Lowering mains voltage

[Hero999]

Yes, or a resistor, or a diode. The diode I²T rating has to be more than the lamp inrush current (10x to 15x the steady state lamp current for a few dozen mS). Do the diode; the lamps will last forever.

These Christmas tree lights are running from a transformer. Never use a diode in series with a significant load connected to a transformer, it will cause core saturation and overheating of the transformer!

If you use a diode the cheap Christmas tree lights will probably be all right but the more expensive transformer will meltdown.

What current do the lights require?

Use a resistor to reduce the extra voltage.

 

[msrtlt]

What current do the lights require?

Use a resistor to reduce the extra voltage.

Each lights string is 25 lights * 7w - approximately 1.5A.

The problem with resistor is two fold: 1. I won't be able to connect another string to it - the current will increase to 3A - but this is not crucial - I can connect the other string in parallel; 2. Often 2 or 3 bulbs burn out more or less simultaneously, in which case, less current through resistor and a higher voltage supplied to the lights - again, probably not that crucial - but that's why I thought that perhaps an appropriate choke would do better.

As suggested, an autotransformer or Kmoffet's idea would be better, although more complicated.

Is there such thing as 20w potentiometer (10 or 20 ohm)?

 

[Willbe]

Put the diode or resistor downstream of the transformer and upstream of the lights.
Brightness is proportional to volts^3.5 and life is proportional to volts^12th or 16th power. Pick your tradeoff.
**broken link removed** has all kinds of power resistors, and a great perf/price ratio.

 

[KMoffett]

...Never use a diode in series with a significant load connected to a transformer, it will cause core saturation and overheating of the transformer!

Isn't "a diode in series with a significant load" on the secondary of a transformer called a "half-wave rectified" power supply? Seems as if this is a common practice. Well, not without a capacitor, but still...

Ken

 

[msrtlt]

A million thanks to everybody!

Amazing amount of workable proposition and also a lot of lessons!

I tend to think that after all, perhaps a simple 20 - 30w resistor may be the simplest and the cheapest.

 

[xanadunow]

zero crossing dimmers

"Dimmers can't be zero-crossing, how could they work?."

This is bit more complicated. In fact I have not seen such a "dimmer" yet. However the statement was brought in by me by transferring the solution from a heater control device.

Traditional dimmers: it is a phase controlled (say it is a triac) device, where - by altering the firing point of the triac within the 0-180 degrees angle we will achieve the 100-0% control of the brightness.

The zero crossing device: It can not use the "phase" controll because it has to turn ON and turn OFF at "zero". Rather then controll the phase, we need to control the number of AC cycles the device is ON and OFF.

For a crude example, if we could turn the load ON at "zero" for just one cycle then keep it OFF for one cycle, then turn it ON again for one cycle e.t.c. .. we will have a 50% power point.
If we keep it ON for two cycles and OFF for one cycle e.t.c. - we will be at around 66%
If we keep it ON for 3 cycles and OFF for one cycle e.t.c. - we will be at around 75%
If we keep it ON for 1 cycle and OFF for 3 cycles - we will be at around 25%

This will work ofcourse but the zero crossing devices do have their own "time delay" - on an average it takes about 8[ms] to ensure the turn ON/OFF at "zero". One mains cycle lasts 20[ms] for the 50Hz frequency or 16[ms] for 60 Hz (2 zero crossings in each Mains cycle).

If we are trying to do the light dimming application we will have to do it pretty fast to avoid flickering, thus it has to be done on a cycle by cycle basis - therefore number of possible power control points in one "control cycle time" is limited to the toal number of (half) Mains cycles the device is OFF without introducing the "flicker".

For a heating application however, this solution is much easier because the time inertia of a heater is larger than the inertia of a lighting bulb.

With heater then - we can easy establish a control cycle of (say) 3 seconds that gives us 3 x 50 x 2 = 300 possible "zero" crossing points for 50Hz and 3 x 60 x 2 = 320 possible "zero" crossing points for mains frequency of 60Hz ( number of zero crossings = one control cycle time lenght in seconds times the mains frequency in Hz times the number of zero crossing in one mains cycle).

Having 300 control points to turn ON or OFF in one control cycle of 3 seconds gives a pretty good resolution for the power control of 100%/300= 0.3%

There will always be ONE mains cycle lost for the turn ON/OFF operation but the benefit of zero crossing is to obvious to pass by.

Also, the larger is the inertia of the load (heater,furnace etc) - the control cycle time can be elongated giving even more zero crossing points per control cycle and even finer resolution of the power control.

This is little above the original subject of reducing the mains power for the XMAS lights but I felt compelled to respond to the query of "zero crossing" dimmers.

Regards,
xanadunow

 

[Hero999]

Isn't "a diode in series with a significant load" on the secondary of a transformer called a "half-wave rectified" power supply? Seems as if this is a common practice. Well, not without a capacitor, but still...

Ken

A full wave rectifier is common practise, a half wave rectifier is less common. It's all right to use a half wave rectifier to power some small load like an LED but a large load will kill the transformer. Transformers can't pass DC and adding a diode results in DC in the secondary and primary which causes core saturation and eventually smoke.