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zero-volt-drop solid-state switch for thermocouple
4

zero-volt-drop solid-state switch for thermocouple

zero-volt-drop solid-state switch for thermocouple

(OP)
I need to switch between 4 thermocouple inputs every few seconds so we can use one thermometer.  First thought is some 555-type ICs and some electromechanical relays.  I desire to avoid big mechanical parts, so a solid-state solution is preferred, but a SSR, BJT, or FET would have a voltage drop.

1) Is there a solid state relay or opamp that will precisely follow the input voltage, and allow me to switch between 4 outputs?

2) Will a thermometer designed to take J-type thermocouples work with a low-impedance buffered signal from an opamp?

Thanks!

 

RE: zero-volt-drop solid-state switch for thermocouple

That does not compute.  Relays are not perfect conductors.  If you're measuring voltage, then you're presenting a high impedance load, and the series resistance of a saturated FET should be insignificant.

And if you're really worried about it, why not throw in a gain stage before the switches?

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: zero-volt-drop solid-state switch for thermocouple

I think that you'd want a proper thermocouple switching system, perhaps from a company such as Omega.

Voltage drop isn't really the issue (the current can be arbitrarily low). The issue is introducing another half dozen unintended thermocouples into the signal path. A correctly designed purpose built thermocouple switch would be carefully design with appropriate materials to minimize unwanted offsets.

RE: zero-volt-drop solid-state switch for thermocouple

2
So true! But, if you want to roll your own, a simple 4066 4x analogue gate works well. The thermocouple output is unipolar so you don't need to go to DG switches.
Also, it is a one or two dollar component. So it doesn't cost much to learn from that mistake.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

Agreed but I'd use a HC4052 and I'd want that used right where reader is.


Otherwise use a commercial T/C multiplexor.

Keith Cress
kcress - http://www.flaminsystems.com

RE: zero-volt-drop solid-state switch for thermocouple

I would convert the signals to 4-20 mA and switch the 4-20. No hesitation. 4-20 mA signals will overcome quite a few Volts of drop and maintain accuracy.   

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: zero-volt-drop solid-state switch for thermocouple

But, Bill - that hurts. That is what anyone would do. Making it as cheap as possible is the name of the game here - I think.  smile  smile  smile

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

Not sure if this is one off or not.

The MiniMux is a commercial box that muxes up to 8 inputs to one output. Indicator LED shows which is active.

http://www.predig.com/PD138/index.php

RE: zero-volt-drop solid-state switch for thermocouple

(OP)
Thanks everyone for alerting me to the potential troubles of the route I'm taking!  

The 4066 is the exact type of answer I was looking for (and as a bonus we appear to have some in stock too), thanks Skogsgurra.

Yes, the goal is to make this as cheap and simple as possible, but I will look into these other solutions.  (The MiniMux looks neat but far too expensive.  The HC4052 apparently needs a binary-coded input to select channels, which is not ideal.)

This circuit will be physically close to the TC reader, and at the same ambient temperature.  

If I use the 4066, would an input resistance of 250 ohms be large enough?  Or will I need to insert an opamp beforehand?

RE: zero-volt-drop solid-state switch for thermocouple

250 ohms! That sounds more like an 4-20 mA receiver. If that is the case, you already have a current output transducer and you can use any switch or relay to switch between the channels.

Are you sure that you only have a thermocouple? Or is it a thermocouple with a signal conditioner? The former is a pair of wires soldered together while the latter is a device with terminals on it.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

(OP)
250 ohms is the stated ON resistance of the 4066.  I am sure we have thermocouples only, no signal conditioner.

I'm looking at the 4016 now, per the 4066's datasheet: "The MC14066B is designed to be pin–for–pin compatible with the MC14016B, but has much lower ON resistance."

Both 4066 and 4016 ON resistances are quite low--am I misinterpreting this parameter?

Either way 250 ohms should be plenty high for a TC measuring 4 ohms, right?

RE: zero-volt-drop solid-state switch for thermocouple

Just one last comment regarding reading thermocouples.  In my experience, I've used a variety of instruments to measure TC voltages, and have never been very impressed with their performance under (what I'd consider normal) operating conditions, i.e. when the temperature environment of the reader must vary from the 68F/20C norm of the producer's laboratory.  Portable/handheld readers have trouble with the tempco of gain setting resistors, and the linearity of their cold junction compensation across any temperature change, even if the temperature is allowed to equalize for up to 8 hours.  One of the worst performers I've found is the classic Fluke 2-channel TC reader (go figure, right Skogs?).  Whenever the accurate reporting of temperature is required, I've never gone too far wrong by grabbing some ice from the lunchroom freezer, and a styro cup with a little water, to form an ice junction.  Put one of your TC's in said cup and read "simultaneously" with the other TC's on the multiplexor, and you'll have at least one point of sanity.

RE: zero-volt-drop solid-state switch for thermocouple

Ryre
Sorry, did you mean channel resistance when you wrote input resistance?

Channel resistance is not a problem if your actual input resistance is 1 or 10 Mohms, typical values. The voltage drop would less than one thousandth or one tenthousandth of the TC voltage.

If your input resistance is low, you cannot use the 4066.

BTB
I see what you are referring to. I seldom use TC for low temperatures. NTC or Pt100 there. NTC mostly. They are quite good in my applications (winding temperatures and such).

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

The main reason thermocouple thermometers are usually significantly more expensive than RTD thermometers is that the contain an RTD thermometer to provide temperature compensation for the reference junction which is inside the thermometer. A thermocouple thermometer can only measure the difference in temp from the reference junction. So if it doesn't know the reference temp., it can't know the measured temp.

RE: zero-volt-drop solid-state switch for thermocouple

(OP)
Skogsgurra, yes I did mean channel resistance.  I was confused since that is the only impedance listed in the datasheet.

Input resistance is measured to be about 2 megaohms, so it shouldn't be an issue.

RE: zero-volt-drop solid-state switch for thermocouple

OK - everything settled then?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

(OP)
Yes, all set--thank you!

The 4066 seems like it'll work--temperature readings are close enough since we don't need absolute precision, and the IC will be at ambient temperature.

RE: zero-volt-drop solid-state switch for thermocouple

One more observation: There can be some crosstalk in 4066 from the control signals to the output, especially during the switching times. According to the data sheets, this may be roughly in the range 10-100mV. This may appear as spikes and/or as a small offset in the output (output voltage slightly different from the input voltage). ( I learned this in the hard way...)   

RE: zero-volt-drop solid-state switch for thermocouple

Yes, there is a picocoloumb charge transfer on every switching edge. But that will not be a problem if the source or sink resistance is low (source resistance in this case). And it will not be noticed in a system where you do not switch very often and where a display is used to read temperature.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: zero-volt-drop solid-state switch for thermocouple

My simple thoughts , accuracy -vs- complexity .

Could a relay or two be useful for killing unwanted channels , as
crosstalk may be a gathering of unexpected sources of interference !

 
 
 

RE: zero-volt-drop solid-state switch for thermocouple

Why would you want to sample a thermocouple (best response time, for a bare bead of say, 28 gauge wire, is still 10ths of a second) at anything near the microseconds or whatever that the switch transients are measured in?  Just switch, allow 10x settling time, take an A/D sample, then switch to the next TC.  Even 16 channels, 0.1 second intervals for each, is only 160 Hz...

RE: zero-volt-drop solid-state switch for thermocouple

According to that post, it supposedly takes 0.1s per channel, so 16 channels take 1.6 s.  

Doing 16 channels in 0.1 s requires ~6 ms read time.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: zero-volt-drop solid-state switch for thermocouple

Take 16 channels, and assume you want to sample each channel at 0.1 second intervals.  16/0.1 = 160.

I purposely did NOT say "0.1s per channel", I said 0.1 second intervals.  I also did not say 16 samples in 0.1 second.  

But in any of your cases, Irstuff, is the mux and its settling time a limiting factor?   

RE: zero-volt-drop solid-state switch for thermocouple

I just having problems with the semantics.  

160Hz sampling is 16 samples in 0.1 s --> 16 samples / (160 samples/s) = 0.1 s, and therefore, each sample is taking 6ms, assuming there's a single meter being mux'd.  If that's the case, then the assumption of 0.1s settling is not valid.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: zero-volt-drop solid-state switch for thermocouple

I never calculated or stated settling time, I just said you are switching the mux at 160 Hz.  Settling time is whatever the system will settle at, and I would take whatever the predicted settling time is and multiply by 10 before taking the sample.  Doing all that within 6 ms sure seems do-able, with the fairly low impedance of a thermocouple.  I know I've done it before...but had a pretty good A/D board to work with.

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