OHMs law experts!

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jseabolt
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OHMs law experts!

Post by jseabolt » Wed Jan 17, 2018 5:09 am

I've looked on the internet for a voltage drop calculator and everything I find is not what I am looking for. Most calculators are asking me for the size and length of the wire. I realize that if you a running a wire, the longer the wire, the more the voltage drop. So the longer the run, the larger the wire required to avoid a voltage drop. I guess they are assuming I am running wiring in my house but that's not what this is all about.

If you had a 6 volt bulb and the supply was 12 volts and you didn't want to overpower and blow the bulb, you could add a resistor either before the bulb or between the bulb and ground. Does it matter which? Typically corroded grounds on our Fiats is what causes voltage drops.

If you want to drop the voltage (DC) by adding a resistor, is there a way to determine what size resistor is needed? Or is it not that simple? Do you need to take into account how much current the device draws? So the more the current the device draws, the more resistance is needed to drop the voltage. So in other words if a device draws 1 amp at 6 volts, you would need a certain size resistor to drop the input voltage from 12 volts to 6 volts. However if that device draws 4 amps, you would need a larger resistor to drop the voltage from 12 to 6 volts.

I wondered if I could install a potentiometer after the supply voltage and adjust it while measuring the voltage with a voltmeter until I get the voltage I am looking for. Then measure the resistance across the potentiometer to determine what size resistor is needed. I just don't think it's that simple.

I realize the proper way to doing it is to use a step down transformer in case the supply voltage was to drift. Just wondering if it's possible using a simple resistor.
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Re: OHMs law experts!

Post by AZRuss124 » Wed Jan 17, 2018 1:31 pm

excuse any errors in this thought. It's been 50 years since I messed with such trivia. ;) Volts=resistance (ohms) times amps. If you have 2 identical resistors in series (daisy chain) , then the sum of the 2 make up the total resistance. When they first started lighting runways, they would connect all the lights in series. Run the wire from the power supply to the first light on one side of the runway to the end and then run the wire back on the other side of the runway and ground at the beginning. Planes kept missing the runway because the lights on the high volt size were brighter than the ones on the ground side. They solved this problem by running the lights in parallel (like railroad track ties), that way all lights saw the same voltage. Your problem is light bulbs have very little resistance, since they are really a coil and not a resistor. Also, if you have other light bulbs in the circuit powered by the same switch or fuse, then your calculation would need to consider the entire circuit. Lets assume your light bulbs is part of a system but has its own ground. What I would do it take another identical light bulb and mount it is series with the your light and hide the new light bulb.
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Re: OHMs law experts!

Post by andy » Wed Jan 17, 2018 1:51 pm

Another factor in Richards analogy is that bulbs in series if one light burns out it would take out all the lights (think of a string of christmas lights).
Parallel would solve that issue to.

As to James' issue, I don't know of a good solution. A step down transformer would not work because this is DC, transformers only work on ALternating current.

About the only thing I can think of is two 6 volt batteries, in series, this would create 12 volts, then a tap located between the batteries would give 6 volts.

I don't have good diagram, but somthing lie this, the center connection to grould should be 6 Volts, the right hand connection to ground would be 12 volts.
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I am not sure how well this would work in the real world, but the theoroy is correct.
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Re: OHMs law experts!

Post by scrapironchef » Wed Jan 17, 2018 2:00 pm

You could either put a 6 volt voltage regulator in line or a string of Diodes, each diode causes a voltage drop of about .6 volts. a little messy but old school hack.

Why for not just change to a 12 volt lamp?
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Re: OHMs law experts!

Post by aj81spider » Wed Jan 17, 2018 3:38 pm

If the bulb is designed to be connected directly to 6 volts then measure the resistance of the bulb. Ohms law will tell you how much current the bulb will draw (V/R=I or 6V/Resistance = current in amps). Hopefully what you calculate will match the bulb spec. With the specs you can also calculate the resistance of the bulb (V/I = R).

If you want to drop half the voltage in the bulb and half in the resistor then you need a resistor the same resistance as the bulb. The wattage rating of the resistor you need will depend on the current. VI = Power, so 6V*Current = the wattage rating.

You can put the resistor on either side of the bulb.

It seems like a lot of work figuring out, purchasing, and mounting a resistor, so I also have the question of why not just buy a 12V bulb?
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Re: OHMs law experts!

Post by rridge » Wed Jan 17, 2018 4:12 pm

As usual James, you are probing the ragged edge of our collective understanding of volts, amps and ohms. The problem with using a resistor to drop voltage is that resistors drop voltage by converting part of the energy passing through them to heat. The classic application in many cars was the multi-speed blower powered by a universal motor and with low speed being provided by dropping the voltage through a resistor bank mounted in the fan's air flow. A simple resistor's heat is probably not a problem on a single low watt bulb.

If you are doing a series of bulbs or a single high draw lamp there are a variety of electronic devices that knock down voltage without generating too much heat. Technically these "step-down voltage regulators" are not transformers but they are DC transformer equivalents. More info including how to calculate the simple resistor size here: http://www.studebaker-info.org/Tech/6-12V/6-12-6.html
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Re: OHMs law experts!

Post by spider2081 » Wed Jan 17, 2018 6:10 pm

Not sure how complicated you want to make this. I would use a 6 volt Zener diode Here is a link to you component values.

http://www.calculatoredge.com/electronics/zener.htm
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Re: OHMs law experts!

Post by jseabolt » Wed Jan 17, 2018 8:13 pm

I was trying to stay under the radar but here's the deal. Ultimately I want to convert my Trabant from a 6 volt electrical system to a 12 volt electrical system. Over the years if I wanted to add something that runs on 12 volts, I'd have to run it through a step up converter. Such as the radio, LED backup licence plate frame, electronic ignition, fuel gauge, and cell phone charger. I've also replaced the 220 watt generator with an 60 amp AC Delco 10SI alternator with an internal 6 volt regulator for more power.

Some 6 volt bulbs are more expensive and have to be ordered from the U.K. because they don't seem to be available in the US. Such as the 6 volt 50/55 watt H4 headlamp bulbs and 6 volt 55 watt H3 auxiliary light bulbs.

I'm waiting for the battery to die before doing the conversion since that will be the most expensive component.

From what others have said, not everything has to be swapped out. The following components have to be switched:

Generator (I could pretty much just swap the voltage regulator in the AC Delco alternator)
all bulbs
Ignition coils (yes two of them)
horn
flasher module
wiper motor (however others have used a solid state reducer like this one without replacing the wiper motor)

Image

The items that you can get away with without changing are:

Starter motor and solenoid (but will spin twice as fast)
Headlight dimmer switch (same type of device used on Volkswagons and Audis, just an electro-magnetic switch that rotates between high and low beams)
Switches and wiring (actually 12 volts will cut the current in half so that's a good thing)

So I was just curious if I could install a resistor between certain components and it's ground to do the same thing. Such as the horn.

So what Richard says is, measure the resistance across the connectors on a device (like the horn), then add a resistor of the same value between the component and the ground.

Could the same be done for the ignition coils?

The light bulbs would probably be the cheapest items to replace. So I'm not really sure why I used that as an example to begin with.

My idea of using a resistor would be to install a male spade connector on one end and a female connector on the other and wrap the whole thing in heat shrink tubing.
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Re: OHMs law experts!

Post by spider2081 » Thu Jan 18, 2018 8:52 am

I think you would be best to measure the current draw for each item operating at 6 volts. Then calculate the wattage for the resistors required. You might find the physical size of the resistors is surprisingly large. If I remember correctly the 55 watt resistor you have in the photo is about 3/4 " wide and close to 4 " long. They should be mounted to a metal heat sink.

Here is another link that can seep up calculations:

http://www.electronics2000.co.uk/calc/p ... ulator.php
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