Does anyone know if this can be used for Pt500 RTD sensors? I have a couple that I have inherited from a friend who extracted from his home oven and want to use in my electric pizza oven (up to 500C).
Also can I get away with one and use a multiplexer (I have a CD4052) to manage two sensors?
The Pt500 RTD you have as stock should work fine. The Pt 1000 measures to the max of 1000 therefor the Pt500 is effectively providing 50% of the available range MAX31865. As I see it.
The CD4052 might work for you with some playing around. The link shows the bridge circuit mostly used with RTD’s.
Bryan
Thanks. The value being either 1000 or 500 refers to the resistance of the sensor at 0 degrees Celsius. From there the resistance of the sensor increases as temperature increases. This may impact what the value of a resistor is if a voltage divider is used to determine output of the sensor. I have been using a voltage divider with a 4700 ohm resistor but an amplifier is recommended as the signal is not regarded strong enough to be reliable.
I have come across commentary before where a Wheatstone bridge is used, so I will read that article and see how it helps.
I also have a LM324 Op Amp but am struggling to understand how to use it properly!
Hi @Dion267210,
I agree with @Bryan160034, It should work just fine. There are plenty of ways you could adapt the PT500 (e.g. placing in series with a 500 Ohm resistor, thus the resistance at 0 degrees is 1000. Then simply scaling it in the software. Not the most elegant solution but an option). A self-made Wheatstone bridge is also an option, though.
Regarding the LM324, I and plenty of other people would be happy to help. There are plenty of resources online, I don’t know how familiar you are with op-amps/electronics in general, but here would be a good place to start.
Additional note: An LM324 may not be a great op-amp for this application, assuming your supply is from microcontroller, you may have a very narrow voltage range supplied by the IC, because the LM324 is not a “Rail-to-Rail” op amp. This means if supplied with [0V, +5V] power, it will generally only be able to produce voltages in the range [+0.5, +3.7] or around there. A better alternative would be a “Rail-To-Rail” Op-Amp.
Hope this helps!
Hi Zach, Dion
I would treat that link “here” with some caution particularly the comments in the “Pros” and “Cons” chart.
In that article no mention has been made of the requirement for a split power supply or a half Vcc “signal ground” for an inverting amplifier.
The 2 basic diagrams are correct although in the pros and cons chart the author says the feedback resistor in the non inverting amplifier is connected between output and non inverting input the schematics correctly the feedback resistor connected between output and inverting input in both cases. That is only one error.
Cheers Bob
Cheers Bob
PS: There is one very important simple rule for an op amp. If you get your head around this nearly all basics fall into place.
An Op Amp will ALWAYS try to do whatever it takes to maintain the 2 inputs to be the same. That is say if the inverting amp basic version has the non inverting input grounded the output will be such that it will maintain the inverting input to me ground also. The inverting input pin in this case is often referred to as “virtual ground”. This same rule applies for both configurations. It is a characteristic of Op Amps.
If you “level shift” the non inverting pin to say half Vcc (as is often the case) the output will be such that the - input will follow and rest at half Vcc so “level shifting” an AC signal to this point and allow single supply operation for an AC signal. This “half Vcc” point becomes the virtual signal “ground”.
Hey Bob,
Cheers for the heads up re: the link.
I’ve been trying to explain that Op-Amp concept to a friend of mine for days! He struggles to understand how it can have approx. equal voltage on both inputs, but no current “flowing into” either, not sure where the confusion is coming from but we’ll get there. They’re certainly interesting.
Hi Zach
The concept of the 2 inputs being equal says most of it. The “equalising” current is supplied by the output.
Example:
Inverting amp. + input grounded.
Non inv input 1VDC applied via a 1k resistor.
Feedback R from output to - input. 2k.
If the - input was Gnd there would be 1mA in series resistor BUT to make that - input ground a balancing current of -1mA has to come from the output.
To do this requires -2V at the output 1mA through 2k.
So we have the 2 currents at the junction of 1k and - input adding up to zero (Kirschoff’s law) and the requirement of the 2 inputs being equal satisfied.
We now have a voltage gain of 2. which turns out being the values of the feedback resistor divided by the source resistance (in this case 1k assuming a generator resistance of close to zero.
Because the input current has been balanced out by the current supplied by the output there is effectively zero input current which implies a very high op amp input Z. Which is the case.
If you were to build up this simple circuit and measured the voltages you would find +1V at the source, 0V at the inverting (-) input and -2V at the output. WRT Ground. An inverted gain of 2.
I think there are lots of complicated Maths involved to describe this but I usually let someone else worry about that and tend to go for the simple explanation as told to me by one of those maths boffins years ago.
The simple approach has been working for me for a long time now and I don’t think I can make it any simpler than above reply and this one.
Cheers Bob
PS: By the way, just to add to the confusion. Because the inverting input is at virtual ground that input series resistor becomes the terminating resistor for the source so that has to be considered when choosing a value.
Thanks!! I didn’t even consider that as an option. It is so simple but makes sense.
Are there any you could recommend that would work with ESP32?
I am sort of there in terms of understanding the theory, but am still struggling with it. I am more of an experiential learner so would benefit more from practical applications. If there is a good device that works with an ESP32, I can have a go at some simple experiments. I set up a simple experiment using my Pt500, a Wheatstone bridge and my LM324 and it simply just didn’t work.
My multimeter confirmed that at 22C, the output of the Wheatstone bridge was 1305mV.
I used 56k feedback resistors and 10k input resistors, so with a gain of 5.6 I should have got an output of 7.48V. The actual reading from the ADC was 2V. @Zach has advised that the LM324 is not suitable for use with microcontrollers.
I have since considered abandoning the RTD and getting some thermocouples instead, as they are better for the temps I am seeking to reach and I can pair them with the MAX31855 amplifier. The other benefit of this approach is the ADC in the ESP32 is not very good, so this will circumvent that problem.
Hi Dion
What was your Vcc (the voltage powering the LM324). For an output of 7.48V you would need Vcc to be somewhere in the region of 10V. If you are only using the ever popular 3.3V then 2V would be about correct. The LM324 would be hitting maximum.
A rail to rail Op Amp would be better but still would be only about 3.2V with a 3.3V supply.
A point to remember here. You are now dealing with an analog scenario so you will have to consider some analog voltages (real ones).
Cheers Bob
Hi Dion
Be aware that board is suited for “K” type thermocouples only. Although the K type is probably the most common there are others around and the MAX 31856 caters for quite a few.
Also if you get a bare K type with only the wires at the end the positive connection is the YELLOW wire and the negative is the RED wire. Don’t ask why, just one of those little annoying things.
Cheers Bob
Hi @Dion267210,
As Bob said above, a rail to rail op-amp will still give you a small range (max around 3.2V), I was just pointing out that an LM324 was particularly not suited for an application where you already have a tight voltage swing range.
The LM324 not working for this case is about your supply voltage, not the combination with ESP32. If desired you could power the LM324 with a separate supply capable of producing the desired voltage levels, but IMO this is more trouble than it’s worth.
Additionally, I found this article showing empirical tests of different microcontroller ADCs, and it seems the ESP32 ADC only allows a voltage range of 0-1V. I don’t have as much experience with ESP32, so take this with a grain of salt, but it may be perfectly possible to use this without an amplifier stage at all. Just food for thought.
Cheers.
Yes, that is what I have ordered. I build small electric pizza ovens as a hobby and K-type is one that can handle temps up to 500C.
Thanks, that’s interesting. When I paired the RTD sensor with my Arduino but couldn’t get the Op Amp to work, I reverted back to taking the raw feed into the ADC through a simple voltage divider. The results actually weren’t too bad. My application is for temps between 400-500C, so I am not to concerned with accuracy <2C. I thought I could just transfer this setup to the ESP32, but the ADC readings and inferred voltage values didn’t make sense. Some research online uncovered that problems with the ADC are common.
As I mentioned to @Robert93820 I have abandoned the RTDs and will try with some K-type thermocouples and the MAX 13855 amplifier.
This is the article that originally got me started with pairing my RTDs with Arduino, but I found the document difficult to follow. I thought they were powering the LM324 off the Arduino, but I think they may be using a separate +5V/-5V source?
https://docs.openenergymonitor.org/electricity-monitoring/temperature/rtd-temperature-sensing.html
Hi Dion
That is quite possible. Wouldn’t it be nice if some of these co called “expert” authors could put the power supply arrangement on the schematic. Would save the guesswork but it is probably too much trouble. most decent professional schematics have this detail.
If I were marking that schematic in a class I would probably give it about 3 out of 10.
Cheers Bob
Hi Dion
Yes, all the work is done for you. I think this is what I used with a small Arduino when I built up a thermometer to check my soldering iron. All the work like cold junction compensation etc are done for you. Comms is via SPI and I think from memory reads the thermocouple temperature and also the chip internal temp. About all the Arduino does is read the SPI and display on a 1602 display unit.
Cheers Bob
The schematic is woeful and punishing for a newcomer to such a concept. You may have noted the designated pins for the voltage followers and the differential amplifier are all the same (2,3,1) ! Also the GND pin (11) is connected to the feedback loop for both voltage followers.
Hi Dion
This sort of thing can be confusing for an experienced person so a nightmare for a newcomer. Yes that ground pin appears to be connected but there is no junction indicated so is just bad drawing. One of the pin 11s is finishing in the middle of a resistor which is nonsense.
The author has used device “a” out of a quad device instead of “a”, “b”, and “c”. All options available in KiCad which implies he does not know much about the software package.
All up a pretty bad effort. Maybe we should make it 2 out of 10.
Cheers Bob
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