555 design critique/troubleshooting - Page 1
Forum section: electronics / projects

author: TonyBurroughs
author_rank: Newbie
timestamp: February 14, 2016, 10:56:44 pm
content: Hi guys! first post here. I'm working on an LED flasher circuit that I'm using a 555 timer for. Requirement is for ~3 second period with ~60% duty cycle. The flasher turns on a FET which allows current flow to the LEDs. I'll attach the schematic so you guys can see what I did and give me any kind of design critique. This is one of my first designs (just finished BSEE degree).We have built a bunch of these things and are finding that occasionally we end up with a unit that flips the duty cycle (2 seconds off, 1 second on). This is also after confirming that the resistors are not swapped. Could something be happening with the FET that might cause it to act backwards? Probably the best thing would be to put a scope on the discharge pin of the timer and compare a good board to a bad board which is something I'll try this week. I mostly just wanted some feedback on the design since you don't get a ton of real world critique in engineering school.ThanksEdit (02/15/2016): All of the faulty boards were repaired by replacing the output FET. Is it possible that digikey sent me PChan FET mixed in with NChan FETs? If the FETs were being destroyed by static during assembly would it reverse their switching order? Lastly, how do you guys go about spec'ing out a FET for such an application? I just picked the cheapest one with a decent ON resistance which had the pin layout I wanted.

author: Seekonk
author_rank: Super Contributor
timestamp: February 14, 2016, 11:16:00 pm
content: There was a discussion once about this. Seems a number of people, including myself, have been shipped diodes that are marked backwards. Add that to your real world knowledge.

author: TonyBurroughs
author_rank: Newbie
timestamp: February 15, 2016, 12:45:05 am
content: mine were purchased as cut tape and only some of them aren't functioning. In your case was it a whole batch of mislabeled diodes or was it just a few here and there? That would surely change the timing. I'll have to check for that too.

author: edavid
author_rank: Super Contributor
timestamp: February 15, 2016, 01:18:57 am
content: There's no need for D1 and D2, since the duty cycle is > 50%.

author: TonyBurroughs
author_rank: Newbie
timestamp: February 15, 2016, 01:43:56 am
content: Quote from: edavid on February 15, 2016, 01:18:57 amAgreed. that is left over from a feature change. Initially they wanted it as 30% duty cycle, then after they saw that in real life they changed their mind. So I swapped the resistors to make that happen but the leftover diodes were already purchased and in the design so I left them.

author: TonyBurroughs
author_rank: Newbie
timestamp: February 15, 2016, 07:40:53 pm
content: bump: found that all faulty boards are repaired by switching out the output FET... what's going on here?

author: edavid
author_rank: Super Contributor
timestamp: February 15, 2016, 08:07:22 pm
content: Quote from: TonyBurroughs on February 15, 2016, 07:40:53 pmWrong FET pinout or blown gate means the 555 output is driving the LED?

author: TonyBurroughs
author_rank: Newbie
timestamp: February 15, 2016, 08:23:39 pm
content: No way the 555 output is driving because the output switches 24V but the 555 output is only 12V. I'm pretty certain I would see the light difference on the LEDs. What are the common failure modes for FETs? Source-Drain short? Gate-Drain short?and it can't be wrong pinout since 90% of the boards work right?