Quote:
Originally Posted by Penthode
I had been powering the chassis and didn't initially pay much attention until I noticed the resistor leads appeared to be blueing! The ac voltage drop across the resistor measured 20 volts. At 2 amperes, the resistor was dissipating a whopping 40 watts!! And it survived!!...
I had an old pack of 1.5 ohm Ohmite ceramic 11 watt resistors. With three in series, I measured a voltage drop of 8 vac which means that the resistors are dissipating each about 10 to 12 watts.
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I'm not sure I follow the second set of power calculations above, but in general, the actual power dissipation in rectifier series resistors is far higher than what you would calculate based on a voltage drop measurement. Since the diodes only conduct briefly at the peak of the waveform (to recharge what has drained off the first filter capacitor), the current during this short interval is far, far higher than the average current. Likewise, the voltage drop is higher than might be expected based on average or RMS AC current for a sine wave. With the current compressed into such a short interval with very high current, and the power dissipation depending on the square of the current, the actual power dissipation is much higher than expected.
So keep in mind that the usual power, voltage, and current calculations only work for DC current or sinusoidal AC current (with RMS value for the sine wave). Once you go to pulsing current in a rectifier circuit, things are quite different!
The instantaneous power, voltage, and current still follow the usual relationship, but the relationship between instantaneous current and average current is no longer so simple.