Why does the resistor go after the led

Hello, I am using five strings of nine LED each 6. The input power supply is Please let me understand the calculations done for the resistors so that the voltage across each LED is more or less the same. Thanks in Advance. I have a science demonstration where I connect 4 equally-spaced two-lead bi-color LEDS one red, one green inside the same LED in parallel on a piece of regular lamp cord.

When you connect these to an AC source they blink red and green alternately and appear yellow if you don't move the bulb. I have always connected them in parallel on a piece of lamp cord and directly connected one end of the cord to a low voltage AC power supply and gradually increased the voltage until the bulbs light. When you swing the cord in a circle holding the end near the power supply the bulbs trace out little arcs during their on times and the colors are alternately red and green.

My question: I have never used any limiting resistor and have done this dozens of times with set-ups that I have had workshop members construct. Luck in which case I'd need 4 R's?? At that point there are no parallel resistances to worry about, only a minor series one between source and drain. Example 2 shows how applications with high current need high power resistors, but I think it should be noted in this tutorial or another that smaller resistors can still be used to the same effect so that a smaller circuit board or device can be created.

Since R1 is 12 Watts, this could be accomplished by 2 - 10W resistors of twice the resistance, or 4 - 5W resistors of 4x the resistance Enough resistors in parallel and you could get down to smaller sizes, although there must be some equilibrium point Which one do I use for Vf? To be saf, use the smallest forward voltage you find listed on the sheet. This will the low end of the range listed until "Typical Forward Voltage". The smaller the forward voltage, the higher the current work the math above to convince yourself of that and you want to ensure that there's no way you're going to exceed the maximum recommended current for the device.

There is a table that talks about Forward Voltage and how its tied to Wavelength nm : Wavelength nm Voltage Green 2. Yes, usually all LEDs of a specific color will have the same chemistry, and so will have very similar forward voltages. So if in doubt just look at the datasheet of an LED of the same color as the unknown one.

One wrinkle here is that "high intensity" aka "super bright" LEDs should really be considered a separate "color" since they will often have a different chemistry than the garden variety LED. Turn on the power, and measure the voltage between ground and the anode of the LED. To determine a "safe" value for the resistor, start with something big, say 4.

I've repeatedly seen the advice to obtain the forward voltage by referring to the manufacture's data sheet, but I suspect that many of us beginners start out buying a grab bag of unlabeled LEDs of unknown manufacture.

Lacking that, in lieu of actually doing the calculation, I have drifted toward using ohm resistors as being more than adequate, but I feel like this is sloppy practice.

Excellent points and a good question. Let's see how it works out You pointed out that many folks will have leds with no data sheets. Now days, 20mA will damn near blind you. I have been loading boards for leds that only indicate if a power supply is on or not, for example, with around 1mA. Plenty bright. And I used to use 20mA typical, no problem with 30mA.

But the vendors have been playing numbers games with brightness lately. And I have found that exceeding this often leads to a short life span. No need to think like "voltage goes into LED first ". Sign up to join this community.

The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Asked 3 years, 11 months ago. Active 3 years, 11 months ago. Viewed 6k times. Dominic James Sibthorp. I did notice that post, however that person has a greater understanding than I do currently, as I didn't understand a word of it.

Add a comment. Active Oldest Votes. The only difference is the potential difference in respect to earth, Instead of looking at the potential voltage going into the led you should look at the voltage across the led.

Decapod Decapod 3, 7 7 silver badges 21 21 bronze badges. According to what I can tell they are the same in both cases. However, there is another factor not discussed here so far. Resistors come in a variety of different ratings based on wattage. As a rule, lower wattage resistors are smaller than higher wattage resistors. I understand the wattage determines roughly how much current it can handle before it burns up.

The common standard power ratings of resistors are 0. Begin typing your search term above and press enter to search. Gr Maarten No difference at all. JimboZA: The pics attached prove it by the way.

A 20 microamp difference! Take a look at the attached screenshot of a circuit I made to tap voltages off from regular leds… If you placed the resistor before the leds in series, nothing changes but it is a little more confusing to look at. Robin2: Since the electrons actually flow from negative to positive But, then, the "hole flow" is from positive to negative. Experimental error 8 Cheap meter