# Resistance??



## naijin (Jul 27, 2013)

Hi there, I have a 48volt dc power supply, one of the switches that connect to it has a 12 volt globe, does any one know what size resistor I will need to drop the 48 volts down to 12 volts?  :thinking:

Thanks in advance
Nick


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## Halligan142 (Jul 27, 2013)

Not as simple as adding a resistor.  You'll need an actual electrical circuit with different components (transistors etc.)  Your requirements (amperage, efficiency, etc) will determine the components.  48v is a common voltage for golf carts so there are many off the shelf products to accomplish this ranging from $10 Chinese questionable quality to hundreds of dollars.  You can snag an inexpensive one of e-bay  http://www.ebay.com.au/itm/250869419023


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## arvidj (Jul 27, 2013)

naijin said:


> Hi there, I have a 48volt dc power supply, one of the switches that connect to it has a 12 volt globe, does any one know what size resistor I will need to drop the 48 volts down to 12 volts?  :thinking:
> 
> Thanks in advance
> Nick



Regretfully more information is needed to answer the question.

We know that you would like to drop 36 volts. Ohms law ... E = I * R ... voltage = current times resistance ... would give you the resistance value with a little rearrangement ... E / I = R ... so all that is needed is the amount of current that will be flowing thru the resistor. Note that the current flowing thru the resistor will be the same value as the current flowing thru the 12 volt globe so it becomes (36 divided by the current requirement of the globe) and you have the resistance value. I'll also note that the '12' is the globe voltage, not the globe current, so be sure to get the globe current to use in the equation.

The other necessary calculation is the wattage requirement of the resistor. Resistor 'drop' voltage by converting the energy to heat. Wattage is determined by W = E * I ... wattage equals voltage times current ... so you have (36 times what ever the current is that you determined earlier) will give you the wattage of the resistor you will need.

So determine the current that the globe will draw, calculate the resistance, calculate the wattage ... I would increase the wattage by at least 25% just to on the safe side ... and you have your resistor requirements.

Good luck, but I suspect you will have some difficulty finding an appropriate resistor.
Arvid

Halligan142 replied while I was typing this. He provides a very practical solution as the 'resistor' solution, though you can do the math, as I indicated, is not practical nor easily obtained.


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## Walt (Jul 27, 2013)

What kind of light source is the globe?

My knowledge of electricity could be fit into a thimble and rattle around, but I think you need to know or measure the resistance of the light, then put a resistor that is 3x the light's resistance. The voltage in a series circuit should drop across each component proportionally to the resistance: if the light is 1/4 of the total circuit's resistance, it should see 1/4 of the total voltage.

LEDs are a little tricky because they typically have a narrow range of voltage they work correctly with, and their resistance drops as they warm up.

Walt


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## Ray C (Jul 27, 2013)

As someone else mentioned, you're be better off getting some type of solid state variable controller for this.  Theoretically, a resister could be used to drop the voltage to 12V but, a good bit of current will always flow through the resistor and it will need to have a power rating (in Watts) of about 2/3 the power rating of the globe.  That's a significant amount of heat that the resistor will always give off.  Also, if you add a resistor and drop voltage across it, the current in the loop will be fixed (largely) by the resistor -and there might not be enough current to make the globe element get hot enough to glow.  Finally, I'll assume the globe has a tungsten filament.  There's one more little complication due to the non-linear resistance of tungsten filaments and that makes it a little tricky to theoretically calculate a series voltage drop circuit...

Ray


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## Tony Wells (Jul 27, 2013)

Possible to switch to a 12 VDC supply, or a 48VDC lamp? 4 lamps in series?


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## Codered741 (Jul 28, 2013)

You could use a DC-DC converter, depending on the amperage requirements.  

Something like this:

http://www.automationdirect.com/adc...(Electrical)/DC-to-DC_Converters/PSP12-DC24-2

Good Luck!

-Cody


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## Ray C (Jul 28, 2013)

Here you go DC-DC converter... couple bucks.  


http://www.ebay.com/itm/DC-DC-HRD-C...o-12v-3A-Switching-Power-Module-/350585958005


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## KMoffett (Jul 28, 2013)

naijin said:


> Hi there, I have a 48volt dc power supply, one of the switches that connect to it has a 12 volt globe, does any one know what size resistor I will need to drop the 48 volts down to 12 volts?  :thinking:
> 
> Thanks in advance
> Nick



Is the globe (bulb?...lamp?) removable? Can you remove it and read the numbers on the metal base? 

Ken


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## jrs591 (Jul 28, 2013)

naijin said:


> Hi there, I have a 48volt dc power supply, one of the switches that connect to it has a 12 volt globe, does any one know what size resistor I will need to drop the 48 volts down to 12 volts?  :thinking:
> 
> Thanks in advance
> Nick



The size of the resistor depends on the current that the bulb draws.


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## naijin (Jul 30, 2013)

Thank you to every one for your suggestions, I think the easiest solution would be to leave the light globe disconnected, after all it is very visible to the naked eye if the switch is or or off, just thought it may look fancy on the control panel. )


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## videotrack (Aug 14, 2013)

naijin said:


> Thank you to every one for your suggestions, I think the easiest solution would be to leave the light globe disconnected, after all it is very visible to the naked eye if the switch is or or off, just thought it may look fancy on the control panel. )



Hi
Just tell us the wattage of the globe, I suspect it is a low wattage indicator lamp? If so, a low wattage resistor will work. Knowing the wattage is also critical for the equation. Tell us the wattage of the lamp, and I'll tell you the exact resistor to use complete with ohm and wattage values. If it is an indicator lamp,this is very easy to do.
Regards
Trevor


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