NE555 PWM circuit - what is limiting the current to my 1W LED load? - Page 1
Forum section: electronics / projects

author: FBMinis
author_rank: Regular Contributor
timestamp: November 30, 2021, 10:35:06 pm
content: Initially I had a resistor in series with the LED to limit the current but I noticed it was not very bright. I decided to experiment and remove the resistor, connecting the LED directly to the 5V. I only measured close to 200mA passing through it, when it should be near 300mA.The LED is sold as a 1W bead with Vf of 3.6V and 320mA current at this voltage

author: thm_w
author_rank: Super Contributor
timestamp: November 30, 2021, 11:02:24 pm
content: The IRF540, your breadboard, etc. will have some resistance. Especially with Vgs at 5V.Aren't you pulsing the FET anyway with PWM to reduce brightness?Do you have a variable power supply you can connect the LED to and observe its Vf/I relationship?Oscilloscope?

author: Ian.M
author_rank: Super Contributor
timestamp: November 30, 2021, 11:10:40 pm
content: AnIRF540is *NOT* a logic level MOSFET. Its max. Vgs threshold voltage is 4V, and the rule of thumb is: you need to drive the gate with double the max. threshold voltage to fully turn a MOSFET on.Also I can just make out that the 555 is a bipolar NE555, so its output cant swing higher than two diode drops from its Vcc supply. As the gate doesn't draw any steady state current, the 555 output high is probably somewhere around 4V. Use a CMOS '555' and its output will swing rail-to-rail.TLDR: change the 555 and the MOSFET for more suitable parts!N.B. put the LED's series resistor back before you test it with a logic level MOSFET, if you don't want aDEDor toelect a pope!

author: FBMinis
author_rank: Regular Contributor
timestamp: November 30, 2021, 11:14:29 pm
content: QuoteNope, I only have one of these connected to an 8xAA battery pack:https://www.ebay.com/itm/DC-DC-Step-Up-Down-Boost-buck-Voltage-Converter-Module-LM2577S-LM2596S-Power-N2-/400985223988I tested the LED by setting the output to 6V and then I started to increment the current gradually (since the module has a constant current function). The LED became fully bright at close to 320mA and I measure the voltage across the LED, which was 3.6VQuoteNot yet, I dont.

author: ledtester
author_rank: Super Contributor
timestamp: November 30, 2021, 11:27:07 pm
content: Without getting any new parts here are some things you can try:- run the 555 circuit at a higher voltage- put a couple of your MOSFETs in parallel

author: FBMinis
author_rank: Regular Contributor
timestamp: November 30, 2021, 11:35:28 pm
content: I have some AO3400 logic-level n-mosfets here, the datasheet looks more promising. With 5V from DS, I only need 2V at the gate for a very high current:http://www.aosmd.com/pdfs/datasheet/ao3400.pdfIs my interpretation correct?

author: Ian.M
author_rank: Super Contributor
timestamp: November 30, 2021, 11:48:05 pm
content: Yep. It explicitly says "RDS(ON) (at VGS = 4.5V) < 33mΩ" and " RDS(ON) (at VGS = 2.5V) < 52mΩ" so you know its going to turn on hard enough, and its min. threshold voltage is 0.65V so the NE555 will be able to turn it off properly.Until you can get some CMOS 555 chips, add a pullup resistor (try 4K7) to the NE555 output to get it above 4V, and you should be golden.

author: FBMinis
author_rank: Regular Contributor
timestamp: December 01, 2021, 01:16:37 am
content: Ok, this is working as it should, thank you so much for the tipsi'll be putting the chip on a socket so it can be replaced with the LMC555 once they arrive.