[LTSpice] Simulate Led Strip  - Page 1
Forum section: electronics / projects

author: lfaborges
author_rank: Newbie
timestamp: August 08, 2020, 02:37:40 pm
content: Hello friends.I'm trying to simulate on LTSpice, a led strip I bought.Voltage: 12VPower: 10W / mWhite Leds: 120 leds / mColor Temp.: 4000KI'm going to use 4.4m (44W -528 leds).How do I simulate this in LTSpice?Thanks

author: OM222O
author_rank: Frequent Contributor
timestamp: August 08, 2020, 03:23:58 pm
content: 1) Why would you want to simulate a led strip in ltspice?2) you can use a constant current sink attached to a 12v source and make it draw the right amount to be 44watts, which is not very useful?3) you can also use a diode + small value resistor to allow a large current to pass through, still not ideal / useful?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 08, 2020, 03:44:14 pm
content: Hi,The answer is like this:I have explained how this works on the model.I have attached the LTspice model.Regards,Jay_Diddy_BLED strip model.asc(1.06 kB - downloaded 342 times.)

author: Ian.M
author_rank: Super Contributor
timestamp: August 08, 2020, 04:16:51 pm
content: You don't have enough data, but we can make some assumptions.To simulate it accurately you'd have to know the Vf of the individual LEDs (and at what If), and the value of the series resistor for each string of three LEDs.From the specs you've given, the If per LED must be approx 20.8 mA ( (10W/12V)/(120/3) ). From that and the Vf at that current, you can work out the dropper resistor for each string of three LEDs.For the actual modelling Jay_Diddy_B has already posted a solution that's better than mine.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 08, 2020, 04:53:04 pm
content: Hi,Ian.M +1There is not enough information to make an accurate model.It looks from the photo that the Resistor is marked 101 which is 100. This is the easy part.The resistor could also be 181 which is 180. It is hard to see from the photo.I just used the LED in the library. To improve the accuracy of the model you have to measure the VI characteristics of a section of the tape.An LED model can be created so that the VI curves of the model and the hardware match.Jay_Diddy_B

author: Ian.M
author_rank: Super Contributor
timestamp: August 08, 2020, 05:03:02 pm
content: Also, if you are driving it with a PWM, its capacitance may be significant to your design. Unless you've got a capmeter that can handle applying a bias current to forward biassed semiconductor junctions, that may be difficult to measure.

author: lfaborges
author_rank: Newbie
timestamp: August 08, 2020, 08:06:04 pm
content: First thanks for everyone for the answers.I need to make this led strip light up in a system that is powered by a constant current regulator (2.8A). I did some field tests and the input voltage was 7V.I will still test the model proposed here in the forum and in the future I will give more answers.

author: lfaborges
author_rank: Newbie
timestamp: August 09, 2020, 02:19:56 pm
content: What I've been doing is using a rectifier bridge and a capacitor, to try to light the LEDs. The problem is that the voltage has been below 12V.The only thing I can't change about the project is the 2.8A current power supplyIn the image, the green signal is the current source 2.8A and the blue signal is the measurement on the LED.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 03:02:18 pm
content: Quote from: lfaborges on August 09, 2020, 02:19:56 pmWhere are you getting the current source from in real life? LED strip is normally designed to work from a constant voltage source.LEDs have a constant voltage characteristic and around 9V is exactly what you should expect from three LEDs connected in series.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: August 09, 2020, 03:20:24 pm
content: Think about factoring in any volt drop on the supply wires to the strip if it is far away from your power source.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 04:09:32 pm
content: Quote from: Ed.Kloonk on August 09, 2020, 03:20:24 pmGood point, but if it's a constant current supply, the voltage drop is less of an issue, as long as it can increase sufficiently increase the voltage to compensate.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2020, 04:51:56 pm
content: Quote from: lfaborges on August 09, 2020, 02:19:56 pmAre you measuring the voltage across D1, just the LEDs, or the voltage across C5?The resistor R1, is part of the LED strip. Each three LED section of the strip has a 100resistor in series with three LEDs.There will be voltage drop along the strip. This means if you feed the strip from one end the LEDs will be brighter at the end you feed than at the far end.Please attach the .asc file to your posts.You only need to run the simulation for 200ms, not 5 seconds.Regards,Jay_Diddy_B

author: lfaborges
author_rank: Newbie
timestamp: August 09, 2020, 05:33:07 pm
content: Quote from: Jay_Diddy_B on August 09, 2020, 04:51:56 pmI am measuring after the capacitor.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 05:56:02 pm
content: Quote from: lfaborges on August 09, 2020, 05:33:07 pmWhat voltage did you think you'll measure?It's what you should expect. M = length = 176, which means R1's value is divided by 176, leaving only 0.57 Ohms, so most of the voltage drop will be due to the LEDs' forward voltage. If you set length to 1, the voltage will increase to 192V.

author: lfaborges
author_rank: Newbie
timestamp: August 09, 2020, 06:51:27 pm
content: Quote from: Zero999 on August 09, 2020, 05:56:02 pmI expect 12V. I tested an LM7812, but the input voltage was low and the output resulted in approximately 8V. I thought about using a TRIAC or a Zener Diode if the voltage went up too much. But I can never find the perfect model to find 12V on the LED.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 08:10:56 pm
content: Quote from: lfaborges on August 09, 2020, 06:51:27 pmThat's hardly surprising, since the LM7812 is only rated to 1A, it would have current limited.

author: ozcar
author_rank: Frequent Contributor
timestamp: August 09, 2020, 11:21:22 pm
content: As stated, the problem is how to drive exactly 528 of the LEDs from a 2800mA CC supply. As somebody already calculated each segment of strip (three LEDs) draws around 20.8mA, so 2800mA drives 2800/20.8 = 134 segments, or only 402 LEDs. 528 LEDs worth will light up, but will not be running at full brightness. Connected like that the choice is 402 LEDs at full brightness, or some larger number of LEDs with diminishing brightness of each LED as more are added.The voltaqge range that the power supply can operate over was not specified. If it can output 24V (I would guess not), you could connect two lengths of strip, each with 402 LEDs, in series, but then you have 804 LEDs rather than the desired 528.As a theoretical exercise, I suppose this could be an interesting challenge.Myself, I would drive the strip from a 12V CV supply as the maker intended.

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2020, 08:56:23 am
content: Quote from: lfaborges on August 08, 2020, 08:06:04 pmPresumably your constant current regulator outputs DC? If so, then you made a mistake in your simulation by using an AC current source.You need a constant voltage source, to power your LED strip. If it's 10W per metre and you're using 4.4m, then that's a total of 44W.I = P/V = 44/12 = 32/3ASwap the 2.8A constant current power supply for a 12V power supply rated to above 32/3A, so 4A or 5A will do.

author: lfaborges
author_rank: Newbie
timestamp: August 10, 2020, 11:54:34 am
content: Quote from: Zero999 on August 10, 2020, 08:56:23 amThe circuit in which I am going to install this LED, has this constant current source of 2.8A . So I said that it is something that I cannot change in the project.

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2020, 12:16:11 pm
content: Quote from: lfaborges on August 10, 2020, 11:54:34 amThen you're out of luck. The LED strip is completely unsuitable for your power supply

author: lfaborges
author_rank: Newbie
timestamp: August 10, 2020, 03:00:44 pm
content: Quote from: Zero999 on August 10, 2020, 12:16:11 pmDon't say that haha ​​I need to find a solution

author: Zero999
author_rank: Super Contributor
timestamp: August 10, 2020, 03:18:09 pm
content: Quote from: lfaborges on August 10, 2020, 03:00:44 pmYou need to use plain LEDs, designed to work off a constant current source.Please post a schematic of the whole thing.