# How to select which resistor is required for my curcuit to reduce voltage

I am trying to connect `arduino` and `esp8266` as like this I know `arduino` runs at 5V and RX of ESP8266 should not have voltage more than 3.5V. If you see my diagram there voltage is large than need on RX of ESP8266. I know resistor can help me for this and ohm law.

``````V = R.I => R = V/I
``````

So I want to understand how should I select what ohm resistor for this cuicuit ? Can anyone explain with some calculation using ohm law ?

## 3 Answers

A voltage divider made with a 4.7KOhm between your +5V and input and a 10Kohm between your input and ground will result in a 10/14.7 ratio, giving you 3.4V on your input.

If you use a 5.6KOhm to +5V instead, your voltage will be 3.2V which may be a bit easier on the ESP8266.

The trick here is not the specific value of any resistor, but the relationship between two resistors. You need to find a divider that will change 5 into 3.3 volts, a ratio of 0.66. Thus, any resistor values where the value of the bottom one in the diagram below divided by the value of both resistors is close to 0.66 will work. My suggestion would work equally well if you chose 1K Ohm and 470 Ohm. Note that a voltage divider should only be used for signals and not for powering the ESP.

• Well, yes this is my question how to decide what ohm is best for my circuit ? how did you decide 10Kohm will give 3.4 V ? – N Sharma May 12 '17 at 7:34
• For a voltage divider, the trick is to find the right ratio between the two resistors. In your case, the 5V potential to ground is split between the two resistors, let's call the one between 5V and your input R1, and the one between your input and ground R2. The current reaching your input has passed through R1, so the potential between your input and ground is determined by the resistance of R2 divided by the sum of R1 and R2. The Wikipedia page explains it better than what I can: en.wikipedia.org/wiki/Voltage_divider – Erik HB May 12 '17 at 7:43
• To follow on from my other reply (ran out of space), you need to find a combination of resistor values that gives you R2/(R1+R2) similar to the ratio of your desired voltage (3.3V) and your original voltage (5V) 3.3/5 is 0.66. any combination of resistors that give this ratio will give you 3.3V, given 5V in. – Erik HB May 12 '17 at 7:46
• Well. "A voltage divider made with a 4.7KOhm between your +5V and input" - Do you mean "input" as RX of ESP8266 ? – N Sharma May 12 '17 at 7:49
• Alright "ratio of your desired voltage (3.3V) and your original voltage (5V) 3.3/5 is 0.66." what .66 ? Do I need .66Kohm resitor ? – N Sharma May 12 '17 at 7:51

A resistor can only drop a certain voltage when there is a certain current - as you know from Ohm's Law. If you have resistance R and current I the voltage dropped is R×I.

So if the current changes the voltage changes - and if the voltage changes the current changes.

So what do we do? Well, we have one known fixed value, and that is the 5V from the GPIO pin. And since that is fixed (at least when HIGH) we could add a fixed resistance to it to give a fixed current, right?

So let's do that. Pick a random value. Experience teaches you that certain values are better than others, but the actual value doesn't matter too much, as long as:

1. It's not so small that too much current flows out of the GPIO pin, thus damaging it, and
2. It doesn't create too high an output impedance, which we will come on to later.

10kΩ is a good figure to start with. Not only does it keep the current low enough, but it's pretty much guaranteed that you'll have one in your bits box somewhere. And we connect it to the GPIO like this (we completely ignore the ESP8266 for the moment): simulate this circuit – Schematic created using CircuitLab

When TX is HIGH it's at 5V. It's connected directly across a 10kΩ resistor, so therefore we can use Ohm's Law to calculate the current - I=V÷R = 5 ÷ 10,000 = 0.0005, or 0.5mA. (Another useful thing with 10kΩ resistors is it makes calculations nice and easy).

Now, what if we were to split that resistor in half, so it was two 5kΩ resistors in series? The circuit would look like this: simulate this circuit

Resistors in series add together, so those two 5kΩ resistors are the same as one 10kΩ resistor. We know that 0.5mA flows through 10kΩ at 5V, so the same must run through 5+5kΩ at 5V. But now we have two fixed value resistors. Because of that we can now apply Ohm's Law to each one to find the voltage drop across it.

So with 0.5mA flowing through a 5kΩ resistor we get V=I×R = 0.0005 × 5000 = 2.5V. That's 2.5V across R1 and 2.5V across R2, since they are both the same. That's exactly 50% of the 5V across each one. At the point in the middle, point "A" on the schematic, you can see that the voltage has been cut in half. Once the current has gone through one resistor the voltage has halved (dropped by 2.5V), and when it goes through the other resistor it drops the rest to 0V, which is the other 2.5V.

If we were then to connect something to point A it would see a logic signal of 0V (LOW) and 2.5V (HIGH) from our 0V (LOW) and 5V (HIGH) signal.

But 2.5V is too low for the ESP8266 to read. Well, the answer is simple. We see that if we use two resistors we divide the voltage into two. The same applies for dividing it into three. For instance: simulate this circuit

The sum is 10kΩ (well, 9.9kΩ but we'll call it 10kΩ for simplicity - a third each resistor, 3333.33Ω). 0.5mA through 3.3kΩ would be 0.0005 × 3333 = 1.66V. Funnily enough, 5 ÷ 3 = 1.66, so we have proved that it splits it into a third. That means at point A 1.66V of our 5V has been dropped (across R1), so 5V is now (5-1.66) 3.33V. Perfect for the ESP8266. Incidentally, point B would be 1.66V (3.33 - 1.66).

So now we have three resistors and the result we desire. However it's a little messy still. Remember that resistors in series add together? So why not add R2 and R3 together into a single resistor? That would be 6.6kΩ. Now I don't know of a 6.6kΩ resistor that's easily available, but 6.8kΩ ones are, though chances are you may not have one. If you did then a 3.3kΩ and a 6.8kΩ would net us near enough 3.3V to be usable.

But as I say you may not have one. So you pick resistor values that you do have. For instance you could use a 2.2kΩ and a 4.7kΩ, or a 10kΩ and a 22kΩ - anything that gives a roughly 1:2 ratio of value (the "upper" R1 resistor about half the value of the lower "R2" resistor).

Now - we get onto that mysterious phrase I mentioned earlier: Output Impedance. You recall we calculated the current that flows through the resistors - 0.5mA. The higher the value of resistors we use the lower the current. Well, that current not only flows through the resistors - a small amount also heads off into the ESP8266. The more that the ESP8266 wants for its RX pin the less there will be available for the R2 resistor. The result? Less current through the resistor = less voltage dropped across it.

The current drawn by the pin attached to point A on our circuit affects the voltage dropped across the resistors.

Too much current drawn and the voltage at point A will droop. Less will be dropped across the (current starved) R2, which means more must be dropped across R1. So point A is now a lower voltage. If that gets too low (too much current is drawn) then the RX pin can't read it properly.

Increasing the resistances decreases the current, but also decreasing the resistances increases the current. If the current is increased then the fraction of the current needed by the RX pin becomes reduced compared to what is available, so the output voltage drops less.

This output impedance, in case you're interested, is measured in Ohms, and is the same as the value of the two resistors in parallel - so (R1×R2)÷(R1+R2).

For instance, I tried connecting an ESP8266 the other day using 3 10kΩ resistors - R1 = 10K, R2 = 20K. It didn't work. You'd expect it to, given the ratios involved, however the output impedance (6.66kΩ) was too high for the ESP8266 I was using. Replacing them for 1kΩ resistors, so it was 666Ω output impedance, made it work straight away.

You can understand it easier if you think of the ESP8266 as just another resistor (it's actually more complex than that - it's also like a capacitor, so it forms a low pass filter which limits the frequency of, and deforms the shape of, the incoming digital waveform). For example: simulate this circuit

I have deliberately shown the ESP8266 as a lower resistance that it would really be (you can expect MΩ) so as to illustrate the problem clearer.

Your R2 now has R(esp) in parallel with it. So the nice clean 20kΩ is now actually (20,000 × 100,000) ÷ (20,000 + 100,000) = 16.66kΩ. That gives us a total of 26.66kΩ, and at 5V that means a current of 0.188mA. Through the 10kΩ resistor we end up with (10,000 * 0.000188) 1.875V dropped. Take that from 5V and you can see that the ESP8266 would only be getting 3.125V.

By reducing the resistors in our network the difference that R(esp) makes to R2 reduces. For instance, reducing it to 1kΩ + 2kΩ gives us an R2 total of (2,000 * 100,000) ÷ (2,000 + 100,000) = 1.96kΩ - hardly any different from the 2kΩ we started with, so the voltage difference will be minimal (3.31 as opposed to the ideal 3.33).

The simple formula for calculating the output voltage from an input voltage and pair of resistors, so you don't have to go long-hand through Ohm's Law every time (though it is useful for understanding exactly what is happening) is:

Vout = R2 ÷ (R1 + R2) × Vin

I can never remember it, though, and usually use an online voltage divider calculator.

• Bravo. If only I was able to explain as clearly I would have been happy as can be. – Erik HB May 12 '17 at 10:25
• @ErikHB The trick is to read the question on your phone in bed when you wake up, then get up (a while later), have bacon and eggs, followed by a big mug of strong black coffee, and then write an answer ;) – Majenko May 12 '17 at 10:28
• @Majenko Awesome!! Thanks a lot :) +1 – N Sharma May 12 '17 at 10:34
• You made my day @Majenko :) Too much useful I went to have cofee and read it while have it :) I will try it once I resolve some issues which I am facing right now. – N Sharma May 12 '17 at 10:46
• This is the best anwer I've seen in a long time. With this simple and easily explained example, we went through many of the basic concepts of the electronics. Super helpful. – Reinherd Jul 5 '18 at 7:16

// According to voltage divider formula,

Vs=(Vo+R1)/(R1+R2), >(whereas R2 is parallel to output).

if Vs=4.5 volts,Vo=3.2V and R1=100 0hms, than R2=246 ohms.

For detail study google Voltage Divider circuits.

• With these resistor values, the current will be much too high. – CL. May 12 '17 at 11:09