Incandescent bulbs may seem like an almost-obsolete component, but they still have an occasional role as current-limiting ballasts due to their nonlinear characteristics.
Now that incandescent bulbs are being phased out in many standard illumination applications, perhaps it’s time to find them another task besides lighting. That was my thought as I saw an article about using these bulbs as current limiters. It’s not a new application at all, ironically: it goes back many decades to the early days of light bulbs. They even had a special name for this role: “ballast lamps”
The article in the June 2017 issue of Model Railroader, “Keeping short circuits at bay with DCC” (DCC is Digital Command Control, a network and protocol standard for model-railroad control) discusses ways to deal with the problem that every power-supply subsystem must expect: short circuits and resultant excessive current flow. The obvious way to cut off the flow of potentially excessive current and so protect both the supply and the load is to use a fuse or circuit breaker; alternatively, all-electronic fuses are an increasingly popular option.
It may seem a crude throwback to use a bulb, but it is a low-cost way to limit current by taking advantage of the very non-linear relationship between current and resistance of an incandescent bulb. When the current is low, the filament’s resistance is on the order of an ohm; when the current increases towards the tens or even low hundreds of ohms, depending on the bulb type, nominal voltage, and wattage rating.
The problem and opportunity when using a bulb as current limiter is that you have to select a bulb with the appropriate voltage and current ratings to make this setup work. Also, you’re putting a resistance in series with the power-supply output, which is always a tricky thing to do since it affects regulation, dissipates power and reduces the available voltage to the load. It’s further complicated since the “alien” series resistance is not linear, but varies with the applied current. In contrast, a fuse, circuit breaker or electronic breaker is also non-linear but in a beneficial way, as each has very low resistance until the overcurrent event occurs, then goes to an infinite or near-infinite resistance.
Nonetheless, there are some advantages to using incandescent bulbs, especially 12-V automotive ones: they are readily available, and they are inexpensive. Further, the bulb gives you a visible indication, albeit nonlinear, of the current level, which you can learn to “read” with some practice. Finally, the bulb is self-resetting, so when the fault condition is cleared, the bulb is ready to go and doesn’t need replacement (unless you pumped way too much current through, which is unlikely if you size it properly.
Although we may think of using a bulb as a current-limiting device to be crude, it is still commercially viable. The NCE Circuit Protector, Figure 1, protects six circuits with 1-A lamps (with 1.75 A as an option) and is certainly easy to maintain, as you can see all the filaments as they glow or even pop.
They are not just for temporary, improvised use; you can buy properly made PC boards which carry automotive lamps to be used as current-limiting ballast such as this six-channel, 1-A/channel unit.
(Source: NCE Corp.)
Interestingly, there is major precedent for using the nonlinear characteristics of an incandescent bulb in a circuit, although not for limiting excessive current on a supply rail. The original, industry-spawning Hewlett-Packard audio oscillator used a Wien-bridge circuit with an incandescent lamp in the feedback loop, to regulate the output of the circuit without causing distortion. Their patent #2,268,872 “Variable Frequency Oscillation Generator” shows both the schematic and explains the operation, Figure 2 and Reference.
Key to the original Hewlett-Packard product, the Model 200A Audio Oscillator, was an incandescent lamp (lower left side) to maintain constant output level despite component and temperature drifts, yet without causing distortion.
(Source: Hewlett-Packard Virtual Museum)
Is the use of ballast lamps a quaint idea from a bygone era, now that we have all-electronic fuses in addition to conventional fuses and circuit breakers? Have you ever used an incandescent bulb as a current-limiting element? How did that work out?
Hewlett-Packard Virtual Museum, Model 200A Audio Oscillator, 1939