Dual / Double Voltage Divider - Page 1
Forum section: electronics / beginners

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:10:45 am
content: Hi All,I am reading a voltage using an analog input pin on an arduino. The max input is 3.3v. Using a voltage divider of R1 & R2 I can then read an actual voltage of 6.6. Beyond that the arduino expresses it's dislike of such voltages by restarting. Is it possible to further split the voltage? Is it done like this??Thank you, John

author: pmbrunelle
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:29:29 am
content: So why don't you just modify R1 and R2 to get the ratio you want?Explain why you would need to go through with this convoluted mess...

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:35:10 am
content: So that I can read 1/4 of the actual voltage and then in code I can say reading * 4 to see what the actual voltage is. So you're saying that I can instead of using two identical resistors I can use R1 and R2 of differing sizes and get the same effect?Thank You, JohnP.S. I don't want a convoluted mess, I just don't know of another way...

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:40:08 am
content: Ok, yes that's what I was looking for. I just need to adjust my R2 and my math. This is going to take a bit more research and testing, but you're definitely right.Thank You, John

author: pmbrunelle
author_rank: Regular Contributor
timestamp: November 17, 2015, 01:45:38 am
content: Yes! You can play with the ratio of the resistors to modify the ratio of the divider.So, I suggest we toss out R3 and R4 from your diagram, and directly connect Vin to the "free" end of R1.Vout = Vin * R2 / (R1 + R2)For something feeding into the analog input of a microcontroller, probably you want to pick R2 = 1 kohm.So now, you could pick R1 to obtain a suitable ratio.

author: MarkF
author_rank: Super Contributor
timestamp: November 17, 2015, 02:14:40 am
content: By the way:If you redraw your original resistors, it will be easier to see that the second pair of resistors will NOT give you the divide by 4 that you expect. I'll leave it to you to do the math.

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 02:17:49 am
content: I'm becoming familiar with this equation, but the problem that I'm trying to figure actually Vin. I've created a DC boost converter driven by microcontroller PWM. I need to know when to slow down the PWM. For this I am reading the Vout. So far it's been relatively simple. Two identical resistors and then I multiply the reading by 2. How can I determine the Vin based on the Vout?Thanks again, John

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 02:36:13 am
content: Mark I really have no idea, I can only guess and that would be an eighth.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: November 17, 2015, 02:42:19 am
content: With two identical resistors you multiply the reading by two. With non-identical resistors, you multiply the reading by a number (that is not equal to two).To calculate this number, you can use algebra to take the original equation and solve for Vin.If that seems difficult, you should set R2 = 1 kohm. Then, try different values of R1. The larger R1 is, the larger the multiplication factor.So maybe grab a variable power supply, start lets say feeding in 10 V to your divider, then you should be able to work out the multiplication factor for different values of R1.

author: Setanta
author_rank: Newbie
timestamp: November 17, 2015, 02:49:10 am
content: Can I suggest using the calculator herehttp://www.raltron.com/cust/tools/voltage_divider.asp.If you want to do the math yourself pmbrunelle gave you the formulaQuote from: pmbrunelle on November 17, 2015, 01:45:38 amRearranging this for working out Vin you get Vin = Vout * (R1 + R2) / R2If you assume you want a Vout of 5V and give R2 a value of 1kohm as suggested then giving R1 a value of 2kohm would give a Vin of 15V - Vin = 5 * (2000 + 1000) / 1000 = 15Setanta

author: bumba000
author_rank: Regular Contributor
timestamp: November 17, 2015, 03:00:14 am
content: Not sure why I didn't see that myself. Thank you very much. I'll be getting to this tomorrow.Thanks again, John

author: MarkF
author_rank: Super Contributor
timestamp: November 17, 2015, 03:57:34 am
content: Quote from: bumba000 on November 17, 2015, 02:36:13 amTake a look at thisResistor in Series and Parallel Combinationsand the Resistors in Parallel and Resistors in Series links at the bottom. Determine the current in each path and then calculate the voltage across R2 to get Vout.Resistors in series: Rtotal= R1+ R2.Resistors in parallel: Rtotal= (R1* R2) / (R1+ R2).Ohms Law: Voltage = Resistance * Current.Try to follow the examples and understand the basics on your own.I'll try to walk you through it if you can't follow it.