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I want to read the full range on my LM34DZ, but I can't seem to find any 1N914 diodes. Is there anything I can replace it with?

Full Range F Sensor

Peter Mortensen
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SpaceCowboy
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    http://electronics.stackexchange.com/questions/166828/difference-between-1n914-and-1n4148-diodes – Wesley Lee May 28 '16 at 23:52
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    1N4148......... – EM Fields May 28 '16 at 23:53
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    You can also just connect the 18K pulldown to a negative rail (-5V or similar), and the circuit will work fine without the diodes. This is just a funky way of getting a (poor quality) 1.2V virtual ground. Basically, the output has to swing below the ground for negative temperatures, so you have to bias it below the ground for it to work. – Connor Wolf May 29 '16 at 00:54
  • If we are getting creative: How about a current mirror using two transistors and a resistor? Or simply use the transistors as diodes. – Michael May 29 '16 at 07:07

4 Answers4

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The purpose of the ground diodes in the circuit are to lift the LM34 above ground so that its voltage drop can become negative. As such, any device that drops voltage can be used as a replacement. However, using a silicon rectifier such as a 1N4148 or 1N400x is likely the most appropriate choice due to the fact that the voltage drop is 1) not grossly dependent on the current (unlike with resistors), and 2) is high enough to be generally useful (unlike with germanium or Schottky diodes).

Ignacio Vazquez-Abrams
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The 1N914 is a very generic "jellybean" small-signal switching diode -- the 1N4148 or any other jellybean small-signal diode (say a BAV99) can be freely substituted for it.

ThreePhaseEel
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The 1N914 might not be available, but you can always find replacement components by looking into the datasheet. This advice applies to any component, by the way. For instance, the 1N4148 might be a suitable alternative if the cost does not matter.

Albits
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    Are you actually suggesting that 14 cents (for a 1N4148) as opposed to 9-10 cents (for a 1N914) is likely to be an issue? – WhatRoughBeast May 29 '16 at 00:09
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    Well, not on a single repair like it seems like you will be doing, but on an industrial applications with maybe thousands of components for each product, it could make a difference in the end. That was just a general comment :) – Albits May 29 '16 at 00:11
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    LOL...You can buy a thousand 1N4148 diodes for a few dollars. Many sources. –  May 29 '16 at 02:55
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    @Sparky256 LL4148 (SMD version) about $5.30 in China for 1 reel (2500). – Spehro Pefhany May 29 '16 at 04:03
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Aside from the many other valid suggestions from others - the 1N4148 and BAV99 being the most common - you could simply use a voltage divider (which wastes more power, so it is not so suitable for a battery source).

The dynamic resistance of a couple of 1N914 diodes at the 75 uA current of an LM34 is around 1500 ohms. Suppose your Vs is 5 V, and you want -1.2 V for the pseudo-ground; you could use something like 4.7 kΩ/1.5 kΩ.

Or use two diode-connected transistors or one transistor connected as a VBE multiplier (requires two resistors).

Peter Mortensen
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Spehro Pefhany
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