"Don Pearce" wrote in message
...
On Sun, 25 Jun 2006 13:40:03 +0100, Jim Lesurf
wrote:
In article , Don Pearce
wrote:
The type of rectifier doesn't make a great deal of difference, although
full wave probably has the edge on accuracy. Don't put a capacitor after
it, just let the meter do the averaging, then do the sums: power = v
squared / R.
I'd be wary of this unless I knew the details of the meter. I've seen some
that can give odd results when asked to give a dc level in the presence of
much ac. e.g. One I recall seemed to only sample the input at a low rate,
so became confused as the level flutuations 'beat' with its sampling rate.
Slainte,
Jim
I reckoned that the chosen method (and prologue) indicated no great
ambition for accuracy, more a wish to get some sort of idea. Now
damping the meter movement with a big cap is fine and it will take the
ballistics out of the equation, but it will now certainly be a peak
reading meter that doesn't correspond to apparent loudness in any but
the vaguest way.
Also, I assumed that this is a real meter with a needle, not a
sampling DMM - otherwise why the question about rectifiers? It could
simply have been used on a AC range. Mine will read Watts or dBWatts
by setting an appropriate impedance in a menu.
d
--
Pearce Consulting
http://www.pearce.uk.com
Hi Don
Yeah, you're right. No great need for any accuracy in this application.
Because of the large numbers of group amps that I'm repairing at the moment,
I've put together a big resistive load, capable of handling a couple of
hundred watts. I've put some heavy duty relay switching in it for 16, 8 and
4 ohms, with automatic power-on default to 16. One or two other useful odds
and sods like a very heavily attenuated little speaker and volume control,
so that you can leave it squeaking away up the corner on a soak test, and a
+ /- LED bar arrangement to give a rough and ready idea of output symmetry,
through socket so that a genuine cab can be substituted for the load, and a
BNC socket for my 'scope. I thought it might be useful to put a very basic
power meter on there too, just to get an idea of how much smoke an amp was
generating. So I had a little play with a couple of spare sets of relay
contacts on the load resistor switching relays, and hung a three way switch
in there too. Three pots and a half dozen resistors later, I had a meter
showing sensible readings for full scales of 10, 50 and 100 watts into 4, 8
and 16 ohm loads, but for straight DC from the bench power supply.
It was just for interest then that I asked the question about whether 'real'
meters read the power in the whole wave, or just one half. So when an amp
says its output is 50w RMS, setting aside all the debate about whether RMS
is valid, just taking it to mean what we all know the manufacturers want us
understand by that, is it 100 watts of power contained in one half cycle, or
one whole cycle. I'm sure you all know what I mean - I'm just not putting it
very well ...
Arfa
Gutless Pommy Wonder
Linear Mode
