[Amps] Measuring RF Power

[R.Measures]

From: R. Measures <r at somis.org>
Date: March 23, 2005 4:10:50 AM PST
To: garyschafer at comcast.net
Subject: Re: [Amps] Measuring RF Power


On Mar 22, 2005, at 2:37 PM, Gary Schafer wrote:

> As an additional note it is interesting that 120 volts DC on that same
> light bulb gives the same 100 watt power as the average AC power.
>
AsI recall, for a sinewave, the average potential is 0.636 of E-pk and 
the root-mean-square value is 0.707 of e-pk.  AsI recall, for a 
sinewave, the average current is 0.636 of I-pk and the root-mean-square 
value is 0.707 of I-pk.

> The statement that you often see, "RMS has the same heating effect as
> DC" gets people confused.
> Where most go wrong with the relationship is that widely used statement
> is referring to RMS voltage or current verses DC voltage or current. 
> Not
> power.
>
> If you read the statement closely in most books it will say something
> like: " a given RMS voltage (or RMS current) through a resistor will
> cause the same amount of heating as the same DC voltage (or current)
> through the same resistor". It does not say RMS power.
>
> We know that DC volts has to give us average power. So then must the 
> RMS
> voltage.
>
> 73
> Gary  K4FMX
>
>
> Bill Fuqua wrote:
>> Now, how about the next example. RMS of a sine wave using only 4 
>> samples
>> but this time use 170 volts peak value and 144 ohms (100 watt light 
>> bulb).
>> I have rounded the following to the nearest tenth to make it easier 
>> to read
>> but if you wish to carry it out to all digits it works perfectly.
>>   phase    V        V Squared           Power into 50 ohms
>> 0            0              0                           0
>> 90          170           28900                    200.7
>> 180         0              0                           0
>> 270        -170           28900                   200.7
>> Total       0               57800                   401.4
>> Average   0              14450                   100.3
>> Sq.root    0              120.2
>>
>> Power = Vrms Squared/R= 14448/144 = 100.3
>> How about that. only 4 samples.
>> By the way that is how most power inverters produce a "modified Sine 
>> Wave".
>> The important point is that the RMS and PEAK values are correct. 
>> Electronic
>> devices with capacitive input filters are happy and so are other 
>> things
>> like light bulbs, motors ,etc.
>>           They use a high frequency inverter (switcher) to get the 170
>> volts DC and then use a pair of H-Bridge power MOSFETs to put the 
>> proper
>> voltages on each of the 120 volt output connectors. One thing, the 
>> cheaper
>> ones do not have a "hot and return" they have two hot terminals that 
>> are
>> 180 degrees out of phase. This is bad news if you are using a hot 
>> chassis
>> AC/DC set. You are going to have 60 Volts RMS on the chassis no matter
>> which way you insert the plug.
>>
>> 73
>> Bill wa4lav
>>
>>
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>>
>
>
>
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org



Richard L. Measures, AG6K, 805.386.3734.  www.somis.org