# Why there are two diodes instead of one used for an LM35 negative temperature circuit?

See the system examples of the LM35 temperature sensor datasheet on page 16, see picture. This should be used for negative temperatures. However, the voltage change is 10 mV/C and it supports -55 C minimum, resulting in -550 mV. A diode (1N914) has a typical drop of 600 or 700 mV .... why are two diodes used, and not one?

(I play to use a 1N4001 or 1N4148 since I have plenty of them). • A "typical" drop is irrelevant here. You need to ask yourself: what is the minimum diode forward voltage when passing the minimum current from the sensor? – Elliot Alderson Feb 7 at 13:14
• Yes, just checked, for an 1N4148 the minimum is around 0.4-0.5V, which could be too less, thanks! It also answers my question that I can use 1N4148 instead. – Michel Keijzers Feb 7 at 13:17

## 1 Answer

The whole point about this circuit is that the 18 kohm resistor is tied to a negative voltage relative to Vout- and this ensures that the LM35 can produce a signal that reflects negative temperatures. Without the bias resistor to a negative supply rail (relative to Vout-), the LM35 only works from +1 or 2 degC.

2 diodes are used because despite what people simplistically call "the standard volt drop of 0.5 volts to 0.9 volts for a range of diodes", the actual volt-drop depends on the current through the diode and, if it is low (as per the case of this circuit - circa micro amps) then you'll probably need two diode drops to produce something in excess of 0.55 volts.

Here's an extrapolation of what a 1N4148 diode might produce at 1uA of current: - Given that an LM35 might only draw about 10 uA at -55 degC, you can imagine each diode dropping maybe 0.4 volts hence two diodes are needed.