Les,First it is a BAD idea feeding a 12 volt fan with 15 volts.
For your LED you have to drop 13 volts across the resistor . (15 - 2) You want 0.02 amps through the LED. So the value of the resistor will be 13/0.02 = 650 ohms (The nearest standard value is 680 ohms which will be near enough. Next calculate the power dissipation in the resistor. This will be 13 volts times 0.02 amps which equals 0.26 watts.
With the 50 ohm resistor that you used it's dissipation would be 13 * 13 /50 = 169/50 = 3.38 watts.
(The current would have been 13/50 = 0.26 amps which would require quite a high power LED Your current value of 0.02 amps would be a suitable value for a normal 5mm LED.) I don't understand your reasoning for your calculations.
Why are you using a 15V regulator for a 12V fan?
It was a misunderstanding on my part. I thought I had to account for a 12V drop by the fan and a 2V drop by the LED. I realize now that didn't make sense. So I made the switch to the 15V regulator but went back to the 12V regulator once I understood. Everything seems to be working great now.If you want to drop the 15 volts down to 12 volts the simplest way is to connect 5 forward biased silicon diodes i series with the fan
Congrats! It's always rewarding to finish a project.Board soldered up and tested!
Thanks, as always, Dennis. You've helped me decide to order from the "safe" suppliers and I've replaced most of my parts.Congrats! It's always rewarding to finish a project.
If you're buying those prototype boards from AliExpress; a word of caution. I've found that they have a defect (shorted pads) rate of at least 10%. After buying several lots of 10, I decided to go for broke and just get 100. The lot I received had shorts on about 50% of the boards. I got a partial refund, but it was still a pain to have to sort through all of the boards to mark the shorts and then send pictures to AE.
There are some parts on Ali Express that are reasonable to use. You just need to make sure that whatever you use is appropriate. Components might not have the correct tolerance or temperature coefficient, but if it doesn't affect your circuit, they can be used.You've helped me decide to order from the "safe" suppliers and I've replaced most of my parts.
More than half of the boards in a lot of 100 had shorts. Some only one or two, but my expectation was 100% of the boards free of defects that prevented them from being used as intended.Let me try to reason how to tell a shorted board: the copper pads touch?
Thanks for sharing your experience and the tip about putting the components on the pad side!There are some parts on Ali Express that are reasonable to use. You just need to make sure that whatever you use is appropriate. Components might not have the correct tolerance or temperature coefficient, but if it doesn't affect your circuit, they can be used.
I bought some lots of 100 AO3400 and AO3401 MOSFETs because I didn't want to pay 10-20X more for parts from more reputable sources (different part numbers). I bought 100 of each and did some random testing of parameters (mainly threshold voltage and 100-200mA drain current) that were important to me before buying more. I thought about buying full reels of them since both parts are end-of-life, but I happened to see a picture of a place in China where people go to buy counterfeit parts and I saw reels of SMT components. That made me decide not to buy a reel.
The counterfeiters will sometimes include a small number of good parts in a "batch" and I didn't relish the idea of having to do 100% testing on thousands of parts.
More than half of the boards in a lot of 100 had shorts. Some only one or two, but my expectation was 100% of the boards free of defects that prevented them from being used as intended.
View attachment 232244
When I use these boards, I usually use them with the pads on the component side. That way I don't need to worry about shorts caused by routing bare wires:
View attachment 232246
Another thing I do is do a board layout so I can optimize component layout and avoid wire crossings (which lets me use bare wire). This is the layout for that board:Thanks for sharing your experience and the tip about putting the components on the pad side!
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by Jake Hertz
by Jake Hertz