Author Options:

Can someone look over this picture on Circuits with LED? Answered

Ok, After countless stupid topics and stupid questions I just made a quick circuit in 5 minutes, to see if whatever I'm thinking in my head is correct. I really do need someone to verify, I plan on doing a bigger project after I understand how resistors work and all that nonsense. According to my calculations, I have 1 LED, Green, which runs 2.1V at 20mA. To power this, I used 2 1.5V AAA batteries. So that means I have 3V powering a 2.1 V LED. I have to drop .9V, so I did .9V/.02mA and got 45ohms. I need 45ohms of resistance to deliver 2.1V of power to the LED. (In the pic I used 3x15ohm resistors, again, I don't know how to wire them either...) Now This is my wiring, I am not sure how you link up resistors, I linked them all to the negative wire which I think was wrong...Someone please verify though.


Yes, it would be best to place the resistors on the "positive" side of the circuit. So called Positive ground circuits can be, um, funny (on such a simple one as yours, it probably doesn't matter so much, but best to learn the correct way from the start). Despite the way the symbols on a schematic point, voltage travels from - to + and not from + to ground as was once believed.

voltage travels from - to + and not from + to ground as was once believed. Pardon me please but voltage does not flow only current flows

Then let me put it another way: electrons (negatively charged particles) are said to move from neg to pos. Voltage is a measure of the energy carried by the charge. Current is the rate of flow of charge. Therefor, there can be voltage without current, but current can not flow without voltage.

Just saying, you didn't have to have 3 resistors, you could have just had 1 with the same power of all 3 connected.

Power is a completely different thing to resistance, that'll just add confusion.

. Looks to me like it ought to work. As others have pointed out, electrically it makes no difference where you put the resistors, as long as they are in series. You could put two of the resistors on one side of the LED and one on the other.
. Most circuits I've seen will place the resistor(s) between the power source (battery) and the load (LED), but, as Goodhart and others point out, that's only convention ("That's the way the guy I learned from did it"). With your simple circuit, it doesn't matter.
. For simple proof-of-concept circuits such as your's, I like to use a potentiometer. Don't need to keep so many resistors on hand or have to figure out how to parallel/series what I happen to have.
. If you haven't already, get a decent digital VOM. You should be able to find a good meter for DIY work for 30-40 $US. I prefer Fluke, but they can be a little pricey.
. I learned a lot from books by Forrest Mims.

And just in case you might not know what a "potentiometer" is... it's essentially just a variable resistor. Break open any discarded piece of old electronics that has a turning knob or a sliding dial on it and desolder those, or clip them off if they have some wire attached (leave as much wire on as possible - it may come in handy). Almost all of those will tend to be potentiometers. As Nacho suggested, If you don't have one yet, get yourself a cheap multimeter (aka VOM, or Volt-Ohm Meter) from your local radioshack or equivalent convenient neighborhood electronics store. Make sure it has a setting to measure resistance (Ohms, often indicated by a little greek Omega symbol). Crank the knob or slider all the way to the right and measure the resistance across the terminals. Then crank it all the way to the left and measure the resistance. If it's not already printed on the back of the potentiometer, write what you found on a slip of paper, and tape it to your new potentiometer.

the circuit im hopeing to build puts the resistor on the negative side. but normally, resistor go on the positive side.


10 years ago

Ok, I see what you guys are saying? Just one more question, I'm a little confused. If I have a complete circuit, with a negative and positive wire, where do the resistors go? On the negative or positive wire? And If I introduced a switch, would it be smarter to put it on the positive or negative wire? Got a little confused by your answer Goodhart. After this I should be all set. Thanks in advance.

Sorry, put the resistors on the positive side of the circuit. Components normally go in the positive "leg" or side. Sometimes the Ground or negative side is called a Ground Rail. No interruptions or parts "inline".

In a simple low-power DC circuit like this, it doesn't matter whatsoever whether you put the resistors on the positive or negative side (or both). As Goodhart mentioned, there is a convention for where to put them in more complex circuits, but the reasons for that convention go well beyond what you need to worry about at this time, and I'd bet the majority of electronics tinkerers here don't even know about it either. So, for once, feel free to ignore Goodhart's advice in this case. It doesn't really apply here, and it's only going to confuse you. "Learn the correct way from the start" is a laudable principle, but I think we're more at the "learn to walk before you run" stage here - no need to lecture a toddler on the importance of proper pacing and hydration during a marathon. ;-)

"Learn the correct way from the start" is a laudable principle, but I think we're more at the "learn to walk before you run" stage here - no need to lecture a toddler on the importance of proper pacing and hydration during a marathon. ;-)

Ok, still once one gets into circuits that are affected by parasitics or power (amperage), it is necessary to think properly, which is why I commented that it will not matter in this circuit presented here, but if he soon does one, for instance an AM receiver, or courts 5,000+ v of electricity, he may be in for a "shock" as it were. ;-)

Safety first, remember :-) And so that means NEVER put the power switch on the ground leg - never.


10 years ago

I see exactly what you're saying. But again, I have too many questions and I have to ask. You said previously that electrons flow from - to +, which is true. But if you put a resistor on the + side, wouldn't like the full 3V fly into the 2.1V LED? (Considering the voltage starts from the - side anyway?)

Electrons are negatively charged, so even though they flow from - to +, the *electrical current* is conventionally drawn as flowing from + to -. In everyday practice, you almost never have to think about the actual electrons - just think of the current flowing from + to -.

Voltage is always measured between two different points in a circuit. Regardless of which side the resistor is on, for your circuit, the voltage across the resistor will be 0.9V, and the voltage across the LED will be 2.1V, adding up to a voltage of 3V across the battery.