led 555 pwm mosfet driver needed? - Page 1
Forum section: electronics / beginners

author: ME
author_rank: Frequent Contributor
timestamp: March 29, 2025, 04:01:32 am
content: I need to drive 100 leds with pwm dimming, I found a cct but the fe is driven directly fro the 555,will it work as is? , the led's are 5mm super bright blue types, i dont have any datasheet so guess about 30ma each?, Thanks.

author: pqass
author_rank: Super Contributor
timestamp: March 29, 2025, 06:00:02 am
content: Apart from the MOSFET drain and source being reversed in the diagram, it should work. See attached below andsimulation here.The 555 pin 7 is an open collector transistor output so it's really just grounding the gate; turning the transistor off. It's the 1K pull-up that is turning the transistor on. A lower value pull-up will turn on the transistor faster but will increase the current through pin 7. I'm not sure what the current rating is on pin 7. Using 1K gives a 12mA current ceiling which should be fine.For 100 LEDs @30mA, that's 3A which is well within the capability of theIRF540. And should only dissipate about 700mW (I^2*Rdson=3^2*0.077).To minimize the number of resistors, you could stack 3 (or 4 if Vf < 3V) LED in-series + resistor and choose the appropriate resistor value to limit the current to 30mA ((12V - (3 * Vf))/0.030A).

author: ME
author_rank: Frequent Contributor
timestamp: March 29, 2025, 06:19:09 am
content: Instead of 1 resistor per led , Can i not use a single higher power resistor to power the whole string/ block ?.

author: pqass
author_rank: Super Contributor
timestamp: March 29, 2025, 06:32:10 am
content: It just occurred to me that you can still run the MOSFET from pin 3; having the full 200mA drive output for max speed.See attached andsimulation here.Quote from: ME on March 29, 2025, 06:19:09 amYou can stack LEDs but their collective Vf has to be less than the supply voltage otherwise they won't light-up. If you're using 12V as the supply, then you're limited to 3 or 4 LEDs in-series per resistor if their Vf < 3V. The only way you can get away with 1 resistor for 100 LEDs is if your supply is just over 300V (assuming Vf=3V) but then you couldn't use the IRF540 as it tops out at 100V Vds.

author: ledtester
author_rank: Super Contributor
timestamp: March 29, 2025, 10:40:10 am
content: Quote from: ME on March 29, 2025, 04:01:32 amI would first test the LEDs at 1 mA and see how well that works for you. You might be able to get away with just a few mAs - depending on the needs of your application.

author: PGPG
author_rank: Frequent Contributor
timestamp: March 29, 2025, 11:10:46 am
content: 470uF shorted by NMOS transistor is not popular solution.

author: ME
author_rank: Frequent Contributor
timestamp: March 30, 2025, 09:34:09 am
content: Quote from: PGPG on March 29, 2025, 11:10:46 amSo what can i do to fix that?.

author: Zero999
author_rank: Super Contributor
timestamp: March 30, 2025, 10:02:24 am
content: Quote from: ME on March 29, 2025, 04:01:32 amC3 should be across the power supply rails, not the MOSFET.The connections to pin 7 and pin 3 are reversed. Pin 3 should drive the MOSFET and pin 7 connected to a pull-up resistor. Here's a similar circuit which is correctly designed.

author: ME
author_rank: Frequent Contributor
timestamp: March 30, 2025, 03:16:07 pm
content: Quote from: ledtester on March 29, 2025, 10:40:10 amthe leds will be in parelell not series, Hence one resistor to power all.

author: pqass
author_rank: Super Contributor
timestamp: March 30, 2025, 04:43:05 pm
content: Quote from: ME on March 30, 2025, 03:16:07 pmDue to manufacturing variation, it's not recommended to put them in parallel. Even with a single resistor, the LED with the lowest Vf will take the most current, eventually die, then the remaining will have to endure more current as the resistor hasn't changed. This is why half of the LEDs in my dollar-store flashlight are dead. Seehere.

author: Zero999
author_rank: Super Contributor
timestamp: March 30, 2025, 08:30:14 pm
content: Quote from: ME on March 30, 2025, 03:16:07 pmIt's generally not recommended to use one current limiting resistor and multiple LEDs in parallel. The problem is current sharing isn't always that great.What's the supply voltage are you using?Assuming it's 12V, then it would be much more efficient to connect the LEDs in groups of three connected in series, each with its own current limiting resistor. That will give you 33 strings of three and one remaining, which will need a higher value current limiting resistor.

author: ME
author_rank: Frequent Contributor
timestamp: March 31, 2025, 02:43:22 am
content: Its going to be for an indoor grow light with 100x red green and blue and purple leds,so 400 totall in 4 controlable banks of 100 leds with 4 555 ic's etc, I have a lab 0-30v 10amp psu so will run it from that.

author: Andy Chee
author_rank: Super Contributor
timestamp: March 31, 2025, 05:24:33 am
content: The application is irrelevant.It is advisable NOT to put LEDs in parallel.That said, solar powered Christmas fairy light LEDs are connected in parallel, but; a) they are not required to be the same brightness, and b) they are operating from a current limited power source.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2025, 07:15:54 am
content: Quote from: Andy Chee on March 31, 2025, 05:24:33 amSolar Christmas light LEDs are also driven at a fraction of their power rating.Quote from: ME on March 31, 2025, 02:43:22 amPutting them in series means less current, therefore thinner wires can be used and less power wasted, both in the current limiting resistors and the MOSFET.It would be better to set power supply to a higher voltage than 12V and put more LEDs in series. If you set it to a bit over 12V, say 14V, then you cam have 25 strings of 4 LEDs. The red LEDs have a lower voltage, above 2V, so could be connected in strings of 7.Heck, I would run the 555 off a separate, lower voltage PSU, or an LM7812 and set the lab power supply to maximum. It's bound to output slightly more than 30V and use as many LEDs in series as possible, with the resistor dropping 10% of the voltage.