What do you make of this. Regarding a LM385.

Discussion in 'General Electronics Chat' started by doug08, Jul 15, 2011.

  1. doug08

    Thread Starter Member

    Jan 30, 2011
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    Line below from a project I am working on.

    "I used a LM385Z, 1.2V voltage reference in the regulator circuit. The circuit does not need a precision reference but I have a few of them on-hand so I used it. A plain Zener diode reference would probably work just as well"



    My question is, what value zener to use in place of the LM385?

    Thanks.
     
  2. Ron H

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    We need to see a schematic before we can give you a good answer. There are no zeners that can replace a 1.2V reference, but there are probably still ways you can use a zener in your circuit, with some other modifications.
     
  3. doug08

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  4. SgtWookie

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    You might use a couple of forward-biased 1N4148 or 1N914 diodes in series with ~1mA current flowing through them via a current limiting resistor to get about 1.2v out. It'll vary over temperature somewhat (negative temperature coefficient), but it's a very inexpensive way to do it.
    [eta]
    The Vf of the 1N4148/1N914 diodes will vary a bit much with the change in current due to the battery voltage drop.

    The LM385 is a micropower voltage reference; it's much more accurate and uses much less current than a couple of diodes in series would.
     
    Last edited: Jul 15, 2011
  5. doug08

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    Jan 30, 2011
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    I'll just use (2) 4148's in series. Anode to ground, cathode to circuit as shown. What size resistor?

    Thanks!
     
  6. Bernard

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    The LM385Z1.2 is far superior to a zener, acts just like a zerer with a brain, & to monitor Battery V , a V referance is much more stable.
     
  7. someonesdad

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    +1 on the use of the LM285 -- I use one as a voltage reference at my bench. It's a good, cheap device -- and has run from three AAA batteries with 1 resistor for over 4 years. It's stable to around 0.1% or perhaps a bit better -- and has a low tempco.
     
  8. doug08

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    the reason why I am here is because I don't have one. If I did, rest assured i would be using it. The person that designed the circuit said I could use a zener in place of it. I wanted to know which zener he was referring to.

    Thanks.
     
  9. SgtWookie

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    They probably were just assuming that Zeners were available down to 1.2v. I don't know of one offhand, particularly not one that low powered.
     
  10. kubeek

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    Look at this circuit, it is similar but doesnt use a hard to find transformer, just inductor. I used it to generate cca 350V for my geiger, T1 needs to be able to handle that voltage and the set peak current through L1. D6-D10 are zeners that set the upper voltage limit, R7 is cca 6.8-10ohm and sets the peak current through L1.
     
  11. someonesdad

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    Understood, Doug. Still, if you don't have the zener or the voltage reference, you'll have to order one, probably from on-line. Since these are cheap parts, the shipping will probably dwarf the part cost. Thus, I'd recommend getting the voltage regulator (they go for about 50 cents).
     
  12. Ron H

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    Doug, what zeners do you have on hand, or have easy access to?
    Also, do you have any LM324s?
     
  13. Jaguarjoe

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    Last edited: Jul 16, 2011
  14. Ron H

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    $0.49 each at Jameco. Of course, you have to pay for shipping.
     
  15. THE_RB

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    It might be a bit of overkill in size, but many people have LM317 regulators in their junkbox. They are about 1.25v and will be a better reference than any simple diode setup.

    Or on the same logic you could use a 7805 5v regulator and 2 resistors to give a voltage divider producing 1.2v output.
     
  16. SgtWookie

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    Well, an LM317 would require a 120 Ohm resistor from OUT to ADJ in order to get a regulated Vref out of nominally 1.25v; so ~10.46mA through the resistor. That's a pretty hefty current draw just for a reference voltage when it's powered by a 9v transistor battery.
     
  17. doug08

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    Jan 30, 2011
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    I don't have a 74act IC for that schematic. I have everything else though. Thanks!
     
  18. doug08

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    Jan 30, 2011
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    I also have several lm317T's.....

    Thanks
     
  19. THE_RB

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    Good point! I only read the OP's posts and didn't load any links to schematics et so I was unaware it used a 9v battery.

    That also means a Lm7805 is not ideal as it will have 7mA or so quiescent current.

    I'd still try playing with the LM317, most of the new datasheets show a 240 ohm resistor as the minimum for Vadj, and I have a feeling since he does not need any significant current or transient response from it he might be able to use a much higher value resistor than that?

    (edit) I checked an LM317 when I was out in the workshop, it produced 1.258v on the Vout pin with resistors of 1k and under, 1.258v at 9.0v input and 1.257v at 7.0v input (so it is regulating). Input current into the Vin pin was 1.25mA with the 1k resistor and 9.0v input.

    Under 1k gave similar results to the 1k resistor, 1.2k and 1.5k caused it to drop out of regulation.

    It could be worth a try, but please test the LM317 there is no guarantee it will be the same as my LM317.
     
    Last edited: Jul 18, 2011
  20. SgtWookie

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    Yep, they don't guarantee regulation on 317's until you have >= 10mA current flow from the output. If the ADJ pin is grounded, at least you won't have the Iref current messing up your output voltage; it's usually somewhere between 50uA and 80uA.

    The 7805's by design sink somewhere between 5mA and 6mA through their GND terminal; but according to all the 78xx datasheets I've read, they require another 5mA load before the guaranteed regulation specifications will be met.

    As you've implied, your mileage may vary - considerably. ;)
     
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