Electronics > Beginners
Diodes in parallel.
Rick Law:
I've something that works in experimental setting. I was thinking of applying it to something that I want to keep more permanently (and potentially more expensive), but I know that I am not knowledgeable enough to identify the draw backs. If anyone can give me their thoughts on this, I appreciate it.
Seeing how schottky diode is used as "power source selector", I applied it to some of the experimental stuff I do...
I had something that I wanted to avoid outage when I need to pull the battery out to recharge, so as with the Arduino's, I decided to use a diode as power-source-selector. I put two sets of batteries+diode in parallel to share/switch between batteries. That worked.
I expanded it - to minimize the lost on diode, for each "battery+diode" set, I decided to use multiple (4) schottky diodes in parallel - I reasoned that multiple diodes in parallel would decrease the current through each diode hence keeping them closer to the (approx) 0.2v drop than the higher current 0.4v drop. That also worked.
I've been thinking of applying that concept (power source switching/sharing and using multiple diodes to lower the voltage drop) to something more permanent (and perhaps more expensive). That leads me to think: "What are the draw backs?", "What if one of the diodes failed?", things like that.
Your thoughts? Thanks!
By the way: While I've been chewing on that thought for a while now and still hadn't done it, the post on ganging up PSU inspired me to ask since the same concept could apply with two PSU's - but I did not want to hijack that thread.
SeanB:
Remember that diodes have leakage current, so multiple in parallel means more leakage current as well, which can, depending on the battery chemistry, be from mildly bad to FOOF level bad. If just pulling the battery out for a few seconds a easier thing is an ultracapacitor or other multi farad capacitor as a reservoir of power to hold up the unit, either in a string in series for higher voltage or just a 5V5 unit for a 5V rail.
wraper:
--- Quote ---"What are the draw backs?"
--- End quote ---
Connecting diodes in parallel is a bad idea (if it's dual diode within one package, usually acceptable). Current won't be shared equally. You should just use diode with higher current spec.
--- Quote ---"What if one of the diodes failed?"
--- End quote ---
They usually fail short, so the whole thing will be shorted.
Benta:
Connecting P-N diodes in parallel is a bad idea, but with Schottkys it will work. But the obvious question here is: why not just use a larger/higher current diode? The result will be exactly the same.
wraper:
--- Quote from: Benta on July 17, 2018, 10:06:41 pm ---but with Schottkys it will work.
--- End quote ---
Proven to be very bad idea by failing TV PSUs. Schottky diodes as well as usual diodes have negative forward voltage temperature coefficient. Which means the hotter diode -> the lower forward voltage -> the higher current flows -> diode heats up even more, Vf drops even more, current increases. Thus imbalance only worsens.
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