This is part of the circuit for reverse current protection or in my case better to say wrong polarity protection. I decide to go with 1 silicone diode in parallel chose 1n4007. protection is done via polyswitch.tripping time of pptc is higher than of simple wire fuse. and as I went through lot of schematic with diode protection they all were using only 1 diode for that purpose.
Knowing tripping time of pptc that is counted in seconds decide to use 3 diodes in parallel.
My setup is 5x1.2v Nimh battery pack. Internal resistance 0.02ohm. Forward voltage at 1A across diode 1.1v as per datasheet .
Ishort=((51.2v)-1.1)/(0.025)=49A
As per datasheet
I2t is 3.7^2A/sec and Ifms (surge current ) 30A.
49A/I^2t=3.5diodes.
Are my calculations right for my case i.e. using pptc as fuse and using minimum 3 diodes? Or is it safe to use 1 diode ? if yes then how 1 diode will hold 49A surge current if its rated for 13A <8.3ms?
surepic:
But whats wrong with protection with diodes in battery operated environment ?
The high fault current, and the much needed fuse to stop that.
Series protection is less crude.
You didn't say what the max curent draw of your circuit is. A schottky diode could be enough (post#3).
A p-channel fet in the + line (or n-channel in the - line) could even have a lower voltdrop (loss) than a schottky diode.
Leo..
I also used diode in parallel, but you need to use a fuse in series, between the battery and the circuit, the diode stays after the fuse, so that the fuse blows when the diode leads
@srnet why to drop voltage If only thing is needed is reverse current protection?
@rtek1000 in my circuit im using pptc i.e. resettable fuse im actually using that kind of fuses almost everywhere where i need overcurrent protection if for example motor starts to stall and starts to draw max current. But as i tried to explained calculations show that diode will get stress cos current will exceed its max forward current more than 19amps. Reaction of pptc depends on voltage and temperature it needs approximately 0.5sec to switch its state from low resistance to high. But diode will hold around 13amps for less than or equal to 8.3ms if i am reading datasheets right. Connecting 3 of them parallel im distributing short current 49amps equally between 3 diodes giving time to the fuse to react and same time holding diodes within their limits.
Agree with wawa about mosfets but also want to know if this circuit will do the job or why mosfet is better option compared to this one.
surepic: @srnet why to drop voltage If only thing is needed is reverse current protection?
Perhaps ask yourself why your cunning wheeze is something I cannot recall seeing being used for reverse current protection.
I have seen that method used for a combination of reverse and overvoltage protection, when the diode is a Zenner, but thats a different requirement.
If the 0.3V voltage loss of a simple diode is a major issue, and it mostly it is not, then use a MOSFET at suggested, reverse protection with virtually no voltage drop and no risk of setting fire to stuff.
