Reply
Thread Tools Display Modes
#1
Old 05-05-2005, 10:19 PM
Guest
Join Date: May 2003
Location: Great Salkehatchie Swamp
Posts: 1,196
Easiest way to make 12vdc into 6vdc

It's been a really long time, but how is the easiest way to make 12vdc into 6vdc. I want to run a 6vdc load off of the cigarette lighter in my car. Can't I just run the supply through a couple of resistors in series? Could someone help me with the math?
#2
Old 05-05-2005, 10:26 PM
Guest
Join Date: Mar 2000
Location: Finger Lakes area of NY
Posts: 1,602
Get to a well stocked auto parts store. Standard Motor Products makes a unit to do just what you want.
#3
Old 05-05-2005, 10:47 PM
Robot Mod in Beta Testing
Moderator
Join Date: Mar 2001
Location: Pennsylvania
Posts: 19,937
Quote:
Originally Posted by hlanelee
Can't I just run the supply through a couple of resistors in series?
Two resistors in series will make 6 volts in the middle of the two resistors (assuming the other ends go to +12 and ground). The problem with doing it this way is that it assumes there isn't anything else in the circuit, so as soon as you connect some sort of load between the 6 volts and ground, the load also becomes part of your voltage divider and the voltage changes. You can minimize this by having the resistance values a lot smaller than the equivalent resistance of your load, but this draws a lot of current and wastes a lot of heat, so much so that it's not practical for any significant amount of current going to the load.

What you really need is a voltage regulator. You could design your own, but you can get one off the shelf cheap enough that it isn't woth the bother (IMHO).
#4
Old 05-06-2005, 12:08 AM
Guest
Join Date: Jul 2001
Posts: 9,175
Shouldn't that be have two resisters much bigger than your load?
#5
Old 05-06-2005, 12:57 AM
Robot Mod in Beta Testing
Moderator
Join Date: Mar 2001
Location: Pennsylvania
Posts: 19,937
Quote:
Originally Posted by treis
Shouldn't that be have two resisters much bigger than your load?
No.

Let's say that your load has an equivalent resistance of 200 ohms.

If you make a voltage divider out of two bigger resistors, say 2000 ohms, then what you end up with is a voltage divider with a resistance of 2000 ohms and a resistance of both 2000 and 200 ohms in parallel. The equivalent resistance of two resistors in parallel is Req=1/(1/R1+1/R2), which in this case would be 1/(1/200+1/2000), or about 181 ohms. Whenever you have a larger resistance in parallel with a smaller resistance, the equivalent resistance is going to be just slightly less than the smaller resistor.

Now you've got a voltage divider of 2000 ohms and 181 ohms, which means your output voltage is going to be 12*181/(2000+181) or just under a volt. A 6 volt device isn't going to work very well being powered by one volt.

If we use two smaller resistors instead, say 20 ohms, then our equivalent resistance is about 18 ohms, and our voltage divider is 12*18/(18+20) so we'll get 5.7 volts out of our voltage divider, which is close enough for most 6 volt devices so we'd be fine in this case.

The problem here is that we have two 20 ohm resistors that are sucking up current all the time. With the load disconnected they are going to draw 300 mA (I=V/R), which is going to be converted into 3.6 watts of heat. Our device, on the other hand, only draws 0.18 watts, so basically our voltage divider uses 20 times as much power as the device we are trying to power. If you try to scale this up to provide a lot of current, you can see that your resistors are very quickly going to become space heaters.

(PS - it's late at night, hope I did my math right)
#6
Old 05-06-2005, 01:23 AM
Guest
Join Date: Jul 2001
Posts: 9,175
Ah yes of course you are right. My apologies.
#7
Old 05-06-2005, 01:30 AM
Charter Member
Join Date: Nov 2001
Posts: 12,684
The use of a voltage divider is unecessay in this case. A series resistor equal to the resistance of your device will give you 6 volts across the device. The current used by the device, or the power (Watts) rating should be listed somewhere on it. Just divide 6 V by the current or 62 by the Watts to get the size series resistor in Ohms.

However, investigate Booker57's suggestion first. A series resistor will be a pain and if considerable power is involved it will be big and will get pretty hot. For example if the current is 1/6th Amp (1 W power) the resistor will also have to dissipate 1 W and something like a 5 W resistor will be needed to avoid high temperatures and even it will get warm out in the open air.
#8
Old 05-06-2005, 07:35 AM
Guest
Join Date: Apr 1999
Location: Ohio
Posts: 10,448
There are four ways to do it.

1. If the load impedance is constant, you could select an appropriate series resistor and be done with it. (The series resistor and load resistance form a voltage divider.) But this technique is rarely used, because it’s usually the case that the load impedance is not constant, which means the load voltage will not be constant. Note that this is also an “unregulated” approach.

2. To improve voltage regulation at the load, you could use an external voltage divider, and then tap off one of the resistors. But this technique sucks if you’re trying to power something. This is due to the Thevenin equivalent source resistance. Normally, if you’re trying to power something, you want the equivalent source resistance to be low (or ideally zero), and the only way to make it low using a resistor divider is to use small value resistors. But this means the resistor divider itself will dissipate a lot of power. Again, this technique sucks if you’re trying to power something.

3. A linear voltage regulator. A voltage regulator is the same as #1, except that the series resistance is automatically adjusted (using a closed-loop controller) so that the output voltage remains constant. Regulators such as the 78XX series work great; they’re simple to use and very inexpensive. The only drawback is that they’re no more efficient than using a series resistor.

4. Switching regulator / DC-to-DC converter. These are the hot thing right now. There are two types… some convert the DC to a high-frequency pulsed (AC) waveform, send it through a transformer, and then convert back to DC. Others use energy storage devices (caps and inductors) and a switching circuit. The former offers galvanic isolation, while the latter is simpler, less expensive, and more efficient. At any rate, switching regulators / DC-to-DC converters are popular because they’re efficient. But they do have some drawbacks… it’s the most expense option, and the output may have some noise spikes in it.
#9
Old 05-06-2005, 08:44 AM
Guest
Join Date: Oct 2000
Location: Yes
Posts: 20,327
Quote:
Originally Posted by Crafter_Man
This is due to the Thevenin equivalent source resistance.
Thevenin/Norton Resistance
#10
Old 05-06-2005, 04:38 PM
Guest
Join Date: May 2003
Location: Great Salkehatchie Swamp
Posts: 1,196
Good ole Squink. That was what I was looking for, to the letter. Thanks.
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 05:40 PM.

Copyright © 2017
Best Topics: scissor springs no big toe intermission films toby keith car synonym lame porn fluffers gattaca meaning boot sole repair hup hup haaa tacky lawn decorations beau brummel baby koala meat hotel detective tiny welder backpage kc escorts coffin rack frog with cigarette patternless crosswords clit penis inside lane steam file shame sign language pirate cat names pronounce monticello cornish hens walmart elephant butt hair piter devries antibacterial irrigation buddhist pronunciation cloves versus cigarettes avatar the last airbender d&d does girls voice change during puberty do female horses race uhaul enclosed trailers for sale feel like i have a fever but temp is low do i refrigerate worcestershire sauce what is the difference between a drone and an rc plane bare feet on desk bruce lee lat spread glass etching cream home depot see you later alligator in awhile crocodile poem left eye lopez nude albuterol substitute over counter the newlywed game in the butt how to change the timestamp on an email i give hope to men i keep none for myself phone jack to aux cable i see you pee joke how long does pepto bismol take to work how to make really hot wings name for male ballerina 5 group cards 1st grade why do i cough when cleaning my ears south park oprah's minge episode alcohol in the freezer how to keep friends from seeing my comments on facebook a fluffer at work when is it too late to transfer colleges how to clean up car oil spills do you have to vote for everything on a ballot mrs susie had a steamboat dragon scale ballistic armor home depot vs menards