PWM LED Dimmer 12V 8A - Page 1
Forum section: electronics / projects

author: gore
author_rank: Regular Contributor
timestamp: August 23, 2015, 09:06:26 am
content: Howdy,I've found this reverse engineeredschematicof a 12V 8A LED dimmer fromEbay. Is it worth giving a shot? Anything fishy about this schematic? What about heat and the mosfet?

author: Psi
author_rank: Super Contributor
timestamp: August 23, 2015, 09:15:25 am
content: You will need a heatsink for 8A. FDD8896 looks to be SMT so a nice amount of extra PCB copper will be fine. 20x20mm etcAlso i dont like the look of that 10k pullup on the gate, seems rather high.220R seems more realistic for a quick turn-on.

author: firewalker
author_rank: Super Contributor
timestamp: August 23, 2015, 09:23:28 am
content: I would add a Gate-Source resistor and reconfigure the gate drive to a totem-pole.Alexander.

author: Zero999
author_rank: Super Contributor
timestamp: August 23, 2015, 12:25:17 pm
content: The LM358 is not very good as a comparator. Swap it for the LM393 and use a pull-up resistor. In fact the pull-up resistor values should be exchanged.

author: gore
author_rank: Regular Contributor
timestamp: August 23, 2015, 12:36:30 pm
content: What should I use as Q1? And the FDD SMD mosfet. I don't have any of those around. Can I use one of the IRF series mosfets instead?

author: gore
author_rank: Regular Contributor
timestamp: August 23, 2015, 01:03:17 pm
content: It's suppose to be a dim control for a 5 meter 12V led strip. Roughly 14.4W/meter.

author: gore
author_rank: Regular Contributor
timestamp: August 24, 2015, 12:00:08 pm
content: Oh, right, what about the Q1? It's not specified.

author: Psi
author_rank: Super Contributor
timestamp: August 24, 2015, 12:10:55 pm
content: BC547 should do

author: gore
author_rank: Regular Contributor
timestamp: August 24, 2015, 12:19:52 pm
content: Alright. I've got all the parts. Thank you everybody for all the suggestions. Gonna do some testing tonight.

author: ralphd
author_rank: Frequent Contributor
timestamp: August 24, 2015, 12:31:00 pm
content: Quote from: Psi on August 24, 2015, 12:10:55 pmOr even 2n3904.

author: gore
author_rank: Regular Contributor
timestamp: August 24, 2015, 10:01:52 pm
content: After doing some tests I've decided to scrap some components. I've settled with this design so far. It works alright. Just wondering if I could make some improvements. Perhaps safety wise.Code:https://www.dropbox.com/s/8iv6uqz6ig79ff7/dimmer1.wmv?dl=0

author: gore
author_rank: Regular Contributor
timestamp: August 27, 2015, 05:21:52 pm
content: Trying to finalize the design.schematicAny improvements I can make? Especially safety wise. It's not going operate in full swing, but there's always a possibility of a fire hazard. I suppose the power supply should be the first link to fail in the chain if something goes wrong.How do you remove the ground plane cut outs on the mosfet pin in Eagle (marked with yellow arrows)? Trying to avoid small traces where high current is present. I want the pin to be connected to the ground plane (high current return), but without the cuts.board3D boardI'd appreciate any help.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 27, 2015, 05:59:22 pm
content: Quote from: gore on August 27, 2015, 05:21:52 pmThese are not cut-outs, they are thermal reliefs. If you remove them, soldering the pins might become somewhat of a nightmare due to how long it will take to warm up the plane before the solder will wet it properly.A 0.5mm and 1mm long wide thermal relief has a resistance of less than 0.5 mOhm and you have three of them. At 3A each, you would be losing less than 5mW there, nothing worth worrying about as long as you have a fuse elsewhere to protect the traces against an output short.If I was doing a LED dimmer though, I would go with a constant-current buck or boost regulator instead of constant voltage duty cycle modulation.

author: gore
author_rank: Regular Contributor
timestamp: August 27, 2015, 06:20:34 pm
content: Quote from: DanielS on August 27, 2015, 05:59:22 pmOh, right. Thermal reliefs is a better way of describing it. Anyhow, if it's not a problem, it's not a problem. Considering the board is double sided with two ground planes, so that makes 6 contacts in total.Quote from: DanielS on August 27, 2015, 05:59:22 pmWhy's that?

author: DanielS
author_rank: Frequent Contributor
timestamp: August 27, 2015, 07:02:17 pm
content: Quote from: gore on August 27, 2015, 06:20:34 pmMore reliable/predictable and produces less EMI.Since diodes have negative thermal coefficient, driving them with a voltage source means the hotter they get, the more current they draw and this becomes a risk of thermal runaway - this is the main reason voltage-fed LEDs need dropper resistors to limit how far this can go. With a current source, you eliminate this problem since even a dead short cannot draw more current than whatever the source is set at.

author: gore
author_rank: Regular Contributor
timestamp: August 27, 2015, 07:28:06 pm
content: I see. Sounds like a better way of doing it. Even though the LED's will transfer the heat to an aluminum profile, so the overall temperature should be really low. On the other hand, those strips are glued onto those profiles. They could come loose after a while and heat up.