Two bulbs of different wattage in one AC circuit

If I connect two bulbs of different wattage in one circuit, can that lead to problems? Is that a bad idea? Does it make a difference if the circuit is serial or parallel?

• In parallel - and they are the same voltage - no problem. – Brian Drummond Apr 30 '16 at 21:16
• For proper operation in parallel, they should be the same voltage, for proper operation in series they should have the same current rating, that's it, as long as that criteria is met, the wattage is not important (but you can find the current from the wattage I=P/V) – Sam May 1 '16 at 0:34

Parallel All the mains bulbs in your house are probably in parallel so that should satisfy you that that works. Each lamp will draw the current it requires. simulate this circuit – Schematic created using CircuitLab

Figure 1. Your house wiring. This system works well because the lamps are fed from a constant voltage supply.

Let's do some easy maths using a 100 V supply. $P = VI$, where P is power, V is voltage and I is current. Using Ohm's law, $V = IR$ we can modify the power formula to get $P = I^2R = \frac {V^2}{R}$. Rearranging the last one gives us $R = \frac {V^2}{P}$. This is hot$^1$ filament resistances were calculated.

Series connected similar lamps

If you put equal lamps in series they will divide up the available voltage between them and the current will be $\frac {1}{n}$, where n is the number of lamps.$^2$ simulate this circuit

Figure 2. In this circuit both lamps have the same resistance so will drop half the supply voltage across each. Since the total resistance is doubled the current will also be halved. Since P = VI then the each lamp will be running at 1/4 power.

Series connected dissimilar lamps simulate this circuit

Figure 3. More voltage is dropped across the higher resistance / lower wattage lamp. It will glow the brightest.

If you put unequal lamps in series the one with the highest filament resistance will limit the current to the others. It may also glow the brightest (if they're all the same voltage rating).

Power calculation for each lamp:

• 100 W: $P = \frac {V^2}{R} = \frac {33^2}{100} = 11~W$.
• 50 W: $P = \frac {V^2}{R} = \frac {67^2}{50} = 22~W$.

Note the possibly surprising result that the lower powered lamp will be brighter and consume more power.

$^1$ Lamp filament resistance increases dramatically with temperature. I have not factored this into the calculations.

$^2$ Again, the current calculation will be higher than calculated here because the filament resistances will be lower when cold.

Assuming that you are connecting two light bulbs of the same voltage rating, but different power, to the same AC source, you have two cases:

• Series: the bulbs are both dimmed and cannot operate at rated power because they are sharing the voltage of the supply (voltage divider); this is usually not done for household lighting
• Parallel: both bulbs will operate at their rated power like normal; this is effectively what happens in a chandelier or when multiple lamps are powered by the same light switch

If you are using constant-voltage bulbs, then you should parallel any mixed bulbs of the same voltage. Constant-voltage bulbs include pretty much any incandescent bulb or packaged bulb-like product or integrated fixture in common use on mains voltage, including whole Christmas tree light strings as a product, and constant-current bulbs packaged to be constant-voltage, such as LED strips.

Mixing is fine. You can parallel any light intended for mains voltage. You can parallel any 12V lighting including LED strips of any length.

If you are using constant-current bulbs, then you drive them in series, so you only need one constant-current power supply. This typically comes up when building an array of raw LED emitters. Most people keep the entire array matched, but you could mix them if current is the same.

Many lamps must be driven constant-current, or they will burn up. Including fluorescent, neon, mercury vapor, metal halide, LPS, and HPS. It may make sense to "series" them to save money on power supplies, but if one bulb blows, they all go out. Some fluorescent ballasts do just that. That's not a big problem for LED emitters, which will likely outlast the driver circuit.

Incandescent lights are a relatively linear load, they work well in parallel but can also go in series if they are matched. For instance Christmas tree lights, electric railway lighting (5x120V=600V) and street lights in a few older cities, notably Detroit.