May 28, 2013
25 years as an engineering tech/associate engineer. 2-½ years university studies for engineering. 4 years as a US Army avionic tech. Job shop machinist prior to that.
Tutorial - Controllable Power Outlet
about 2 years ago
Some people want a SPDT relay. You can re-layout the pcb to include the SPDT relay footprint. However, the NC contacts are rated much lower than the NO contacts. I would recommend the 1.0W relay, since it also has ballast and tungsten ratings. The ballast ratings are for magnetic (transformer types), not for electronic ballasts unless otherwise stated. An electronic ballast can have as much as 10 times the in-rush current of magnetic (because electronic ballasts are capacitive), so derate appropriately. If you re-layout the PCB, be sure the traces can handles the current that the relay contacts can pass. I agree with the author for not using the SPDT and don’t personally recommend using it or find a relay that has the same 30A ratings on both the NC and NO contacts. For those that like to program, try using a latching type relay in this same footprint. They typically have a much higher current rating. You just need to pulse the coil ON and pulse them OFF to LATCH and UNLATCH them.
The 1K Ohm for the LED was probably chosen, because the LED is a high brightness type. Also, the LED is mounted inside an enclosure an will not typically be seen by anyone. So, the author probably felt the LED necessary, but didn’t need it too bright or to consume more power than needed for his project. At 5VDC, the froward voltage of the LED is 3.4VDC max, which gives 1.6VDC remaining. 1.6VDC/1K Ohm = 1.6mA, bright enough for the high brightness variety. If the absolute maximum current is desired, then 1.6VDC/20mADC = 80 Ohms. Personally, I’d either get rid of it or stay with the 1.6mA the author designed the circuit for.
I would recommend using a PN2222 or PN2222A, since these have the same pinout, package and can sink 600mA. Looking at absolute maximum for the relay (the 1.0W relay) and the maximum 20mA (if a tech wants theirs brighter) LED, that gives 220mA on a 200mA rated component. The PN2222(A) have roughly the same voltage ratings, have the same pinout and use the plastic TO-92 case. Probably cost about the same too! The values for R1 and R2 can stay the same.
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