opamp out put - Page 1
Forum section: electronics / beginners

author: m3vuv
author_rank: Super Contributor
timestamp: September 15, 2019, 11:07:40 pm
content: hi all,just wondered if a MC34071P op amp output current is enough to power a regular led?

author: MarkF
author_rank: Super Contributor
timestamp: September 16, 2019, 12:07:27 am
content: That is why they have datasheets.Better question: Why consider an "Obsolete" part?Do you have 1000 of them?

author: m3vuv
author_rank: Super Contributor
timestamp: September 16, 2019, 01:21:41 am
content: no,there in a psu i have,just wondered if the output is enough to drive an led direct without transistors as want to have a cv indicator on it.

author: Jwillis
author_rank: Super Contributor
timestamp: September 16, 2019, 02:42:32 am
content: I'm learning Opamps as well and I hope that some of this is correct.By regular LED I will assume that its a Standard 5mm. Typically it will have a Maximum current rating of 20mA . LED's can and will draw more current than they are rated for if you do not have a limiting resistor in series with the LED and the source.First off you will need to know 2 things . The voltage you will operating the LED and the maximum suitable current your LED will operate at. In your case the source is the output of the OpAmp that you have chosen for your experiment.Lets choose a current that is below 20mA to be on the side of caution.Lets say 15mA. You can fine tune that to your liking later.You need to measure the output voltage of your Opamp .Use this formula to find a suitable resistor .R = (VS - VL) / IR is the resistor value .VS is the voltage source ( the output voltage of your opamp you will need to measure the voltage at the output ).VL is the LEDs forward voltage ( different colors of LED have different forward voltages. Typically Red: ~2V,Amber: ~2V,Yellow: ~2V,Green: ~2.5V,Blue: ~3.5V,White: ~3.5V)I is the current you what you want your LED to run on.Less than 20mA for 5mm stardard LEDs is best.Now the source (your Opamp) Lets look at the Data sheet which can be searched very easily.https://www.onsemi.com/pub/Collateral/MC34071-D.PDFLook at page 4 At "Output Voltage Swing" .The output swing is relative to the supply voltages.We want to know how much resistive loading can be put on the output and still have it function properly.LEDs are diodes and do not have a linear resistance the same as a regular resistor. But for the sake of simplicity lets assume that the LED has A zero resistance and we'll just use the resistance of your limiter resistor in series with the LED we discussed earlier .So if you have,for example ,a 2k resistor you can see VCC = +5.0 V, VEE = 0 V, RL = 2.0 k ohm and Out put voltage swing is 0.3 V .Ohms law V/R=I 0.3/2000 = 0.00015A = 0.15mA Not enough to power a 15mA LED and keep the Opamp stable.Although it says that the opamp has a Output Short Circuit Current of 10mA -30mA that won't guarantee stability.If you used a small transistor to pass current that is controlled by the opamp then you can light your LED.If I have anything wrong here then one of the more experience folks here are very welcome to correct and/or comment. I'm learning as well so any extra information would be beneficial.

author: MarkF
author_rank: Super Contributor
timestamp: September 16, 2019, 04:24:24 am
content: Quote from: Jwillis on September 16, 2019, 02:42:32 amAll BS.You do not start with a resistor and work you way backwards. You calculate the resistor after all else is determined.You need to know the op-amp supply voltage and how near its output voltage can be to that voltage.You will need the op-amp output voltage swing to be higher than the forward voltage of your selected LED.At this point if you have a high enough op-amp output voltage, you can select a resistor such that you have a 15mA current as a starting point:R = (Vout_max- Vled_forward) / 15mA.Also, this op-amp is capable of a 30mA output. Twice what a typical LED would need.The problem arises because an op-amp is not a digital device. You will have an output voltage swing dependent on its input voltage. Knowing this, you can set the resistance based on the maximum output voltage you expect to see (Vout_maxin the previous equation). Your LED will be brightest at this maximum signal and dimmer for lower levels.If you don't like the variable brightness, you could add a transistor or comparator to drive the LED.So, the op-amp is capable of driving a LED provided the MANY other requirements are met!

author: Zero999
author_rank: Super Contributor
timestamp: September 16, 2019, 08:47:26 am
content: 15mA is quite likely more than enough for a modern 5mm LED. Quite often 1mA is sufficient, so to answer the original question: yest, the MC34071 is more than capable of driving a standard LED, without a transistor.The MC34071P (P denotes the through hole package) is obsolete, but it's still widely available in surface mount packages.https://www.onsemi.com/pub/Collateral/MC34071-D.PDF