Simple Temp Sensor Circuit using TMP36 and LM358

Ian0

Joined Aug 7, 2020
9,667
Not so. At 69 ma the zener impedance is 10 ohms at 1ma it's 400 ohms. The input of a LM358 is not going to load it down.
It's not about the LM358 loading it down. The current changes as the supply voltage changes. The reference voltage changes as the current changes (due to the zener impedance), AND it changes with temperature.
 

Ian0

Joined Aug 7, 2020
9,667
I have seen that tlv431 can withstand only 20mA. will it be ok?
Yes, it only needs 100uA (if I remember correctly). A normal TL431 needs at least 1mA, but nowhere near 20mA.
Keep the current as low as possible - otherwise it will flatten your batteries.
I'd be looking for an op=amp with a lower operating current than a LM358 because it needs 500uA. You don't need any great speed, so you can probably find something that will work at 40uA - have a look at Microchip's offerings.
 

sghioto

Joined Dec 31, 2017
5,376
If the TS doesn't require any high degree of accuracy two 1N4148 diodes in series with a 1K resistor would make a decent reference. The battery and temp indicators are adjustable.
1640382741176.png
 

Ian0

Joined Aug 7, 2020
9,667
If the TS doesn't require any high degree of accuracy two 1N4148 diodes in series with a 1K resistor would make a decent reference. The battery and temp indicators are adjustable.
View attachment 255969
If he didn't require any degree of accuracy, he'd use a 1N4148 diode as the temperature sensor. He's using a temperature sensor that's typically accurate within ±1°C.
 

crutschow

Joined Mar 14, 2008
34,280
If the TS doesn't require any high degree of accuracy two 1N4148 diodes in series with a 1K resistor would make a decent reference.
I don't know if I would call about -4mV/°C drift in the reference voltage a "decent reference".
Why use that when a TL431 is readily available and not much more than the cost the diodes?
 

Thread Starter

Lucky-Luka

Joined Mar 28, 2019
181
I have tried to improve the schematic.
I have noticed that in the simulation, using 3.6V input as low battery state, the op amps don't give me the rail voltage at the output. Is that normal?
Any thoughts about the improved schematic?
Thanks
 

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Audioguru again

Joined Oct 21, 2019
6,671
The old LM324 quad and the LM358 dual both have terrible crossover distortion, lots of noise (hiss) and a frequency response/slew rate that has trouble with frequencies higher than 2kHz at high levels. Their problems are because they are one of the first opamps that have a fairly low idle current.

Most modern audio opamps have no crossover distortion, low noise and work well to 100kHz and higher. Their idle current is 5 times to 10 times more which is still fairly low.
 
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