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Hi all,
I need some advice on how to read a temperature "on the cheap"
I have a PLC, it allows for 0-10volt input "analogue", I know than you can some way have a RTD, NTC or somthing similar, maybe in series with a fixed resistor.
I have a 24volt DC power supply
I am looking for a control temp of 120C
( I know that you can but a converter, but are expensive in this application)
cheers
Mad
How cheap? Do you want it to be independent of the the PLC or must it also be an input?
I have plenty of inputs, so permanent is required, if i can do one then I would add others (for no other reason that more numbers look better)
CHEAP, a few bucks ($5-10)
Something like this MF?
http://www.electrocontrols.co.uk/pro...sheets/D02.pdf
Or maybe one of these?
http://www.electrocontrols.co.uk/pro...sheets/H15.pdf
it needs a converter eh, no way to use the thermister alone?
That is an odd resistance. If there was one with a nominal 1k@25C, it would make sourcing the thermisters easier.Quote:
Originally Posted by frank
Hi Mad,
In my experience PLC analog input almost always uses an industry standard sensor (usually PT100) with an associated transmitter, to convert the sensor resistance to the correct Vdc or mA signal. I have never seen any form of resistive temperature sensor connected direct to PLC input, however I am familiar with certain PLC's having input cards suitable for direct connection to thermocouple (with inbuilt cold junction compensation). Typically thermocouples are very cost effective, but probably only worth the expense of the TC input card if multiple inputs are required. Hope this helps.
I have in the past always work on industrial gear with PLCs, and yes use PT100 and converters/transmitters, money not really an issue. most PLc manufactures also have specialist moduals.Quote:
Originally Posted by HVACRsaurus
How ever i have just found out how to achieve my desired results. Very cost effective, accuracy is not great, but more than good enough for what i am doing.
12 volt input to NTC
NTC output to PLC 0-10V input
PLC input to resistor input
resistor output to ground
Resistor size to be calculated
Glad I finally can help you.
Adding a resistor to a PT100 will not work MF because a current will flow through your PT100, it will heat up and measure faulty temperatures.
I advise using an LM35 temperature sensor( cheap, very accurate, linear mV/°C signal, from -55°C till 155°C,..)
If you can't use the mV and ramp it up in your PLC, you only need to add some very cheap electronics to make it work in the range.
[QUOTE=Peter_1;243409]Glad I finally can help you.
Adding a resistor to a PT100 will not work MF because a current will flow through your PT100, it will heat up and measure faulty temperatures.
I advise using an LM35 temperature sensor( cheap, very accurate, linear mV/°C signal, from -55°C till 155°C,..)
If you can't use the mV and ramp it up in your PLC, you only need to add some very cheap electronics to make it work in the range.
Don't you just love google translation. Very funny. Good circuit, thanks Peter
The cheapest way is a TC connecting to a analog/digital converter like Maxim 31855 and bring this in a microchip with the proper code. But that's far beyond my expertise.
Thanks Peter, your a little gem! (sorry for the delay in response, been out of town) i will investigate these.
I was looking at 10K NTC, not a PT100, but as you stated the NTC are not linear.
I will investigate LM35.
Building a circuit board from scratch, is the long term cheapest way, but i am not at that stage.
Also I do not need the accuracy, as much as my settings are rated conservatively in the first place.
cheers
Mad
Look at a small IC the XTR102 it will give you high precision with a Pt100 and it has an mA output signal, but with a simple resistor of 500 ohms you will get exact 2 - 10 volts.
you can measure better as 0.01 Celsius if PLC has enough bits input.
It is even possible to have multiple inputs.
want to know more send me an email to shooter@home.nl
These IC are used in the little heads on top of sensors but then the cost is 100 bucks or more.
the XTR is avaiable at RS and farnell.
Paul, If you put the system together as you suggest, what would the rough cost be? It would help me as well as Mad.Quote:
Originally Posted by shooter
The XTR105 is costing around 10 $ the rest is some resistors to set the gain and offset
put it on a small breadboard and it will work
so within your budget.
look on internet for this circuit XTR105 Burr brown www.ti.com/lit/gpn/xtr105
several settings in table.
if needed more just give message.
The 31855 is only 4 to 5 $ and the TC is much cheaper then a PT100
Quote:
Originally Posted by Peter_1
Peter, I got a Trojan Horse from your link. Is there another place to get it?
OK, I am still not clear on this, Peter and Paul, assuming I would like to use a standard PLC or any controller with an analog input, I will need to have some sensor that can be either a PT100, PT1000 or similar (PT1000 is common for me to use). I will then need the signal converter, XTR102 or equiv. Is this not a lot of work when you need to make 100-300 units? Lets assume this to be used in the control of a custom unit (HP or AC or .....)
With the LM35, you still will need to pot the IC and make some small amount of electronics. Peter have you done this?
Mike, the more your need, the cheaper it will come but then you have to go - i guess - for a PIC.
Tell me exactly what application you need it for. You want to use a PT1000 - which is expensive- but what do you need as an output or to what device do you want to connect it? IS this for 300 pieces?
I never done it myself but I know pretty well how these things works and what is possible. And I'm extremely interested in electronics.
For the practical side, I have a friend who is very experienced in electronics, especially PIC programming and all the peripherals around it like WiFi, Zigbee, GPS, ... He translates my wishes in a working PCB.
This is even much more than refrigeration a very specialized job.
Peter
Post 10 AVG just blocked this link for me too. Due to virus ect, I did not see MikeHolm post to later.......
some PLC can read a mV signal, but most sensors have only very small signals.
only the ones with some sort of amplifier are direct connectable. but they are expensive, or the card in the PLC is expensive
standard input is still 10 volt, and i do not know ant cheap sensor that can do this.
If you need a fixed point sensor look at a clixon.
If you do not need high precision you could try it with a simple resistor and 24 volt, however this resolution will be about 50 points max, as you loose a lot.
You can also use a simple 555 and measure the time.
xtr105 is very precise and exactly made for a Pt100, for pt1000 also speciall made IC.
You also can use a dedicated tempcontroller like a elliwel.
my favorite is the xtr105
however you could use any opamp, this one has the advantage of running of 24 volts single source and gives you 10 volts back. so no conversion.
PIC, yet another skill to learn. I have been making control panels from relays for 25 years, and i know i could save a lot of time and materials with PLCs, or PIC controllers but the time it takes to learn to program stips me dead. Just not enough time in a day. I am looking for something to use with the 20-30 custom systems I make annually and the price of add ons for the PLCs can be high.Quote:
Originally Posted by Peter_1
Therefore this is a very good thing to be able to produce, as well as a pressure signal processor for the PLC if you have a circuit for that.
are the PT100 thermisters less than $5 ea? I get PT1000 for about $12 but in quantity it gets expensive.
I have a low level PLC, reads at stages 0.02 volts or maximum possible readings of 500 of the 0-10v range, and practically more than likely 400.
Accuracy is never going to be great, and I for heat pump water heating, high accuracy is not really needed.
Because I do have a number of spare inputs, and a small operator interface, i would be nice to show a few temperatures. (only one is sort of critical, discharge temp!) but this still set conservatively in the scheme of the mechanics.
By adding NTC, lets say a 20Kohm (which in NZ are only around $12 (6euro), and a 1000ohm (???) resistor only a few cents, and is easy to install.
Ok you need to do some maths in the PLC, which again will be very low level, loosing more accuracy.
It would seem that we will end up with a reading of of +/- 2.5C across the predetermined ranges, if you want the whole temp range covered this will be higher
Mike, I have at least 20 books about microcontrollers, from PIC's to GWBasics, to BX-20 processors, Basicstamp,...Quote:
Originally Posted by MikeHolm
All because I want to learn more about this but don't have the time. I followed courses for PIC, given by my friend. But we have to realize, this is really specialized work and if you want to do it on a certain level, you have to do it every day. There are so much tricks you have to know that it takes months before you reach a decent level.
If you want to start learning the basics of a microcontroller, then you should visit once the website of Parallax. You can download there freebooks explaining programming PIC's via some sort of Basic.It really starts at a low level and its written for kids in the technical schools in the age of 15 years.
I could realize after 1 week, each evening several hours making a thermostat which showed on an LCD actual temperature, my name , set temperature, DT,... I felt so happy that I understood finally how the controls of a multiline text- or graphic LCD works.
I have a small cupboard full of breadbords, led's ,IC's, Capacitors, resistors,..
Look once to my post of mid 2004 http://www.refrigeration-engineer.co...ighlight=stamp I haven't controlled the links.
There's a lot to find about microcontrollers.
Look also once at www.tri-plc.com (cheap and very reliable OEM PLC's, I have used some of them) and the very good and cheap product of www.unitronics.com, found in many machines, also Coca Cola uses these.
But, for 20 to 30 units/year, I think my friend can make something for you. Contact me once in PM and explain what you need. He already made a PCB for a small heatpump manufacturer/installer in The Netherlands for only 20 units/year. It reads several temperatures, pressures, a multiline text LCD, connected to the internet via IP, and freely programmable. It will cost in the range of 350 to 400 € (for the PCB)
At Unitronics, if you don't need graphics, only some text, then look for the Jazz range. They have controllers which accepts immediately RTD's and TC. Very good and cheap.
For example: a JZ10-11-UA24, 9 digital inputs, 2 digital/analog inputs (you can choose), 2 analog inputs (LP and HP pressure for example), =total of 13 inputs + 2 x PT1000 or TC,
Output5 relays + 2 transistors + 2 analog, (p.e. VFD) , 2 PWM's,
2 lines of 16 characters LCD
Price beginning 2010: +/- 180 €.
These are interesting prices compared to a PLC where you have to add additional I/O's
And...software is free!!http://www.unitronics.com/Content.as...nloads&CatId=3
I'm not related to Unitroncs at all.
Hi Peter,
was the price you indicated for the model you indicated, or just the price indicated for the very basic unit?
cheers
Mad
MF, it was for the model I gave but without taxes and transport. The basic model is 113 €
The model I gave, second last one on page 16 http://www.unitronics.com/data/uploa...re_09_2011.pdf (their page 16, printed on the pages)
I see now my prices are from mid 2009, not 2010. But this will not give a big difference I guess.
Jazz
JZ10-11-T10 Jazz OPLC, Disp 2x16, 6DI, 4TO 113 €
JZ10-11-T17 Jazz OPLC, Disp 2x16,10/7IO,AN 131 €
JZ10-11-R10 Jazz OPLC, Disp 2x16, 6DI, 4RO 113 €
JZ10-11-R16 Jazz OPLC, Disp 2x16,6DI, 2D/AI, 2AI, 6RO 131 €
JZ10-11-R31 Jazz OPLC, Disp 2x16, 16DI, 2A/D, 2AI, 11RO 161 €
JZ10-11-T40 Jazz OPLC, Disp 2x16, 16DI, 2A/D, 2AI, 20TO 173 €
JZ10-11-PT15 Jazz OPLC, Disp 2x16, 3DI, 3A/DI, 3PT1000, 1TO 222 €
Thanks Peter, that is awesome
Thanks Peter, this is awesome and will take some time to digest. I sometimes can't believe myself. 28 years ago I obtained an electronics engineering technologist certificate (3 year course) and since then i have forgotten 99% of it. I always had a phobia around programming. To me, if I can't see it it ain't there. I'm pretty good with relays and such but i couldn't program my way across the street...:D