12V / 6V DC Question

A pair of zener diodes with say 6.2 volts reverse breakdown could be used to stabilize the voltage for questionable loads if the expected current draw was not too great. That would solve the issue of one bulb burnt out killing the other bulb.
 
rabler said:
A pair of zener diodes with say 6.2 volts reverse breakdown could be used to stabilize the voltage for questionable loads if the expected current draw was not too great. That would solve the issue of one bulb burnt out killing the other bulb.
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It could, provided the supply does not rise above 12.4 volts. The other problem would be power limits on zeners. If the bulb is 2 amps, and the diode is having to drive a 2 amp lamp, it will have to dissipate 12 Watts of power (as heat when the other bulb burns out). If these are tiny 1/10th amp indicator lamps, then it is a more workable solution.

The original poster did not indicate if the lamps were for illumination, or simply indicating a "condition".
 
If the lamps are the same (number?) they are additive. The limiting factor is current rating. They must be the same. Incandescant lamps have a curious resistance curve, the lamp being near zero when it's cold and increasing as it gets hot. As an example, when I wire my model trains for 12 volts at 1/2 amp(500mA), I use an 1157 turn signal lamp as a series current limiter. For normal running, the motor draws so little (relative) current, the lamp does not light. During worst case short circuits, the lamp lights at 1.5 amps, limiting the current to the value for that lamp. Using only one filiment lowers the "turn on" current. I use a number of 'scenery' lamps around the layout. I purchase 5 volt lamps, usually LEDs, from offshore. Connecting 3 per string in series, I feed with 12-15 volts auxillary power.

There are some shenanigans that can be accomplished with different value lamps, where 2 or 3 are paralleled to draw a specific current and then series with a different lamp of the same (total) current to double the voltage. I call it shenanigans because the calculations involve some electrical (DC) theory and are specific to each lamp. It is not something to be done arbitrarily.
den-den said:
Yes for two identical incandescent bulbs, NO if there are any electronics involved (example: LED)
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In most cases, LEDs can be run in series. They drop around 2-1/2 volts each, varying up or down depending on color. Most LEDs light at 10 mA to 20mA, there are some exceptions for larger sizes. Current is the deciding factor, not voltage, 120 volt 'drop' lights, portable work lights, will have 2 or 3 strings of 30 or 40 LEDs in series. There will be a resistor in series with the LEDs as a current limiter. The Milwaukee drop light I stripped down for parts used 100 ohms at 1 watt. A common size in cheapo flashlights as well. An imported 'Cob' light rated for 240 volts had the LEDs mounted in groups of 6, which were then connected in series. As a rule, I do not follow color temps, but there are several grades of white. Bright white, cool white, daylight, etc. Similar to flourescent tubes. . . Each such colored LED will have a slightly different voltage drop. They can be mixed as needed, but differences in voltage must be allowed for, possibly adding or subtracting a few to reach the desired voltage. The only limiting factor is current, they must be near the same.

Complex electronics can be, and occasionally are, run in series to use a higher voltage. This is done in cases where the running current is the same for both modules. They MUST have the same current draw, any change or imbalance will take one out. And other factors using inputs and outputs where the reference or ground must be fully isolated. It can be done, but is considered poor practice.

For 6 or 12 volts, I assume as DC only, there are 3 legged fuses regulators that will serve the same purpose. A 7805 or LM340-T5 will regulate up to ~40 volts down to 5 volts. It can be increased by placing a diode in series with the center (ground) lead. Each diode so placed will increase voltage output by ~0.7 volts. In use, a 6 volt lamp run at 5 volts can be seen as an indicator but is not so bright that it can be used for illumination. But will last almost forever.

7800 series regulators(TO-220) are available from 5 volts(7805) to 32 volts(7832). There are many such from different manufacturers, having different number series. The issue to keep in mind is that the difference in voltages is dropped across the regulator. To explain, assume a 7812 being driven by 24 volts with a current load of 500mA(1/2 amp). The regulator is dropping 12 volts at 500mA. The regulator is dissipating 6 watts of heat. (12 volts times 1/2 amp) A TO-220 package will require a heat sink to dissipate that much heat.

To go further involves a great deal of math and electronics theory so is best left alone. I will be happy to follow up if asked, it gives me something to think about. But takes a lot of space. It's an involved subject. . .

.
 
Thank you everyone for the great replies!
The application is wall art. My friend operates a garage and he loves nostalgic Americana type car stuff. He has the entire front end (sheet metal) of a 1939 Dodge truck high up on a wall in his garage. All of the lights on it are 12v dc and run off of a 120v ac to 12v dc power supply. He has 2 period correct amber fog lamps that are 6v that he wants to add to it. All lights are incandescent.
I will see if I can get a picture of it.
 
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