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Old 08-11-2008, 02:26 AM #1
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Default Voltage Divider help

I need to split a power source directly in half.
Im using a 12v 1a power supply and am making a laser engraver.
the laser will run on 6v 500ma and so will the cnc.

I have been reading online, and a voltage divider seems the easiest way.
the math i used is from here:
http://www.wd5gnr.com/basiccir.htm

and im using this as a reference as to where to connect the 2 sets of 6v output wires:
http://i180.photobucket.com/albums/x...scort/Fig2.png

The math i have done:
6v drop voltage, divided by current draw of .5a = 12 ohms

so as illustrated in the picture above, R1 and R2 would = 12 ohms and i can connect the wires as shown.
the immediate problem i noticed, is getting 12 ohms. the closest ive gotten so far is four 47ohm resistors in parallel = 11.75ohms.

is this correct? would this be the best way to split 12v @ 1a into 2 power connections?
i am using a DDL driver on one of the connections with a red 18x burner diode... if i have 6v 500ma going to it, it should be fine with the current regulator correct? the other power connection has two stepper motors that ran fine at 6v 500ma.

Any help is greatly appreciated!


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Old 08-11-2008, 06:10 AM #2
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Default Re: Voltage Divider help

Voltage dividers are not a good way to go, because as the load changes it affects the other load and you don't get the predicted voltage divider you wanted before. *The resistors draw power as well.

Instead, you should use some power op-amps or regulator circuits to power your laser mill. *That way each line will be better isolated from the other. *Here is an example circuit for a voltage follower. *Just replace the "sensor" pot with a high-impedance pot, and set it to the halfway mark.

If you're still hell-bent on using a voltage divider to evenly split up the voltage, just choose two resistors that are equal in value (or a pot turned to 50%) and hook your leads to the two resistors. *E.g.:

[pre]
VCC
|
+------- A
/
\
/ R1
\
/
+------- B
/
\
/ R2
\
/
+------- C
|
GND
[/pre]

Have R1 = R2, and connect the laser between A and B, and the mill between B anc C.

I would not suggest you do this though, especially for these high currents, because the value of the resistors will affect how much power they waste, and how much power can be delivered to your loads. *As mentioned before, the load (your laser and CNC mill) will also affect the voltage divider, which is not good. The op-amp circuit (or a regulator circuit below) is usually the best way to provide a buffered source of power to devices, so they're not affecting the power source.

On another note, you can set up a regulator circuit as well, especially if you have some of those old LM317 kits laying around. *Just set it up in a voltage-regulation configuration (there are examples in this, see figure 1), for 6V. *Then you can just use that 6V to power your two devices. *It might be easier than the op-amp circuit as well. *

Make sure the regulators and power opamps are well heatsinked, as they are managing a lot of current.
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Old 08-11-2008, 01:59 PM #3
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Default Re: Voltage Divider help

well im looking for a simple one but i dont know much about the power fluctuations.
The article says to use an op-amp and resistors that are 2%-5% of value. i dont know what an op-amp is.
i got lost in reading, ive been reading nonstop for over a day now and am getting tired of trying to figure this out. every time i come across some new way of accomplishing the same thing... so i dont know whats best now.

i need a schematic with parts list if you can please... if at all possible, i would like to use simple parts i can find at the local radio shack.
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Old 08-11-2008, 02:02 PM #4
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Default Re: Voltage Divider help

the last time i tried connecting the DDL circuit to a power supply that was powering another device as well, the DDL circuit nearly caught on fire! literally. so i know i need to do something.

one thing i dont fully understand is what will happen when the load changes?
for example the laser should stay constant as a load (maybe change a lil as it heats up).
the CNC will be changing all the time, it activates 8 coils on 2 stepper motors independently so theres stuff always turning on and off hundreds of times per minute.

so would the CNC turning on and off all the time affect the laser?
if so, then i can understand why i would need a buffered uhh &quot;splitter&quot;? (not sure what to call it anymore)

what would you recommend?
And sorry im not the most experienced, but is there anyway i could get a schematic and parts list specifically for my application please?

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Old 08-11-2008, 02:46 PM #5
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Default Re: Voltage Divider help

yeah you're trying to draw way too much power from a simple resistor divider (these are really only useful when you just need a voltage reference in a circuit with little to no power draw)

I would recommend getting either a 6v Power supply and just be done with it, or get some voltage regulators and set them to 6v (LM317s can handle up to 1.5 amps when properly heatsinked)
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Old 08-11-2008, 03:23 PM #6
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Default Re: Voltage Divider help

so i would need a dual lm317 circuit since i need 2 outputs?
any way i can get a wiring diagram please?
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Old 08-11-2008, 04:15 PM #7
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Default Re: Voltage Divider help

If you have you 12v supply sorted, buy a 6v power supply for a car, that is what I have done on a couple of projects.....The one I have linked to needs to plug into a cigar lighter, the ones I have used in the past had flying leads....
http://cgi.ebay.co.uk/DC-CAR-POWER-A...6.c0.m14.l1318

The picture shows at bottom left 240vAC to 12v adaptor, which runs the fan, multivoltage regulator (set at 3v in this application) drives the Laser.
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Old 08-11-2008, 04:59 PM #8
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Default Re: Voltage Divider help

ah i understand what you mean! thats a great idea!

i live in the US though... this looks like it would work fine? are &quot;charges&quot; a bad idea or are they pretty much the same thing as you described?
http://cgi.ebay.com/XM-6V-DC-CAR-POW...d=p3286.c0.m14
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Old 08-11-2008, 05:01 PM #9
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Default Re: Voltage Divider help

or, i was just thinking... the bluray diodes needed atleast 6v but i suppose the 18x red would only need about 5v.
would this work fine too?
http://cgi.ebay.com/XM-RADIO-5V-DC-C...d=p3286.c0.m14
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Old 08-11-2008, 05:39 PM #10
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Default Re: Voltage Divider help

if the car power adapter says it is a 6v 1a adapter... will it consume all of the power from the 12v 1a adapter or just take what it needs?

i found a 6v 1a car adapter at radio shack.
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Old 08-11-2008, 06:19 PM #11
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Default Re: Voltage Divider help

Quote:
are &quot;charges&quot; a bad idea or are they pretty much the same thing as you described?
Though not always the case, chargers tend to be CC where as regulated supplies like this are usually CV and draw the required current.

So you need to be sure of what you need, I see you are using a DDL driver, in which case CV will do the job well.

Regards rog8811
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Old 08-11-2008, 06:37 PM #12
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Default Re: Voltage Divider help

cool, i went ahead and bought the 6v 1a car power supply from radio shack.
ill be connecting this tonight and testing it.
i will post the results

Thanks for the help!
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Old 08-11-2008, 07:10 PM #13
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Default Re: Voltage Divider help

also, the perseid meteor shower peaks tonight after midnight!
always a good show, look toward the north east.
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Old 08-11-2008, 09:12 PM #14
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Default Re: Voltage Divider help

I think it is still cloudy here, is it meant to be a particularily good year for them? Last years shower was sparse....

Good luck with the build

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Old 08-11-2008, 11:37 PM #15
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Default Re: Voltage Divider help

i dont know if they can predict that or not... but then i havent heard anything except for what the viewing conditions are going to be like in my town. we should have a clear view with the moon setting at midnight.
i have to work in the morn so im going to watch it till maybe 1am.
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Old 08-12-2008, 07:24 PM #16
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Default Re: Voltage Divider help

well i didnt see many last night... also didnt get a chance to work on the power supply.
i guess tonight is supposed to be the best viewing night, the observatory here is having a Perseid party at sundown.
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