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LM318 Op-Amp and TMP36 Temp sensor question

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Andy1845c

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Hello,

I want to use an analog temp sensor like the TMP36 and interface it with a 0-10V card on a PLC. I just happened to have a TMP36 laying around so I mocked it up and am getting ~.7 volts from it at the target temp (basically room temp).
I'd like to raise the voltage in a linear fashion to something more workable. I tried to use a LM318 (again, had it laying around) and hooked it up as shown in my attached schematic.
I cannot get an output of any kind from the op-amp. I have my .7 going in and nothing coming out. I have tried several ICs trying to rule out having a bad one. I've never used an op-amp before so I may very well be missing something basic.

I've studied AudioGuru's circuit (http://www.electro-tech-online.com/attachments/opamps-gif.16569/) and mine matches his non-inverting single supply circuit minus the voltage divider.

Anyone have any suggestions?
 

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alec_t

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For a single-polarity supply, as you have, the LM318 cannot operate properly with an input voltage less than about 3.5V, according to its datasheet.
 

Andy1845c

Active Member
Thanks for the reply Alec. Where on the data sheet is the minimum input voltage specified? I see an "Input Voltage Range - Min 11.5v". I don't see the 3.5V figure anyplace. Is there a better op-amp you would recommend for this?

Also, if anyone is so inclined - if I wanted to translate the .7 volts into a 4-20 mA signal - would that be exceedingly complicated? I don't know that I want to, but I have been pondering it in my head.
 

MikeMl

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The LM318 is a very old design, and is not rail-to-rail in or out. Even the crummy old LM324 or LM358 would work in your application because it's common-mode input voltage range goes down to -0.5V with respect to the V- pin.
There are now hundreds of opamps that are rail-to-rail in and out

63.png
 
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alec_t

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Andy1845c

Active Member
What would happen if I change the supply voltage? I did try it at a variety of voltages up to 18v and got no output.

0.7v would ideally be about 12mA or something. Mostly I am just wondering where I would start if I wanted to play with that route.
 

Andy1845c

Active Member
The LM318 is a very old design, and is not rail-to-rail in or out. Even the crummy old LM324 or LM358 would work in your application because it's common-mode input voltage range goes down to -0.5V with respect to the V- pin.
There are now hundreds of opamps that are rail-to-rail in and out

View attachment 109217
Ill see if I by chance have either of those around. I've never used op-amps before so I haven't really stocked up on them
 

MikeMl

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What would happen if I change the supply voltage? I did try it at a variety of voltages up to 18v and got no output'...
To make that circuit work with the '318, you would have to add a negative supply of at least 4V. This makes for split supplies, +15 and -4V.
 

Andy1845c

Active Member
To make that circuit work with the '318, you would have to add a negative supply of at least 4V. This makes for split supplies, +15 and -4V.
Thank you Mike. I clearly need to study how op-amps work :)
 

MikeMl

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alec_t

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