# Voltage Divider

Thread Starter

#### Lightfire

Joined Oct 5, 2010
690
Hello,

Just read the voltage divider on AAC ebook and found it is interesting.

Now, can somebody look at my schematic and check if the lights would light...

Thanks everybody....

#### crutschow

Joined Mar 14, 2008
24,749
I don't see that the resistors serve any useful purpose. The three bulbs are in series with the 60V battery. The resistors only serve to draw addition current from the battery without performing any other function.

Don't know why it's called a voltage divider.

#### Adjuster

Joined Dec 26, 2010
2,148
Parallel resistors might in principle be used to allow lamps of different current ratings to be used in a series circuit. In practice however the different lamp types might heat up at different rates, and due to the large change in resistance with temperature, some lamps could suffer excessive voltages at every switch-on, shortening their lives.

#### wmodavis

Joined Oct 23, 2010
739
It depends on the specifications of the bulbs, i.e. their resistance, and voltage and or power rating and how you define 'light' (from dim to normal).

You show current/voltages in the diagram but those are based on the resistances of the resistors only with the bulbs not being present. The bulbs are in fact resistors too and you have not defined their specifications. Unless you know their characteristics you cannot determine from the information given what will happen.

Incandescent lamps will likely have resistance values much lower than the resistors you show in parallel with them so they will therefore be the main determinors of the voltage drops around the loop and the resistors will not have much effect.

In short the voltages and current shown are not correct unless the bulbs are removed from the circuit shown. And it is not possible to determine if the bulbs will light unless you provide their specs. Bulbs usually have a voltage and wattage associated with them.

#### Adjuster

Joined Dec 26, 2010
2,148
I should have looked at the resistor values before commenting! Something you need to understand is that ordinary incandescent filament lamp bulbs generally require at least some tens of milliamps to operate. Often their current ratings are much higher than that. Here are a couple of examples:-

A very small indicator lamp, of a kind that was more common before LEDs were available, might be rated at 12V, 20mA: a power rating of 0.24W. That is equivalent to a resistance of only 60Ω.

More powerful 12V bulbs use more current, and so are equivalent to lower resistances, for instance a 12V 60W car headlamp bulb would draw about 5A, and would be only around 2.4Ω resistance.

Thread Starter

#### Lightfire

Joined Oct 5, 2010
690
Heya,

The bulbs specifications are theer. the specs of the resistors are not indicated there.

Lamp 1 specs=0.0027 amps / 13.636 volts
Lamp 2 specs=0.0027 amps / 24.5448 volts
Lamp 3 specs=0.0027 amps / 21.8176 volts

Actually, I want the bulbs lights normal.

Actually in my calculation, each resistors would produce the same ampere and voltage as the lamps need, respectively.

need confirmatioon pls.

#### crutschow

Joined Mar 14, 2008
24,749
Parallel resistors might in principle be used to allow lamps of different current ratings to be used in a series circuit. In practice however the different lamp types might heat up at different rates, and due to the large change in resistance with temperature, some lamps could suffer excessive voltages at every switch-on, shortening their lives.
Yes, that's obviously what the circuit is trying to do. I missed that completely.

But for the given values of bulb voltages and current, the resistors serve no purpose. The bulbs would draw that current and drop those voltages without the resistors.

How can the bulb voltage be spec'd at 3-4 decimal places? Those voltages appear to be what was calculated, not what the bulb requires. What are the actual bulb specs?

#### ifixit

Joined Nov 20, 2008
650
Hello,

Now, can somebody look at my schematic and check if the lights would light...
Hi Lightfire,

No. All the bulbs are burned out. Only the resistors are drawing current.

Regards,
Ifixit

Thread Starter

#### Lightfire

Joined Oct 5, 2010
690
Yes, that's obviously what the circuit is trying to do. I missed that completely.

But for the given values of bulb voltages and current, the resistors serve no purpose. The bulbs would draw that current and drop those voltages without the resistors.

How can the bulb voltage be spec'd at 3-4 decimal places? Those voltages appear to be what was calculated, not what the bulb requires. What are the actual bulb specs?
Actually, the bulbs' specs are exactly the same as what the resistor would give volts and amps..
need clarification,pls.

#### thatoneguy

Joined Feb 19, 2009
6,359
Actually, the bulbs' specs are exactly the same as what the resistor would give volts and amps..
need clarification,pls.
After the first resistor, there wouldn't be any current for the next to bulbs to light if the first bulb failed.

Since they are different voltages, you should wire the bulbs in 3 parallel sets, each bulb has it's own series resistor, like an LED chain.

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