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?
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
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
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
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
- Don Klipstein ( firstname.lastname@example.org)
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.