Thermocouple voltage boost circuit needed.

jpanhalt

Joined Jan 18, 2008
11,088
I want to turn on the LED when the pilot is lit as an indication it is functioning. The current produced by the TC is used for other circuits.
If you want an on/off function, rather than various degrees of dimness, keep the comparator or op-amp wired as comparator as shown by Sghioto.
 

sghioto

Joined Dec 31, 2017
2,691
True the LM358 does have a offset voltage of about 90mv with the gain set at 101 but that's not really an issue. Using the other half of the chip as a comparator compensates for this by allowing the trip point to be set higher as needed.
 

DickCappels

Joined Aug 21, 2008
8,135
True the LM358 does have a offset voltage of about 90mv with the gain set at 101 but that's not really an issue. Using the other half of the chip as a comparator compensates for this by allowing the trip point to be set higher as needed.
The idea of using an LM358 in a circuit to watch such a small DC voltage just doesn't seem right, even with an offset adjustment in this day and age when opamps with offsets of well less than a millivolt are pretty inexpensive. The venerable OP-07 (Vos =< 60 uV) can be had for less than 50¢ these days. Have a look at some of the other dual opamps. \rant

Whatever you do, use a DIP version so you can use a socket and change the opamp if the one you initially select doesn't work out.
 

DickCappels

Joined Aug 21, 2008
8,135
My first reaction to @Dodgydave 's post was "I can't believe that Analog Devices makes an opamp that drifts at such a rate!" They don't. It is a thermocouple amplifier for type J and K thermocouples and the sensitivity is 10 mv/°C. *whew*
 

Ian0

Joined Aug 7, 2020
4,051
True the LM358 does have a offset voltage of about 90mv with the gain set at 101 but that's not really an issue. Using the other half of the chip as a comparator compensates for this by allowing the trip point to be set higher as needed.
That depends on how many you are making. OK for a one-off, but if you want to make a batch of fifty, the technician that has to trim each one will kill you.
Or if you choose the AD594 at £22 a piece then the accounts department might kill you instead.
 
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Ian0

Joined Aug 7, 2020
4,051
The venerable OP-07 (Vos =< 60 uV) can be had for less than 50¢ these days. Have a look at some of the other dual opamps.
OP07 is the one I first thought of, but I don't think it is happy on a 0-5V supply with its inputs at 0V. TLV07 seems to be a newer version which is happier on low voltage supplies. Vos isn't quite as good at 150uV but good enough to avoid any potential violence from the technician.
 

Ian0

Joined Aug 7, 2020
4,051
No, same circuit, but it is a single not a dual. The other half of the circuit doesn't have to be that good. If you want a “one package” solution there are plenty of dual precision rail-to-rail op-amps out there. TI’s website is always a good place to start to look.
 
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