i'm building a 16 ch nterface for reading 16 TC type K. I want switch the 16 TC only on positive wire and negative in common on ground with 74hc4067. Aftert the mux i place a AD595 and after a max 187 for reaDIN ON PARALLEL PORT on PC. Somebody has already made this? Do you think that is function? Any tips?
Ciao, Massimo
Why do you want to "switch the 16 TC only on positive wire" ?
Conventionally one would use a two CD4067/4051's etc or a CD4052 to switch _both_ leads of the thermocouples. Typically this would be followed by an amplifier. And one of the inputs would go to a temperature reference to provide "ice point" reference.
But the AD595 (which I haven't used) incorporates amplification and ice point, so you would seemingly just need to assure that the thermocouple leads and the AD595 are at the same temperature but I don't know what the " thermocouple failure alarm" circuitry consists in (that might be a complication -- I don't know).
Or did I misunderstand your question?
Marc Marc_F_Hult
Hi Marc,
Thanks forthe answer.
Yes the AD 595 contain an Alarm circuit that control if thare is a open TC(2 wire open) but isn't necessary to use it.
I tough to switch only the positive laed of TC and connect all 16 negative lead togheter with the common ground of the circuit ..?ve seen that some rotary selector for TC switch only 1 wire and the other are connect togheter to gound. I?ve seen bad? This can create some error? My problem is that i dont't know how to control via Parallel port 2 4067 mux. Yes i'll keep the ad595 and leadsTC at the junction at the same temperature "Marc F Hult" ha scritto nel messaggio news: snipped-for-privacy@4ax.com...
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This is OK if you are not measuring temperatures below 0 C (The "ice point' is typically set to 0 volts) *AND* you have adequate input protection.
The solution is simple: both multiplexors are controlled by the _same_ 4-bit signal so both thermocouple wires are switched at the same time.
Are you planning to use a single (+ only) or bipolar ( + and -) power supply? The digital switching is slow enough that it doesn't add significant noise and the AD595 is rated for a single 5.0 vdc supply so one could use a single 5vdc supply for both analog and digital. Conventionally, one uses bipolar power supplies if temperatures below freezing need to be measured because freezing (0 C) is often set to 0 volts.
It's a good idea to have overvoltage protection at least at the outputs of the multiplexors. If you were following the multiplexors with a differential amplifier as is conventional, I would suggest that you put 100Kohm resistors in series with each thermocouple lead between the lead and the multiplexor input. But I don't how those series resistors would affect the thermocouple error circuit in the AD595.
For bi-polar supplies, use two zeners in 'reverse parallel' (one in each polarity from signal to ground) at each output (4 zeners total). Together with the 100Kohm input resistors, they provide good protection.
On pc boards used for sensor input and external control I like to put the first IC's in the signal path leading onto and from the board in sockets. That way the IC's can be easily replaced if they are damaged for whatever reason. These IC's are cheap, so it is a practical solution.
An input multiplexor board along this design is needed for the supplemental,
16-bit-resolution analog input on the hard-wired lighting controller I describe atHope This ... Marc Marc_F_Hult
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