I'm looking for a little more specific advice on selecting type of components and/or values for my application. I'm a little confused about whether to use a standard fuse or a PTC fuse; whether to employ a Schottky diode or a simple rectifier diode...
Currently my prototype is built on a 2-layer PCB and uses an AMS1117-5.0 voltage regulator (SOT-223 pkg) for 5 volts. It uses about 32mA on average (fluctuates between 22 and 42mA). I'm not sure what the startup current is or how to measure it.
Based on the datasheet for the regulator, with a dropout voltage of between 1.1 and 1.3, I am speccing the device as requiring between 6.3 and 15 volts. (If I use a Schottky, I'll need to increase the minimum accordingly.)
What I would like to accomplish is the following:
- Protection against inverse polarity
- Protection against overvoltage and overcurrent (exploding parts, fire, etc.)
Typical operation will utilize 6 series AA cells, either NiMH or Alkaline (7.2 to 9V). These have current capacities of 2300-2500 mAH (though I'm not sure if such cells can actually deliver more than 2.3 to 2.5A).
I'd like for polarity reversal to do no damage and not require a fuse replacement. Overvoltage and overcurrent conditions can blow a fuse. I'd also prefer a minimum of parts and cost as another design goal is small size. Surface mount components preferred.
Current schematic with no protection:
So my question is thus:
- Of the components: Schottky diode, rectifier diode, "normal" fuse and PTC fuse; what combination of these (or other suggestions) would best serve my requirements?
- What criteria should I use to select appropriate values?
It's been suggested here and elsewhere that doubling the normal current use may be a valid starting point for selecting a fuse. I found a Littelfuse 1210L005 resettable PTC which has a hold current of 50mA and a trip current of 150mA, but I am not sure if these are desirable values.