Zener Diodes

Discussion in 'General Electronics Chat' started by IGill, Feb 12, 2013.

  1. IGill

    Thread Starter New Member

    Jan 5, 2013
    27
    0
    Can Zener diodes be wired in parallel?

    I have some zener diodes diodes with Vout = 5.1 Volts and Izm= 80 mA. In a USB circuit I am designing, for an output of 500mA, the Iz = 88 mA which is above the Izm value.

    If I wire 2 zener diodes in parallel; will this give me an Izm of of 160mA? But will it regulate the current at 2*5.1 - 10.2 Volts?
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You generally don't run 5.1 volt zeners in parallel because due to the voltage tolerance both zeners will not conduct at exactly the same voltage so the current will not be equally shared.

    If you wire (2) 5.1V zener diodes in parallel the regulated voltage will still be 5.1 volts. (2) in series would regulate to 10.2 volts.

    Can you post more details on your circuit?
     
  3. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    Perhaps you can put a 560 ohm resistor in parallel with the zener? That will dump 9 milliamps and the zener will still be in control of the voltage.

    Following this line of thought, you can put even less resistance in parallel if your circuit will be OK with that.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    We really need more details on what you are trying to do. I suspect that zeners will not be involved in an optimum solution. They seldom are, where voltage regulators are concerned.
     
  5. IGill

    Thread Starter New Member

    Jan 5, 2013
    27
    0
    I am building a circuit to enable me to charge a Garmin 305 from a dynamo hub on my bike via a USB socket. The garmin is designed to be designed to be charged from a USB and so I am trying to replicate a USB power output.
    The dynamo hub provides between 6 to 6.5V AC. I am use a bridge rectifier from 4 4001 diodes, some capacitors to smooth the peaks and troughs, a zener diode to regulate the voltage and a resistor to end up with 5V DC at 500mA.
    I haven’t got the calculations with me as I did them at home last night, but it turned out that with the resistance to produce 500mA in the circuit gave me an Iz of 88 mA and the maximum for the zener diode is 80.
    I can get about 350 mA with an Iz less than 80 with a larger resistor. This will probably be enough to charge my Garmin.
    The obvious thing to use would be a 7805 voltage regulator but the input voltage needs to be 2V above the output voltage (I think) and I haven’t got the required input voltage.
     
  6. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    My resistor idea will work, but you could also glue the zener to a bit of metal to get rid of the extra heat, or buy a 1 watt zener.
     
  7. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    all that stuff for 5V? probably easier to get a small 5V reg like 7805 at XYZ (no radioshack's in UK i presume).
     
  8. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    He already said he can only count on 1 extra volt. Not enough to run a 7805. Maybe a low drop-out regulator?
     
  9. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    digikey 1016-1674-ND, not a 7805, but many to choose from
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The series resistor for the zener regulator must supply 500mA to the load, plus enough current through the zener for regulation (80mA in this instance). If the Garmin were disconnected, but the dynamo were turning, all 580mA would pass through the zener, causing over 3 Watts to be dissipated by the zener. The series resistor will always dissipate a little less than a Watt, regardless of the load.
    A low dropout regulator is probably the best bet. It will only deliver as much current as the charger requires, so it will probably run cooler. A switching regulator would be more efficient, but may be overkill for this application.
     
  11. #12

    Expert

    Nov 30, 2010
    16,343
    6,828
    If only we were provided with a schematic and the information that the Garmin would be unplugged part of the time, we wouldn't be guessing like this.
     
  12. IGill

    Thread Starter New Member

    Jan 5, 2013
    27
    0
    Thanks for the help. A low dropout regulator may be the way to go.

    However, am I going OTT with this? If I limit the current using simple resistors, is it likely that damage will be caused by using 6 Volts rather than 5 Volts if the current is limited?
     
  13. IGill

    Thread Starter New Member

    Jan 5, 2013
    27
    0
    It turns out that all this is moot. I built up a circuit on a bread board and taped it to my bike to test. It turned out that the voltage kept dropping below 5V when it comes out of the bridge rectifier.

    My next plan of attack is to remove the bridge rectifier and use the diodes to produce a voltage doubler which I can pass through a 7805.

    I think it is going to be trial and error because I can not reproduce the power supply other than on my bike.
     
  14. DC_Kid

    Distinguished Member

    Feb 25, 2008
    638
    9
    6vac dynamo hub probably looks like crap on the scope, probably rough sawtooth. i guess you need to scope it to see what it really looks like (or have manufacture supply the datasheet). a basic volt meter is based on good sinusoidal source and probably in the 50-60Hz region. once you know what the source looks like to can start on a good design to get regulated 5vdc, etc.

    maybe instead of 0v/5v you can use -2.5v/+2.5v design?
     
    Last edited: Feb 14, 2013
Loading...