voltage and current are in phase, and the power
consumed is simply the product of the two. e.g. 20V across a 10 ohm load gives
a 2 Amp current, and you are dissipating 40 Watts of real power.
If on the other hand the load has a COMPLEX impedance (there is some capacitive
or inductive component) then the voltage and current will be out of phase. This
means that the power dissipated in the load will be *less* than the answer you
get if you multiply volts and amps together.
WHY??? I hear you cry! The reason is that capacitors and inductors don't
dissipate (much) power. They store it as electric charge and magnetic field,
respectively. Thus the power is not all "used up".

Summary: 	1 Watt = 1 Volt * 1 Amp, in the special case where Voltage and 
		Current are in phase (at DC, or AC into a purely resistive
		load).

		1 VA = 1 Volt * 1 Amp, but here we are talking about the
		MAGNITUDE of the COMPLEX product of the voltage and current
		phasors. I realise this may be greek to you - a decent
		electronics textbook should explain about complex power (VA), 
		real power (W) and the relationships better than I can.

2) Testing your supply...

First measure the open circuit voltage at the outputs. You may see a bit more
than 20 Volts. Don't worry unless it's a lot more (about 30V). This checks
there is juice coming out. If no voltage - it's toasted, but make sure you
check all of the outputs before you junk it, as there may be two wires
connected to the same supply rail.
I wouldn't recommend trying to measure the current with your meter alone, as
this basically shorts the power supply, and the current may burn out the
secondary winding on the transformer (or your meter!). Instead, get a 20 Ohm
resistor rated for 20 Watts (This will get 1 Amp out of your supply) or a 10
Ohm resistor rated at 40 Watts to get 2 Amps out of the supply. Remember, you
can put two 20 Ohm, 20 Watts in parallel to  do this. Hook this load across
the supply and measure the voltage. This is the case where VA = Watts, and you
should be able to check you get what you need out of this sucker.

Hope this helps!
--
*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*
+    Michael Thomson   + The opinions expressed here, while definitely NOT  +
+    BNR Europe Ltd.   + those of my employer, may OR MAY NOT be mine!      +
+ Phone +44 628 794416 + --------------------------==========-------------- +
+   Fax +44 628 776017 +        This space intentionally left blank         +
*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*