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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 14th Sep 2021, 7:57 pm   #1
Skywave's Avatar
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,311
Arrow Measuring power in a known load with a 'scope.

Measuring power in a known load with a 'scope - the easy way. *

So you've got your repaired audio amp. on the bench ** and you need to check its output watts when it is connected to its appropriate load - typically a 8 ohm resistor of adequate power rating - and your audio sig. gen. is connected to the amplifier's input.

The common method is to measure the peak-peak voltage across that load R with the 'scope and then do the necessary arithmetic:
1. Measure the Vp-p.
2. Calculate Vpeak: Vp-p 2
3. Calculate Vrms: Vpeak 1.414
4. Calculate the watts in R: (Vrms) R.
Unless you're outstanding at mental arithmetic, that involves a fair bit of work with a calculator or W-H-Y.

Here's the easy way:
Power in R = (Vp-p) 8*R
which for a common 8 ohm load reduces to (Vp-p) 64.

A numerical example; method 1 (assumes R = 8 ohms):
1. Vp-p = 10 v.
2. Vpeak = 5v.
3. Vrms = 5 1.414 = 3.536
4. Watts in R = (3.536) 8 = 1.56.

A numerical example; method 2 (assumes R = 8 ohms) :
1. Vp-p = 10 v.
2. (Vp-p) = 100
3. Watts in R = 100 64 = 1.56.

Now I could have reduced all of the above by only simply stating:
Power in load = (Vp-p) 8*R without the details, but I went into detail to justify and illustrate my opening claim and to try to avoid the need for any detailed explanation. (But I am happy to explain if required).
Finally, as to why (Vrms) R = (Vp-p) 8*R, I leave that up the reader to justify that for himself: it's not difficult.

* All of this post assumes that you do not have an instrument to measure the output power directly - such as the Marconi TF893A power meter.
** All that follows from here is obviously not limited only to audio power amps.: my choice of such is just a typical example.

Al. / Sept. 14th.
Some people have cats and go on to live normal lives.
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