Series bulbs with rectified AC

[duke37]

14 months ago, I was volunteered to be the light string repairer. It took ages to find the dead bulbs or poor connections so I thought 'blow this for a lark' and I placed a 1N4007 in the plug to run the bulbs slightly cooler.
This Christmas one of the strings gave a short flash and died. The string is open circuit.

What would the effect of a diode be? Would the lower temperature give a longer life or the pulses vibrate the filament to destruction?
This technique is used to reduce the heat generated in AC/DC valve radios by using a lower dropper resistor.. Are the filaments likely to suffer?

 

[Externet]

I would say the filaments would run cooler and the 'vibration' would be half the amplitude, with at least 5 times life.
I use backyard incandescent 230V-200W bulbs fed by 120V instead of 120V-75W bulbs and so far, 10 years with no replacements. Did the same for a 120ft tall antenna tower red beacon.

If you feel uncomfortable with the diode, splice 2 strings in series, no diode, to the same voltage source.

 

[Harald Kapp]

Get a centennial light.

 

[dorke]

In general,the life time of an incandescent bulb is mostly dependent on the number of it's turn-on times..
This is due to the fact that a cold bulb has much lower resistance than a warm one (by a factor of about 15!).
The much higher turn-on(cold filament) current shortens the life of the bulb dramatically.
And that is also the reason that bulbs mostly fail when they are turned on.

Putting a series diode in the AC path will create a half cycle feeding voltage to the bulbs.
Effectively halving the RMS voltage to the bulb.
It will increase life time dramatically while reducing the light intensity dramatically .

I think the pulsed nature will have no meaningful effect .
Reason being the bulb filament will not cool significantly in the half period time the diode isn't conducting
(10mSec ,for 50Hz).

A better solution would be to use LED strings.

 

[Harald Kapp]

Effectively halving the RMS voltage to the bulb.

Actually the factor is 0.7, not 0.5.

 

[dorke]

Thanks Harald for the correction!
0.7 it is.

 

[duke37]

With half wave rectification, the filaments will run cooler and will have a lower resistance so the light does not go as low as might be expected.
Valve radios often use 6.3V heaters and dial bulbs, I replace failed bulbs with 8V bulbs and have not seen another failure but the use is not intensive.