Instructables

Really basic Resistance / Amperage question:

before I begin: yes, I should know this already, but I'm really sleep deprived right now, and therefore slightly muzzy on my electronics knowledge... My circuit has two loops running in parallel off one, limited, 18v power supply: [*] Loop 1 requires the majority of the supply's power to run [*] Loop 2 is simply a 7805 voltage regulator supplying 5v to an analog input on an Arduino, so needs almost no current running through it. [*] when Loop 1 and Loop 2 are both active, Loop 1 does not get enough power. if I place a resistor in Loop 2, in series with the 7805, will it increase the amperage to Loop 1? I think the answer is yes, but, once again, I'm MASSIVELY sleep deprived. any help is appreciated.

pyper6 years ago
I'm guessing your using 9v batteries, in which case your supply impedance (internal resistance of the battery) is probably quite high and that's what's causing your trouble. Try buying better batteries, or switching to rechargeable (Ni-MH) 9v batteries.
gschoppe (author) 6 years ago
guessed and checked, and 100-150 ohms seems to be the correct range... I need to keep this a two battery setup for size, and unfortunately, I'd have to totally redesign to center tap, due to the safeties I've built in, but I think this will work reliably, if not efficiently.
gmoon6 years ago
Try running the 7805 off the "center tap" of the two batteries (i.e., off of 9V.) The higher the voltage you supply to a 7805, the more current is wasted in the regulation process. Or try a decent switching regulator in place of the 7805 (or double your batteries, as NM suggested.)
NachoMahma6 years ago
. It will reduce the current in Loop 2, which should allow more to go through Loop 1 (assuming you're overloading the PS). Just make sure you don't drop so much voltage across the R that the 7805 isn't getting enough V (calc using L2 I, not total I). . Probably help to reduce I in 5V sensing line (if it's a switch closure, R can be usually pretty high)
gschoppe (author)  NachoMahma6 years ago
Thanks a million... i was almost sure, but to tired to fully trace my logic... any thoughts on resistor value? I'm using two 9v batteries, in series, as the source, but L1 is a total black box... its the transformer from a negative ion generator... I'm using it for a 1/4" spark gap, with continuous spark... I'm thinking "guess and check" is gonna be my catch phrase.
. I'd try four 9Vs (2x2). If the voltage still drops too far, you're probably SOL.
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. No idea how to size R for L2 w/o knowing current. Yeah, "guess and check." I'd start at 10K, but that's probably too high. A volt meter on 7805in should tell you a lot.
. Just noticed that the uC input is analog. Probably can't do much with that, but I'm no expert.