Hey folks. I got a question for you guys.  Can 50, 80, 100 Watt mercury vapor bulbs be used in a fixture with a ballast rated for 175 Watt bulbs?  Or will it smoke the ballast?

Hey folks. I got a question for you guys.  Can 50, 80, 100 Watt mercury vapor bulbs be used in a fixture with a ballast rated for 175 Watt bulbs?  Or will it smoke the ballast?

The ballast would be OK, it would feed the bulbs by it's 175W (all MV's have about the same arc voltage of around 110V, so the ballast would see the same load as the rated 175W lamp).
But the bulbs won't be happy with that, I guess... (ka-boom) :-D.
With discharges the ballast is the thing what define the current (so power) delivered to the lamp, not the lamp itself. The lamp rating mean the power/current level the lamp is capable to accept for long time and capable to convert it in satisfactory way to the light. But if the ballast feed more, the lamp would run at higher actual wattage and vice versa.

Somewhere here is a picture of 175W MV bulb, what someone run in 400W fixture - the arctube was slightly bulged as a result, but it still survived it for a few years, so the 100W would most likely survive it for some while, the 80W for a short time, but the 50W would melt the arctube and explode very soon, factor of 3.5 overload is really too much...

The ballast will power the lamp at close to what the correct lamp would take. The 175W ballast will power any lamp (that it can start), even the 50W one, at not far from 175W

A lamp powered at too high power for it, will not work well (it will burn out fast, possibly kick off randomly, overheat, maybe even explode or damage the fixture...) and sure can damage the ballast

Discharge lights (this include mercury, metal halide, sodium and fluorescent) fixtures should be used only with the exact lamp they are made for, or compatible replacement lamp. It is not like incandescent or halogen fixtures where you can install a lower power lamp and it would be ok too



The reason is, that the power that the lamp gets depends on the properties of the lamp and of the ballast. The ballast is made to power the correct lamp with the correct power, but it is not taken into consideration what would happen if you use it with the wrong lamp - as this should never be done

If you try, for example, to power a smaller lamp which has lower voltage, with a simle inductive ballast :

The lamp voltage is lower, therefore the ballast coil voltage is higher (the sum of both is constant for each ballast - they are the ballast's OCV)

The higher ballast coil voltage causes higher current

On the lamp, the voltage and current, multiplied by each other, determine the power that the lamp is going to get. The lower voltage and higher current, together usually give power not too far from what the correct voltage and current would give. So the small lamp is getting about the same power as what the large lamp would get, and is significantly overloaded

On the ballast, the power lost as heat is the wire losses (current in square multiplied by the ballast wire resistance) + the core losses (which depend on the current as well). As they all depend on the current, and it is higher than should be, the ballast will overheat and will be damaged. Not necessarily within short time or with smoke, but it will overheat, degrade, short out etc

So correct me if I am wrong.... I could run a 100 Watt bulb and it probably wouldn't last as long as the 175, but would still get good life.  But if I ran 80 or 50 watters they would disintegrate almost immediately?

The 100W may work for some time, but still it may explode, there is no guarantee the 100W bulb would take the 175W.

If you want to power the 100W bulb and you are looking for a ballast, you may use the "70W pulse-start MH" ballast. This feed the lamp at about 0.95A, what mean about the 100W into the 110V arc lamp (the MH have 77V only, so the power is lower; the ballast more-less maintain the current)
With the MH ballast you only have to deactivate the ignitor.

In similar way the 80W MV could be powered from 50W MH ballast. The 50W MH ballast current is about 0.75A, 80W MV lamp need 0.8A, so it would be slightly underdriven (but this is of not much issue for the MV).

The 50W MV need 0.6A, here I do not know exactly, what to use. 35W MH ballast feed 0.5A, so it may be marginally usable.

And as with the 100W MV, the ignitors have to be deactivated.

There are other problems to running lamps on the wrong ballast. For example the electrodes in the lamp can evaporate and blacken the arc tube glass, reducing the amount of light you will get (and causing the lamp to overheat even more) - to the extent where you'd be better off with a halogen without any ballast anyway

The 50W merc might work with 2 fluorescent ballasts in parallel ? Perhaps 2x 18W T8 ballasts ?