Any electricians? / Electrical engineers?

[DSM Storm]

Im curious if anyone can tell me what kind of resistor I would use to drop a 12 volt current to 6 volt? It would be great if you could explain why that resistor and how to choose the correct resistor, thanks.

[Viprklr]

Voltage is not current. Voltage creates current.

R = V/I

Voltage = V (6 in your case)
Current = I (I'm assuming your using a car batt. use amps)
Resistance = R

You find out your current (I) and you'll know what resistor you need. Take that number (R) and you'll have ohms.
That'll be the resistor you need.

You could also hook up a 6 volt light bulb up in there in series. That'll divide your voltage in half.

Or you could just go to the store and buy a 6 volt battery. :D

[Dynamic]

I'm sorry but I have to disagree with that.
The formula you wrote is not complete.

First you have to know how much current the thing needs you want to supply.

With this information you can do the maths.
R=U/I, where I (currenct) and U (Voltage) is known.

If you know the R of the thing to supply use this to calculate current.
I=U/R
then you can use the R=U/I

Let me give you an example:
You have a bulb needing 1 ampere at 6 V.
Your battey supplies 12 V.
You already know "I" (current/ampere): 1A
So the resistance has to be:
R=U/I = 6V/1A
= 6 Ohm
Another example:
You have a bulb with a resistance of 10 Ohm and needs 6 V.
Your battey supplies 12 V.
First the current:
I=U/R = 6V/10Ohm
= 0,6 A
Then the resistance:
R=U/I = 6V/0,6A
= 10 Ohm

This calculation are only effective if you put the resistance in series and not if put parallel.
Means one contact of the resistance is connected to the battery, the second contact is connected to one contact of the 6V lamp and the second contact of the 6V lamp is connected to ground.

ATTENTION IMPORTANT!!!!! YOU ABSOLUTLY HAVE TO CALCULATE THE ELECTRICAL POWER ON THE RESISTANCE!!!!
Do so: P = V x I
For the first example: U=6V, I=1A
P = 6V x 1A = 6W, so you HAVE to buy a resistance that holds at least 6W power. DON'T BUY SOMETHING SMALLER. It will get hot and could burn you car down. BURN DOWN!!!!!!
Also check that the wires are big enough!!!!!

And finally two little trix:
If you need 6V at your lamp and have 12V coming from the battery take a Meter to measure the resistance of the lamp and take the resistance. Because = Rn=R1+R2, where both have 6 V and the same current, so they must have the same datas. (Don't forget the power!)

If you have variable resistance at the lamp then go and buy a electronic voltagechange. It is a small electronic with a transistor that holds stable 6V.

Sorry if I didn't use the right terms but my technical dictionary lies in my office.

Hope to help,
Thomas (Communications Engineer, Germany)

[jimmoltz]

Depending on what your application is, will decide if you need a regulator or not. I agree with Dynamic's calculations and examples, but he left out two other ways to get the voltage you need. If you take two equal value resistors and connect them in series between battery and ground and tap from the middle between the two, the voltage is half of the supply. This is called a voltge divider, the value of the resistors depends on the current you need. But the output voltage changes with supply/load.

The second way is much better, there is a semiconductor device called a voltage regulator, and it provides a constant output voltage with varying loads, just like your alternator uses, but in a smaller package. You need to provide more info for us to better help you.

[Viprklr]

How's the formula incomplete? It's the same as yours. The only difference is you're using U for voltage where I'm using V for voltage.

As jimmoltz said we need more information.

Regardless, I hope this all helps you dsmstorm.

[Dynamic]

Vipklr, I agree with you on this point. It is indeed the same formula. It was quite late yesterday. I did not think of the formula beeing incomplete but the calculation way.

Jim, this is exactly what I wanted to say. As we develop only in German language here I am a bit rusted with the english terms in electronics.
I tought of the voltage divider with the first tip I gave. Look at the formula I wrote there.

[Edited by Dynamic on 07-01-2001 at 06:29 PM]

[GermanDSM]

Originally posted by Dynamic
...Thomas (Communications Engineer, Germany)
sorry German below...

Hallo Thomas bist du es??....Thomas Schlitzer aus Köln???
Wir haben glaub ich email-Kontakt gehabt.....bist doch der mit dem GVR4 oder???

achja ich hab schon den zweiten Unfall mit meinem DSM hinter mir und immer noch keine Chance zum tunen gehabt! :( :( naja zur Abwechslung geht meine Klima nicht mehr, wahrscheinlich kein Kühlmittel mehr. Hast du ne Ahnung wieviel mich der Spass kosten könnte???

meld dich mal hier im Forum....wir könnten ja in der Lounge nen deutschen Thread für uns auf machen! ;) ;) naja hoffentlich hat das keiner der Moderatoren verstanden ;)

[Dynamic]

Hallo Asif, ja ich bins. Defekte Klima, laß uns einen neuen Thread in der Lounge aufmachen, um den Platz hier für diese elektronischen Probleme freizuhalten.
Mach doch einen auf und mail mir die Adresse. Grüsse, Thomas

To all in this thread I also have to excuse me for this short trip to German.

Thank you all for the space.

DSM Storm, could you please give us further information about your problem or tell us you have got all information you needed.

[greddyhu9]

you have to take in account also how you are doing the calculations..where teh resistor will be located. Do you know if the item is in series or parallel with other items? voltage stays the same in parallel.. and splits in series. So make sure you arent soldering a resistor in parallel. If the item is in series with something in parallel.. then you have to take teh other item in account.. Its confusing to say it like that.. just done with my circuits 1&2 classes.

[Fully Spooled]

I would have suggested the simple voltage divider as jimmoltz did in his post. I think Dynamic was going the same direction but just with alot more theory.

[dsm93talon]

You can do it...
Voltage drop from 12v to 5-6v

Put a 680ohm resistor on the + wire, and you're set.

[FastGSXauto]

wrong, you need two resistors to divide voltage. If you want 6 volts, put two resistors of the same resistance in series. 6 volts will be right between them.

[dsm93talon]

putting the load BETWEEEN the reesistors is useless :| as power flows 1 way, from thee sourcee, through the load, to thee ground. Putting 1 reesistor first is the samee as putting one before and one afteer thee load.