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.
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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