Diode selection

Discussion in 'General Electronics Chat' started by geoffers, Nov 1, 2012.

  1. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Hi all,
    Probably a v simple one, I was ordering parts last night and needed some 1n4148 diodes, I'm designing a new pcb and thought I'd use surface mount diodes instead of through holes to save space. One of the diodes is a half wave rectifier for a FDX-B rfid reader chip, the other four are used for 'diode feedback'. I noticed there are different types of 1n4148's? The through hole ones are signal diodes and some of the surface mount are down as rectifier diodes. This made me wonder a. which ones I should chose? b. could a different 'type' of diode improve the perfomance of my circiut? I picked the 1n4148's as thats what was on the data sheet, but only picked a signal diode type as that was the one that would plug in my breadboard! The function of the diode feedback is somthing I've yet to figure out, I've asked here before but know one really seems to know. I've no formal training so this may seem basic, I dont know much about diodes! Any advice would be welcome.
    Thanks Geoff
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,363
    All standard diodes are rectifier diodes. "Rectification" refers to the function that the diode is being used for.

    The 1N4148 is a popular signal diode, meaning it is used in applications where the current is relatively low (under 100mA).

    If your rectification need is under 100mA the 1N4148 will work. If it is greater than this and less than 1A then a common 1N4001 or 1N5817 (which gives lower voltage drop) would be a better choice.
     
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  3. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Thanks for that, I guess the 1n4148's being reffered to in slightly different ways is down to each manufacturer, I just wanted to be sure I would be getting the same thing in a surface mount.
    Thanks again Geoff
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The 1N4148 and 1N914 are switching diodes; their Trr (reverse recovery time) is extremely brief; on the order of 4nS-5nS. Schottky diodes are about the only other way to get that kind of Trr in a diode; 1N5817 thru 1N5819 are very common. These are all axial leaded diodes though. BAT54 are SMT Schottky diodes with a very low Vf.

    You'll find that standard silicon rectifier diodes like the 1N4001-1N4007 are very slow by comparison; they will work OK for rectifying 50Hz to 400Hz (or so) signals, but don't turn off quickly enough for higher frequency applications. You need extremely fast diodes if you're working with a RF signal.

    Keep in mind that you can mount 1N914/1N4148 diodes vertically; one lead just long enough to go thru the board and soldered on the other side; the other bent in an upside-down "U" shape, going into the board right next to the diode body. This way you can take up less space than even an SMT/SMD diode:
    [​IMG]
     
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  5. geoffers

    Thread Starter Active Member

    Oct 25, 2010
    239
    6
    Thanks, my signal is around 125kHz so I suppose I will need one of those two diodes, thats a good idea with the mounting, I'll have to play around with my pcb software. Regarding the diode feedback, I'd love to know how it works, I just know it does. Here is the circuit.
    Diode%20feedback.jpg

    Its a rfid reader, coil1 and coil2 produce a 125kHz signal across the coil, the transponder loads the coil to transfer its data. It all works without four diodes and two resistors but adding them increases range, I assume by increasing the magnetic field? Any ideas.
    Thanks Geoff
     
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