# Reactive AC power / amp / heat questions

Discussion in 'General Electronics Chat' started by itsme83, Jul 11, 2011.

1. ### itsme83 Thread Starter New Member

Jul 11, 2011
6
0
Hi everyone,

I need some clarity on power and heat dissipation issues for the following amplifier setup...

I have a class AB amplifier driving a mostly reactive load. Let's say that the amp has a dual rail supply of +200/-200V, and that it provides a sinusoidal output voltage at the full 400V pk-pk. Let's also say that the apparent power I'm measuring at the amp's output is 3VA and the real power I'm measuring is 1W.

Of course this means that 1W is 'used up' by the load, that's easy enough.

What I'm not sure about are the consequences of the remaining 2VA of reactive power, specifically in terms of a) demands on the amp's power supply rails, and b) heat dissipation in the amp's heat sink.

Does this 2VA get 'burnt' entirely by the amplifier (i.e. is it converted entirely into heat)? Or is it 'recovered' by the power supply? Or what?

In other words, using these known values:
i) apparent power of load [3VA]
ii) real power of load [1W]
iii) amplifier efficiency [say ~80%]
iv) assume no quiescent current in amp for simplicity

..I want to be able to state the following:
ii) Heat dissipated in amp heat sink [W]
iii) RMS current drawn by each amp supply rail [A]

Hope that's clear. Can anyone help?

2. ### itsme83 Thread Starter New Member

Jul 11, 2011
6
0
Actually any info about reactive loads on AB amplifiers would be useful. I'm not looking for someone to come up with the values above

Feb 5, 2010
3,795
951

Jul 11, 2011
6
0
5. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,795
951
That is a lot of theory and math to be in understanding of...

Hint: Reactive power is supplied by a generator but is not 'wasted' in the load.

Which portion of your circuit arrangement could be considered 'the generator'?

6. ### billbehen Active Member

May 10, 2006
39
1
Driving a reactive load from a linear amplifier is maybe asking for trouble. The output power from the amp comes from the emitter of the power transistors, which is a PN junction, and thus only passes current one way: out for NPN, in for PNP.

If MOSFETs are used, these have an intrinsic reverse body diode in them that may tolerate the reactive current!

This reactive current, assuming it is tolerated in some way, is mostly not dissipated at all, but just sloshes back and forth, (somewhat) out of phase with the drive signal of the amplifier.

As most speakers, if this is an audio system, work by magnetic induction, some reactive current is to be expected. So, maybe the amp is designed for a reactive load? I would check the specs and schematic of the unit to see what it says/looks like. Also, if it's made by a company that's still in business, they might have application engineers to answer questions....