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Electronics => Electronics => Topic started by: cooldog on January 03, 2010, 08:32:36 PM

Title: what size diode for reverse polarity protection
Post by: cooldog on January 03, 2010, 08:32:36 PM

so i had this thread
http://www.societyofrobots.com/robotforum/index.php?topic=6236.0 (http://www.societyofrobots.com/robotforum/index.php?topic=6236.0)
but i figured i would make a new one instead of bring it back.

my questions is about diodes for reverse polarity protection.
my supply voltage is 9v and i know i would need to put the diode between the battery and the 5v reg.
voltage drop isn't to much a problem for me

my questions are:

• are there different sizes for diodes?
  
• what size should u use?
  
• if possible does anyone have a digikey/mouser part # for one i should use?
  

Title: Re: what size diode for reverse polarity protection
Post by: SmAsH on January 03, 2010, 08:51:39 PM

Anything like a 1N4007 should do for that.

Title: Re: what size diode for reverse polarity protection
Post by: cooldog on January 03, 2010, 08:55:55 PM

http://ca.mouser.com/ProductDetail/Diodes-Inc/1N4007-T/?qs=e/Rqmsgwm9iVtgJYUW23AA%3d%3d (http://ca.mouser.com/ProductDetail/Diodes-Inc/1N4007-T/?qs=e/Rqmsgwm9iVtgJYUW23AA%3d%3d)

like this?

Title: Re: what size diode for reverse polarity protection
Post by: dunk on January 03, 2010, 08:58:42 PM

hey cooldog,
just pick one that is rated to a higher voltage than you are using and a higher current than you are likely to draw.

dunk.

Title: Re: what size diode for reverse polarity protection
Post by: cooldog on January 03, 2010, 09:03:22 PM

thanks alot, helps alot

Title: Re: what size diode for reverse polarity protection
Post by: Soeren on January 03, 2010, 11:53:56 PM

Hi,

Quote from: cooldog on January 03, 2010, 08:32:36 PM

my supply voltage is 9v and i know i would need to put the diode between the battery and the 5v reg.
voltage drop isn't to much a problem for me

If the 9V comes from a PP3 (the little box shaped 9V batteries) voltage drop does matter for you!

A regular 7805 needs around 8V in and an LDO needs around 5.5V (at the very least).

A 9V battery has got 6 cells. A single cell alkaline is considered flat at 0.9V, so to use a 9V battery up properly, your circuit need to work down to 5.4V, but with an LDO you can use it almost completely (forget about using a regular 7805 though).

If you add a diode, you're starting to throw away good power. With 1N400x, you will drop 0.7V, if you go with a Schottky diode, you can get down to 0.3V, using a power MOSFET you can get down to some mV, but if you add a fuse from eg. the positive input and then mount the diode (after the fuse) with the cathode towards the fuse and the anode towards the negative side ("ground"), you avoid the voltage drop completely. In case of wrong polarity, replace the spent fuse (after removing power).

Title: Re: what size diode for reverse polarity protection
Post by: FrenchyDude on January 04, 2010, 01:41:34 AM

LDO was new to me... here's the link to wiki I found on it

http://en.wikipedia.org/wiki/Low-dropout_regulator (http://en.wikipedia.org/wiki/Low-dropout_regulator)

Frenchy.