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Topic: Quickie question on voltage dividers. (Read 1 time) previous topic - next topic

mabvs

Okay.. So as I peruse the forums here, I see a lot of advice involving voltage dividers, typically something along the lines of "+12v---10k---+6v---10k---ground"

My question revolves something I learned my 1st year as an electrician. Electricity always follows the path of least resistance. Using this as a voltage divider, what would prevent voltage drop at the 6v if you hook up a larger circuit with a fair bit of resistance? Why wouldn't more current just take the  shortcut through the second 10k resistor to ground, since that would be "easier" than going through your project?

retrolefty

Quote
My question revolves something I learned my 1st year as an electrician. Electricity always follows the path of least resistance. Using this as a voltage divider, what would prevent voltage drop at the 6v if you hook up a larger circuit with a fair bit of resistance? Why wouldn't more current just take the  shortcut through the second 10k resistor to ground, since that would be "easier" than going through your project?


Yes, any circuit wired to the junction of the resistors that represent a current path to ground or +12vdc would effect the divider and that point would no longer be +6vdc. A more through circuit analysis would be required to calculate the voltage at that point caused by the new impedance being added to the circuit.

A typical application for a voltage divider used in a Arduino is to wire the junction point to a analog input pin. Because a analog input has such a very high input impedance, it has no practical effect on the voltage divider, so the input just reads the voltage presented to it (of course your example of 6vdc would damage the pin). It's no difference if you used a digital volt meter to measure the junction point of the divider, because the volt meter has such a high input resistance it doesn't effect the divider (it doesn't 'load it down') so the meter will read one half of the applied voltage.

That make sense?

Lefty

mabvs

It does. So in effect I am right, and voltage dividers needs to be "dialed in" so to speak depending on the circuit attacked. I was just in another thread where somebody was talking about driving an arduino in a car, and some suggested they use a divider to power it. To me, that just seems a little sketchy. I am about to put my LED project in my car, but I am using a 7809 and a couple caps to power it.

retrolefty

It almost never makes sense to try and power something using a voltage divider. The supplied voltage would never remain constant because the load resistance would change as different devices are turned on and off by the arduino, which would change the dividers values. That's why there is almost always a voltage regulator involved, either the on-board +5vdc regulator or one external to the arduino powering the project.

Lefty


Bill_Kerr

Hi mabvs

Valid question.

The quick answer is as long as the effective resistance attached attached across the divider resistor is greater then 10x the resistor in the divider, in your case 10k it would have to be 100k, then there is neglible effect. 

Following a divider with an op amp design with a gain of 1 usually provides a resistance in the meg ohm of isolation.

Regards  Bill
Don't you just hate ONE D 10 T problems?

jmknapp

Electricity doesn't just follow the path of least resistance, it follows every path available, the amount in each path depending on the resistance (impedance). In other words, if you had a 1 megohm resistor in parallel with a 1 ohm resistor, some current would still go through the 1M resistor.

Joe

Techone

@mabvs

So you are planning to use the Arduino in the car. The Arduino board ( the barrel voltage input ) can take 12 V. Like retrolefty say, it contain a 7805 on-board, The 7809 will be use to power LED's via transistors or other parts.

Anachrocomputer


somebody was talking about driving an arduino in a car, and some suggested they use a divider to power it.


A voltage divider isn't going to work in this application. The current drawn will affect the voltage setting, and on top of that, the current drawn by an Arduino (or any microcontroller) is variable, which makes it impossible to set the divider correctly. Use a three-terminal voltage regulator such as the LM7808 (for 8V to Vin) or LM7805 (for direct 5V).

scottyjr

I get the impression that voltage dividers, for the most part, are used to obtain a reference voltage. Am I on the right track? - Scotty

Anachrocomputer

Yes, they're used to generate a reference voltage, but that voltage is just a proportion of the voltage that they're supplied with. If that supply voltage varies, then the output voltage will vary, too. So this makes them a poor choice if you need an absolute voltage reference, such as for an ADC (analog-to-digital converter). A better way to make an absolute voltage reference is a band-gap device such as the LM385 or TL431.

mabvs

@ Techone,

Yeah, the on board 7805 can handle 12v, however, I get anywhere from 12.2 to 13.5 in my 69 El Camino. Old car, and though a lot of it has been computerized (ignition, air intake control, etc), the alternator puts off varying voltage depending on temp, rpm, how hard the bass is hitting my subs and whatever else may or may not be running off of it. Sounds odd, but I had the same issue with the old alternator setup as well before I switched to the current 300amp model.

Anachrocomputer

A car's charging system can go up to 14.8V or so when the battery's fully charged, and the voltage will drop below 12V when the engine's not running and the battery's low.  So, the best way to run an Arduino is via a three-terminal regulator of some sort.

tkbyd

Guide to voltage dividers....

http://www.arunet.co.uk/tkboyd/ec/ec1voldiv.htm

Grumpy_Mike

Quote
I learned my 1st year as an electrician. Electricity always follows the path of least resistance

You might have learnt it but it was rubbish. Totally wrong.

Techone

@mabvs

Quote
So, the best way to run an Arduino is via a three-terminal regulator of some sort
<--Anachrocomputer is right.

Supply the "12" V line into a 7809 and the 7809 feed the Arduino. And don't forget to place capacitors filter ( at V In & at V out ) of the regulator, also may be an inductor ( C - L - C <-- a PI configuration ) at at the Vin of the 7809. Because the "nice" & "cool" classic 69 El Camino <-- a GM car produce a lot of noises & spikes in the "12" V line.

Quote
A car's charging system can go up to 14.8V or so when the battery's fully charged, and the voltage will drop below 12V when the engine's not running and the battery's low.


Yes it does. Wost case : going low to 8 V ( yes, it happen to me in the winter, when I start a diesel Ford E-350 School Bus at - 20 C ) when it fully charge, it about 13 V -14 V. <--- That is Normal operation.  At start-up, the battary can go low 9V to 10 V. Well to be accurated, mesurments of the battery with a DVM.  In general, It 12 V +/- 2 V for normal operation.
   

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