Large ballast on small lamp

[wooden_reefer]

For florescence lamp, it seems that a large ballast rated for high wattage bulb can operate well on a small wattage lamp.

It seems that a large ballast, say rated for 4 foot 40 W lamp, can operate a smaller bulb, say 18 inches 18 watt.

The voltage may differ a little, a few volts, but it seems OK. May be the life may be shortened somewhat.

Any truth in this?

At one time I thought that a ballast gives high voltage, but it gives AC in the 40-50 volt range.

 

[cindre2000]

Overdriven normal output fluorescent bulbs were fairly commonplace up to the mass production of HO and VHO. It does shorten the life span of the bulbs, and the light gain is only about 70% efficient, but you can do it.

 

[wooden_reefer]

Overdriven normal output fluorescent bulbs were fairly commonplace up to the mass production of HO and VHO. It does shorten the life span of the bulbs, and the light gain is only about 70% efficient, but you can do it.

But using a ballast rated for higher wattage is not necessarily over-driving, IMO.

It would be over-driving if the ballast gives a higher voltage, or the closed loop voltage does not drop by as much as designed.

I think the close loop voltage does not drop much because the wattage is small anyway.

 

[sowellj]

You have your physics a bit confused ... Power, measured in watts, is the product voltage (Volts) and current (Amps).

 

[wooden_reefer]

You have your physics a bit confused ... Power, measured in watts, is the product voltage (Volts) and current (Amps).

The answer is not in this type of consideration.

Do you think higher watt lamps use higher voltage?

The larger ballast can dissipate heat more readily when running a higher wattage lamp, certainly, but the higher high dissipating capacity may not be used when a small lamp is run.

 

[sowellj]

Not trying to be difficult, but I still think you are confused. Different ballasts will have different current draw, Higher wattage ballasts will pull more current. Voltage will be ~120 in this country. So a 250watt ballast will pull ~2 amps (2 amps x 120 volts = 250 watts). A 400 watt ballast will pull roughly 3 ampls (3 amps x 120 volts = 400 watts). Ballast characteristics are independent of the bulb.

You can overdrive bulbs to some extent, at the cost of bulb life. A good example of this is the 400 watt radium, which is rated for 360 watts. However, ballast performance as far is heat dissipation is independent of the bulb plugged in. Also, you would never want to over drive to an extreme, in other words, running a 175 watt bulb on a 1000 watt ballast would not be wise. Hope this helps.

 

[billsreef]

But using a ballast rated for higher wattage is not necessarily over-driving, IMO.

Using a higher wattage ballast on the lower wattage bulb is overdriving by definition It has to do with how the ballasts work. They do not produce output according to what bulb is attached. Ballast output is the same, no matter the bulb. So a ballast that outputs properly for 110 watt VHO lamp will still provide the same output to a 40 watt NO lamp and hence be overdriving it.

 

[wooden_reefer]

Many ballasts have a range of wattage that is applicable by design.

Sometimes a ballast is good from 14 to 22 watts for example.

Obviously when a ballast runs a small lamp, it uses less watt, otherwise it will heat up.

 

[wooden_reefer]

At least for the old magnetic ballast for the flurosence lamp, it is an induction coil, not a transformer.

The load is still the lamp itself, plus some heat in the ballast. This is no step up or step down of voltage from 120 VAC. The coil changes the instantaneous voltage.

The power is governed by the load, which is the lamp plus some heat loss of the ballast.

Interesting.

 

[wooden_reefer]

The reason I started this thread is that the previous owner, the DIYer, used ballasts designed as for 30 to 40 watt T8 and T10 for a G25T8 UV.

 

[sowellj]

Many ballasts have a range of wattage that is applicable by design.

Sometimes a ballast is good from 14 to 22 watts for example.

Obviously when a ballast runs a small lamp, it uses less watt, otherwise it will heat up.

Again, not trying to be difficult, but this is just plain incorrect. If you don't believe this, go plug a 175W bulb into a 1000W ballast. If the bulb were what determined the power delivery, there would be no need for different power ballasts.

 

[wooden_reefer]

Again, not trying to be difficult, but this is just plain incorrect. If you don't believe this, go plug a 175W bulb into a 1000W ballast. If the bulb were what determined the power delivery, there would be no need for different power ballasts.

Perhaps MH does not work in the same way. may be there is a transformer and the voltage differ for a smaller lamp. So one cannot generalize.

Many flourescent ballasts have a range of applicable wattage.

At least for the old magentic flurosent ballasts, the voltage does not differ for different wattage lamp. There is an induction coil, not a transformer. The voltage applied is the same, the ballast is just a choke on the current.

It seems that the G25T5 should run the F40T10 ballast, but I am actually using an old magnetic rated for 18 watts to run this tube. It has a starter.

 

[wooden_reefer]

I think I have the answer, as least for magnet type florescent ballast.

The ballast is simply an induction coil.

The voltage is the inductance times the rate of change of current.

So whether the instantaneous voltage is higher for a larger ballast depends of the inductance and the rate of change of the current. The inductance is based on the magnetic field and the number of turns in the coil.

If the instantaneous voltage is the same, the same ballast should run a lower wattage lamp.

Don't know if MH works the same.