Basic DC electrical/resistor question. Voltage drop
Thread Starter
Le Mans Master
Joined: Oct 2003
Posts: 6,126
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From: Streamwood illinois
Basic DC electrical/resistor question. Voltage drop
I've been toying w/ electronics for years but just recently I got into it more and am trying to learn more merely as a hobby. I purchased an Arduino and have been futzing around and have a basic question when I'm having issues understanding. I understand Ohms law but what I don't exactly understand is the exact effect a resistor will have on a supply voltage. IE, I've got +5vDC feed into a 220 Ohm resistor and I see about 1.7vDC out of it. How can I actually calculate what that voltage output after a given resistor value would be? I know the input voltage and I can input the resistance value but I just don't know how to calculate what the voltage drop will be. I've toyed around w/ multiple resistors and input voltages but just am scratching my head here. I'm betting there is more to it than just using ohms law which is why i'm asking here.
Thanks
Dave
Thanks
Dave
Last edited by T_Vert; Dec 2, 2016 at 02:22 PM.
Tech Contributor
Joined: May 2003
Posts: 7,035
Likes: 134
From: North Augusta, SC
http://www.bcae1.com/ is a great site to learn stuff like that.
If I understand the question you're asking, and am remembering it right, the simple way to explain it....
You need to know the resistance of every load in the circuit. Voltage drop across each load is proportional to that resistance compared to the total resistance of the entire circuit. 10v supply with 2 loads in series, if 1st is 2ohm and second is 8ohm, then 2v across first and 8v across second. 5v source and 2, 8ohm then 1v and 4v.
What voltage do you want? If the 220ohm is your load, and you want to feed it 3v, then you'd have to put ~150ohm in front of it.
If I understand the question you're asking, and am remembering it right, the simple way to explain it....
You need to know the resistance of every load in the circuit. Voltage drop across each load is proportional to that resistance compared to the total resistance of the entire circuit. 10v supply with 2 loads in series, if 1st is 2ohm and second is 8ohm, then 2v across first and 8v across second. 5v source and 2, 8ohm then 1v and 4v.
What voltage do you want? If the 220ohm is your load, and you want to feed it 3v, then you'd have to put ~150ohm in front of it.
Thread Starter
Le Mans Master
Joined: Oct 2003
Posts: 6,126
Likes: 19
From: Streamwood illinois
Yeah that is what I'm asking. So you must first calculate the load of the entire circuit? Do you do this w/ datasheets generally? How do they compensate for any resistance from within the circuit traces itself? That seems like a ton of work to just get to a voltage. I'm going to guess that most people use some type of voltage regulator rather than a resistor but I was hoping I could figure out a way to calculate what resistor I could use to get say 5V to 3.5V for example.
In my circumstance my load is 1 LED which I do not know the specs of. I did send 5Vdc through a 220ohm resistor to the LED and checked the current and the total of that circuit was 9.29mA which brings me to 538Ohms. So am i safe in saying that the LED has a resistance value of 318 Ohms? 538-220 Ohms from resistor)
This site here explained it well for me. It seems I do need to know the LED specs prior to designing.
Dave
In my circumstance my load is 1 LED which I do not know the specs of. I did send 5Vdc through a 220ohm resistor to the LED and checked the current and the total of that circuit was 9.29mA which brings me to 538Ohms. So am i safe in saying that the LED has a resistance value of 318 Ohms? 538-220 Ohms from resistor)
This site here explained it well for me. It seems I do need to know the LED specs prior to designing.
Dave
Last edited by T_Vert; Dec 3, 2016 at 11:15 PM.
Tech Contributor
Joined: May 2003
Posts: 7,035
Likes: 134
From: North Augusta, SC
If you want 3.5v into the LED, then your resistor needs a 1.5v drop.
Assuming the LED is 318 ohms.
1.5/5 = .3
.3 = x/(x + 318)
x=136
Idiot check... 136+318=454, 136/454=.3
Put a 136 ohm resistor in series with the LED and the LED will have 3.5v
Assuming the LED is 318 ohms.
1.5/5 = .3
.3 = x/(x + 318)
x=136
Idiot check... 136+318=454, 136/454=.3
Put a 136 ohm resistor in series with the LED and the LED will have 3.5v
Tech Contributor
Joined: May 2003
Posts: 7,035
Likes: 134
From: North Augusta, SC
I don't know how to tell by looking at a diode what type it is (or if you can). Google may be able to help with that.
You'll have to know something about it in order to calculate what you need. If you don't know the manufacturer specs of the LED you can always hook it up in a test circuit and take measurements. For an existing circuit you'll just have to poke at it with a meter.
What are you trying to do?
What are you trying to do?
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