Light Bulbs
The following topic are covered in this section
1) LIGHT BULB CURRENT RATINGS AND DCC COMPATIBILITY
2) PROTECTION BACKGROUND
3) LIGHT BULBS PROTECT BY REDUCING THE VALUE OF THE SHORT CIRCUIT CURRENT
4) LIGHT BULBS ARE NOT CIRCUIT BREAKERS
5) WHICH IS BETTER? DCC CIRCUIT BREAKER OR BULB?
6) BEST APPLICATIONS FOR LIGHT BULB BASED CIRCUIT PROTECTION
7) HOW DO BULBS WORK?
8) HOW ARE BULBS RATED?
9) HOW DO BULBS PERFORM CIRCUIT PROTECTION?
10) BULBS REDUCE THE CURRENT. THEY DO NOT LIMIT IT OR CLAMP IT.
11) WHAT IS REQUIRED OF A BULB TO ACT AS A SUCCESSFUL CIRCUIT PROTECTOR WITH DCC?
12) HOW DO I PICK A SUITABLE BULB?
13) ADDITIONAL INFORMATION
1) LIGHT BULB CURRENT RATINGS AND DCC COMPATIBILITY
2) PROTECTION BACKGROUND
The application of concern is dealing with a short circuit where hazardous current are flowing. Hazardous current are current of sufficient level that they can cause real damage to equipment or wiring. Basic Circuit protection involved detection a hazardous current and removing or modifying it such that no damage can result. The simplest form of circuit protection involves the concept of using a simple ON-OFF switch that is wired in series with the circuit that is subject to a short circuit event. If you can turn the switch off, you can stop the hazardous current from flowing. The most common form of this type of protection can be found in you household AC power panel in devices called "Circuit Breakers". The name implies the actions of a switch that somehow is automatically thrown in the event of a fault resulting in hazardous current.
3) LIGHT BULBS PROTECT BY REDUCING THE VALUE OF THE SHORT CIRCUIT CURRENT
A Light Bulb when wired in series with a section of track can be used to implement a circuit protection function. When a short circuit occurs, the current must flow through the lamp. Bulbs are like resistors in that they can REDUCE the value of the current flow to something much less than the value of current that flowing when the short circuit started.
4) LIGHT BULBS ARE NOT CIRCUIT BREAKERS
A Light Bulb are similar to a circuit breaker but are not a true circuit breaker. Why? With a short circuit, the circuit is never broken which means the current never stops flowing. A True Circuit Breaker (ON-OFF switch) turns off and "breaks the circuit" which forces the current to zero value.
5) WHICH IS BETTER? DCC CIRCUIT BREAKERS OF BULBS?
Depends on your protection goals, train operating conditions and risk one is willing to take.
CIRCUIT BREAKERS OFFER ZERO DAMAGE TO EQUIPMENT NO MATTER WHAT IT IS. No current flowing means zero heat generated anywhere.
BULBS OFFER THE LOWEST OUT OF POCKET COST AT SOME RISK OF DAMAGE TO EQUIPMENT.
Under a short circuit, current flowing versus none may or may not be hazardous depending on what is being protected. Heat sensitive parts or small wires involved in the short circuit creation path that must carry the bulb current may heat up and deform or worse burn up. You cannot assume the only the bulb will get hot.
BULBS CANNOT SUPPORT A LOT OF ENGINES RUNNING.
If used as a power district protection, there become a operating problem if the trains has more than one engine (HO using #1156) bulb. The problem appears since train current must run through the bulb too even though there is not short circuit. If enough current is drawn by the trains or trains, the bulb will start to glow resulting in filament resistance going up. The bulb will begin to drop some of the track voltage across itself resulting in less track voltage for the train. You will actually see the bulb glow vary with the speed of the trains. The loss of track voltage means the train will lose power limiting the trains speed and pulling power. The throttle does not seem to control the trains speed very well. If you could temporarily bypass the bulb with the same train running, you would see a big difference in the trains performance. The trains speed will suddenly increase with the bulb out of the picture and respond to throttle speed changes as you expect. Conclusion: only use bulbs as power district protectors if you have simple trains with single engines.
6) BEST APPLICATIONS FOR LIGHT BULB BASED CIRCUIT PROTECTION
1) Lower Current DCC systems: Most DCC circuit breakers only work with high current booster (5Amps typically). Many can work at a lower current level. However when the current level gets really low as found in small DCC systems (2Amps or less), there are no DCC circuit breaker options.
2) Frog Power Protection: The need for this occurs when a turnout uses a power frog who's polarity depends on the position of the pointrails. A train going against the turnouts pointrails will have the situation where a metal wheel enter a frog who polarity is set to the opposite value of the rail the wheel is riding on entering or leaving the frog will create a short. What happens when the same wheels reach the point rails....well that is a different problem. There are two options here.
2A) By using a small bulb wired in series with the frog only results in the bulb lighting up and no short circuit stopping the train.
2B) DCC frog Juicers. These are specialized devices that will power a frog automatically addressing both polarity issue of the wiring and a standing short circuit on a frog. The cost of a singlel frog juicer is the same as a basic DCC decoder so they are expensive solutions compared to a frog. They are compatible with low power DCC systems. Frog Juicers are made by "Tam Valley".
7) HOW DO BULBS WORK?
Bulbs have resistive filament which is a very special type of wire that when sufficient current flow through it will glow and create light. The filament get very hot with the temperature of the filament directly related to how bright it gets. Since there is no oxygen inside the bulb, the filament cannot actually burn and be consumed. The filament simply takes the heat and glows.
8) HOW ARE BULBS RATED?
Given the filament is resistive, a voltage applied to the bulb will result in a current value flowing through it. The current flowing times the applied voltage on the bulb is the watt rating of the bulb.
Volts x Current = Watts = Heat. ---> BULB WATTAGE = BULB BRIGHTNESS
Hence this why bulbs are rated in watts for a given standard voltage like 120V for houses or 12V for cars. Higher wattage bulbs will have more current flowing in the filament which allows the filament to get hotter and glow brighter. Hence we associate bulb brightness with it Watt rating.
9) HOW DO BULBS PERFORM CIRCUIT PROTECTION?
Simple ON-OFF switches work by offering TWO bi-polar opposite resistance values to the current flow.
a) When a switch is turned on, the resistance is almost zero.
b) When the switch is turned off, the resistance is almost infinite.
Like a switch, the bulb filament resistance has TWO resistance values. The Filament resistance depends on the filaments operating temperature. The two common temperatures are:
1) COLD FILAMENT when the bulb is OFF. This is a very low resistance value. This value is normally associated with the bulb inrush current value which is not an electrical parameter of concern here but important in how it works.
2) HOT FILAMNET when the bulb is ON. This is a resistance value that is 10X or more higher than the cold value.
KEY POINT: The 10X change in bulb resistance mimics the resistance values of a ON-OFF switch. It is these TWO VERY DIFFERENT RESISTANCE VALUE STATES that allows the bulb to offers itself as protective circuit device.
10) BULBS REDUCE THE CURRENT. THEY DO NOT LIMIT IT OR CLAMP IT.
A time zero when a short circuit is created, the current flowing is above the maximum booster current rating. The bulbs low cold filament resistance is not enough to limit the booster current. However, the high current flows through the cold filament starting it to heat up. The filament glows and the filament resistance rise above the cold filament resistance value. The process continues until the bulb reaches is maximum brightness and at the same time it highest filament resistance value. Only when the resistance rises above a level that can reduce the booster current flow below the booster current rating does a successful bulb protection event take place. The point is the current is reduced down to a lower level equal to the bulbs current rating and that it takes TIME for this to happen.
To say the bulb limited or clamp the short circuit current it INCORRECT. These terms imply that at no time did the bulb ever allowed the current to exceed the bulbs current rating. That is simply not not what is physically happening. Full booster current flows at the start of the short circuit which is well above the bulbs current rating.
11) WHAT IS REQUIRED OF A BULB TO ACT AS A SUCCESSFUL CIRCUIT PROTECTOR WITH DCC?
It must be characterized to be compatible with the DCC boosters circuit breaker function. Circuit breaker have two parameters.
a) Current Trip Value which is the same as the booster current rating.
b) Delay time to trip and shutdown. Unspecified time parameter unique to booster.
A DCC circuit breaker works by sensing the current flow that is above the trip level. It starts a countdown timer and when the timer expires it checks to see if current is not fallen BELOW the trip. If it has not fallen below the trip level, the booster shutdown resulting an loss of layout power. If it has, the power is not lost and the layout can continue operating as if nothing happened. These are the constraints the bulb must work under to be successful.
This means the bulb must have a:
a) hot filament current that is less than the booster current trip level.
b) low filament heating delay time (gets hot fast) BEFORE the booster timer times out.
Simply put the bulb must react faster than the booster in terms of controlling the short circuit current. It is a race between the bulb and the booster to see who will address the short circuit first. You want the bulb to win!!
12) HOW DO I PICK A SUITABLE BULB?
STEP 1: DETERMINE THE APPLICATION
a) A bulb implementing power districts need to have it current rating that is close to the booster rating but never equal or greater.
b) A bulb implementing frog protection needs to have it current rating about the same as the current draw of a single locomotive.
STEP 2: CALCULATE A GIVEN BULB CURRENT.
Since we are talking automotive light bulbs for DCC applications, the bulb voltage is always the same. Knowing the bulb wattage and the bulb voltage, we can use ohm law to calculate the bulb current.
Bulb(Current) = Bulb(Watts) / 13V (12V is the lowest voltage with a fully charged battery. 14V when being charged. 13V nominal)
Check the current value against your requirements you chosen in step 1.
STEP 3: TEST THE SELECTED BULB.
Connect the bulb to the track to light it up. If the light bulb lights up WITHOUT the booster shutting down, the bulb will be successful. IF not then the bulb is taking to long to heat up. Try again with a different of lower current bulb.
13) ADDITIONAL INFORMATION
Marcus has a section on using light bulbs and his experience along with others.