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Topic: voltage divider help (Read 1 time) previous topic - next topic

Oct 03, 2011, 03:54 pm Last Edit: Oct 03, 2011, 03:57 pm by mblackwolf Reason: 1
Hey guys,

I am going to be using a 6V battery for my project and need to step it down to other voltage for different components (i.e. 5V for arduino mega and my controller, 4.8V for servos, 3V for some other components). I am thinking of using a simple 2 resistor voltage divider since I dont know other ways minus a transformer. I know the basic equations (i.e Vout = (R2/R1+R2)Vin) but the thing I am stumped on is choosing the resistors. I know I can calculate the resistor values using the basic equations but I am not sure on if it will be better to use a combination of lower resistance resistors vs. higher resistance assume I can find a high reistance and low resistance pair that will do the same step down. Also, If I can find the right resistor pairs how do I choose the right wattage for the resistors as I have seen they typically come in 1/8, 1/4, 1/2, etc watt resistors. Thanks for any help also any suggestions on better ways to step down the voltage are greatly appreciated.

Just ran a quick calculation and wanted to throw in a example of the different resistor combos i.e. to step down 6V to 5V you could use R1=100 ohms R2=500 ohms or R1 = 1k R2 = 5k

Grumpy_Mike

Do not use a potential divider for generating voltages for powering stuff. It is the totally wrong thing to do. Use voltage regulators.

Noorman

You do not have to step down the voltage for the arduino to 5V there is a regulator on board.
The Arduino itself has a pin that can provide 50mA at a voltage of 3.3, so no need there as well.

Basically you do not need to step down a lot, check the specs for the servo, they tend to operate at a voltage of 6V.

Btw, using resistors for this purpose is not a good idea because of the power loss in the resistors.

Use voltage regulators instead, they are made for that purpose (78xx series).

Cheers, Peter
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... bonding electrons and bits!

OK. Sorry I have a very limited electrical background do you typically build voltage regulator circuits or buy themm for specific voltages?

Grumpy_Mike

You normally buy them for a specific voltage. You only need to add two capacitors for a compleat circuit.

MarkT

No, you need a voltage regulator chip.  A resistive divider is only used for signal levels as it wastes lots of power and is hopeless at "regulation" - this means as the load changes it cannot keep the voltage constant.

A voltage regulator (checkout 7805, LM1117-50 and LM1117-33 for examples) keeps the voltage extremely constant as the load current changes, this is what you need.

Regulators come in various voltage and current ratings, and various packages.  If powering a 5V regulator from just 6V you need a "low dropout" or "LDO" regulator.  You may find your servos tolerate 6V anyway.  Don't share a regulator for the Arduino and the servos, BTW, servo motors are liable to produce a lot of noise and spikes on the power line which can disrupt (or even damage) sensitive electronics.
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Thanks alot for all of the help. I will definately look into the regulators then. Hopefully I can find them for what I need. I have to look intot he servos because I know some of the smaller ones only run off 4.8V but some of the larger ones run up to 6V. So for the arduino I shouldn't need to step down the voltage as there is an onboard regulator? So I can just attach the 6V battery directly to it? If not that is fine as I still need a 5V reg for the controller I am going to build.

Also, I was planning on tearing apart a couple of old laser pointers I had to do something with. I dont have specs but they run off 2 AAA batteries which in series is about 3V so would the pin that operates at 3.3V from the arduino work for these or would I need a 3V reg from the battery directly too them with a electronic relay connected to the arduino to control any on/off commands.

AWOL

I don't have one, so I can't check it, but does the Mega's on-board reg work down to 6V?
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

I believe 6V is the bottom limit of the input voltage on the mega. I just realized this so I may need to step up my battery. I just choose a 6V because I already had one available so it would save me buying a new one but the recommended voltage is 7V to 12V. Anyone got any idea on using the 3.3V to power the laser pointers? Also, could someone point me to a good place to get the LDO regulators from?

Grumpy_Mike

Quote
Anyone got any idea on using the 3.3V to power the laser pointers?

Yes don't it is not as simple as that, please read:-
http://www.repairfaq.org/sam/laserdps.htm

Well reading through that really quick he recommends maybe taking the button cells of the off-the-shelf laser pointer and finding the current and making a circuit where the current can't go over this. Do you guys think I could just put electrical relays between the batteries and the laser pointer and switch them on and off with the arduino. The  laser pointners I have are pen laser pointers and actually run off of 2 AAA's and not button cells. So I could easily make/buy a 2AAA battery pack so that I am not changing anything in the pointer other then replacing the on/off push botton with a relay switch that allows the arduino to press it.

Grumpy_Mike

Yes that would work, providing you don't want to turn it on and off too fast.

#12
Oct 03, 2011, 05:41 pm Last Edit: Oct 03, 2011, 05:47 pm by mblackwolf Reason: 1
no it wont be fast on off switches. More of a button press to initiate a loop that will turn it on until a button press ends the loop to turn it off. Any suggestions on good relays that could be used?

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