Dimming a 12V LED strip with a mosfet and PWM - Page 1
Forum section: electronics / beginners

author: andete
author_rank: Contributor
timestamp: September 23, 2012, 04:51:40 pm
content: This week I spent some time playing with dimming 5m of 12V LED string with a mosfet and PWM.I documented my findings on my blog athttp://joost.damad.be/2012/09/dimming-12v-led-strip-with-mosfet-and.htmlAs I'm self-learned and relatively new to EE I'd love to hear what people think about my observations especially if they are rubbishJoost

author: Luckymat
author_rank: Contributor
timestamp: October 09, 2013, 07:39:20 am
content: Why you cant use just regural transistor?If you want to connect mosfer watch video on youtube from afrotechmods about transistorsAfter you watch it you can easly connect it

author: tszaboo
author_rank: Super Contributor
timestamp: October 09, 2013, 09:25:08 am
content: Your observations are accurate. Putting a shunt resistor is not wise though. Capacitor is better. It is actually good to inform people, that gate capacitance with mosfets are not negligible. And you have ringing because the LED strip has long wires, and it acts like an inductor, because the self inductance. I'm not sure if you need this sharp edges becuase your eyes are fine with some Khz PWM, no need for nanosec rise time.Quote from: Luckymat on October 09, 2013, 07:39:20 amusing PV junction transistor is usually results an inferior design in power electronics.

author: Luckymat
author_rank: Contributor
timestamp: October 09, 2013, 11:04:22 am
content: So whats the diffrent between normal transistor resistor an a small cap?Does it really metter (i mean its his setup MUCH better?)Sorry for asking becouse im beginner and i want to learn something too xD

author: zapta
author_rank: Super Contributor
timestamp: October 10, 2013, 04:21:52 pm
content: Quote from: NANDBlog on October 09, 2013, 09:25:08 amThe sharp edges reduce the heat on the mosfet. Either zero voltage or zero current thus zero power dissipation. During the ramps the mosfet has non zero current *and* voltage.Mosfets are better than regular transistors because in the on state, they have lower voltage drop and thus dissipating less power.OP, another alternative is to use off the shelf low side mosfet driver, e.g.http://ww1.microchip.com/downloads/en/DeviceDoc/22092E.pdf(there are many others), and if you want to get really fancy, you can get one of those addressable led strips (e.g.http://amzn.com/B008KEO540), they allow you to control the intensity and color of individual leds using a serial protocol. Imagine all the cool patterns you can generate;-)

author: tszaboo
author_rank: Super Contributor
timestamp: October 10, 2013, 04:46:55 pm
content: Quote from: zapta on October 10, 2013, 04:21:52 pmSure. We have 12V and 800mA, 9,6W. If this is on the MOSFET (huge overestimate) for 600ns, 1000 times a second (1Khz PWM), that is about 6mW dissipation. Does it matter?

author: owiecc
author_rank: Frequent Contributor
timestamp: October 10, 2013, 06:52:43 pm
content: Quote from: Luckymat on October 09, 2013, 07:39:20 amMosfet is a regular transistorIn this application it does not matter much but why not go with it if you have a spare one laying around. Plus at 800mA LED current a NPN will dissipate around 0.5W while this MOSFET will dissipate only ~30mW during conduction.Andete, this MOSFET is actually not very good for this circuit. You supply the gate driver with 5V. Subtracting the voltage drop on BJTs you are closer to 4.5V in the gate signal. If you check the typical output characteristics at 25°C for you MOSFET (Fig. 1 in datasheet) you will see that at 4.5V gate voltage the transistor performs much worse than with higher voltages. Still not a very big issue with the power levels you have there but you must be careful with it.

author: RSTPhysics
author_rank: Newbie
timestamp: October 10, 2013, 08:14:33 pm
content: 800mA is very little tho and that package probably wouldn't need any cooling anyway.And as said for PWM-ing a led strip you probably wouldn't need to pay much attention either to ringings and rise/fall times and such.But non the less it was a interesting project and you probably learned something in the process.