Quote:
Originally Posted by Kat Kononov
If I add a diode to the IR board, will it prevent it from breaking in case the polarity is accidentally reversed? If so, where should I add it?
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IMHO, a small series fuse in line with your +12V input and a reverse-biased (cathode to the +12V, anode to GND/return) diode in parallel with your +12V/GND leads at the input.
How to pick a diode:
1.) Low forward voltage drop when conducting otherwise your IC still sees too large of a negative voltage across it's terminals
2.) Non-repetitive forward current rating - need to match this value (and the time it can withstand this value) to your fuse to ensure the fuse will blow before your diode does!
3.) V(br)r (reverse breakdown voltage) > the voltage you are sitting across when not conducting
How to pick a fuse:
1.) Fast-acting, since this board doesn't have a lot of inrush current (inrush current = initial current spike when powering on and charging up capacitors and associated IC's) or current spikes where you wouldn't want the fuse to blow.
2.) ~1.5x your expected maximum current draw to avoid nuisance blow, as in 1. Easy to spec, since the LM78L05 can only source 100mA, anything above that is an error condition.
3.) Small - no giant breaker or glass BUSS fuses, lets stick it right on the board
Possible solution:
Diode -
SD103AW,BW, or CW
Fuse -
0603FA250-R
Alternatively:
Use a series diode (heck even the one mentioned above) with an LDO (Low dropout regulator) instead of the standard linear LM78L05. Something such as the
MIC2950 which has a dropout voltage (as Al S. mentioned) of 0.3V@100mA would mean that your circuit would work down to 5V + 0.3V+0.6V (forward voltage of the diode above) or 5.9V! If your backup battery is down to 5.9V you're in trouble
At the end of the day, after typing this up someone will find a rule or a new Q&A stating we can't modify the boards anyway
