Ping Don Klipstein

[Jeff Wisnia]

What happens if a 120 volt incandescent bulb is powered by 240 vac with an inline series diode?

Will the bulb consume the same average power as it does when run from

120 vac without the diode? (Ignoring the 0.7 volt diode drop.)

Or, will the two conditions differ, and the bulb running from 240 vac with a series diode quickly fail?

Thanks,

Jeff

[Don Klipstein]

240V AC halved by an "ideal diode" (zero voltage drop) will have the same average voltage as unmodified 120V AC. However, the RMS (root mean square) voltage will be different - higher by a factor of square root of 2. That would be 169.7 volts. The RMS voltage is the effective voltage for simple resistive loads such as incandescent lightbulbs and resistive heaters.

Think of it this way: An ideal resistor (resistance not varying with temperature) receives 1/4 as much watts when voltage is halved. Not only is voltage halved, but amps are also halved, and that means watts are quartered.

But a diode merely disconnects the load from the line for half the time, so wattage is merely halved. By Ohm's Law, the effective voltage of AC after a diode is then the original voltage times square root of 1/2.

Incandescent lamps do have the complication of their resistance varying with temperature, but the effective voltage ("RMS voltage") is still the same as if resistance was constant. An incandescent lightbulb receiving

240V AC through a diode is effectively getting 169 volts. After the effect of resistance varying with temperature, power consumption by the lightbulb will be close to 1.65-1.72 times that at 120V rather than double.

Keep in mind that a voltmeter will read a voltage other than about

169 volts unless it is a "True RMS" type. It may read 120 if it has fullwave rectification or possibly either 240 or close to zero due to having mere halfwave rectification. On a DC range, a non-true-RMS voltmeter will read halfwave rectified 240V or fullwave rectified 120V as about 108 volts - the actual average voltage. (Nominal voltage of AC is the RMS voltage, about 11% higher than the average for a sine wave.)

According to what I consider a "usual 1-size-fits-all" rule, life expectancy is reduced by a factor of about 60 - and actual life expectancy results can vary significantly, even greatly.

I would caution that burnout may be more spectacular than at 120V, possibly even unsafe, especially for a diode unless it is big enough to not protect your fuse or circuit breaker by blowing first (possibly explosively) should a "burnout arc" form.

And at 70% overwattage, the bulb may overheat and break - especially in an enclosed fixture or a recessed ceiling fixture.

Lower wattage 120V lightbulbs, such as maybe around or under 25 watts (?), may have their filament vary enough in temperature over each AC cycle when used with a diode and 240V to change results somewhat from results with actual 169-170V - probably for the worse in terms of life expectancy.

- Don Klipstein ( [e-mail])

[Jeff Wisnia]

Many thanks, Don.

Jeff