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!
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
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
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
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
Otherwise use a commercial T/C multiplexor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: zero-volt-drop solid-state switch for thermocouple
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: zero-volt-drop solid-state switch for thermocouple
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 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
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
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
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
ht
See page 7.
RE: zero-volt-drop solid-state switch for thermocouple
RE: zero-volt-drop solid-state switch for thermocouple
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
RE: zero-volt-drop solid-state switch for thermocouple
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
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 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
RE: zero-volt-drop solid-state switch for thermocouple
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
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
RE: zero-volt-drop solid-state switch for thermocouple
TTFN

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

http://www.Hi-TecDesigns.com
RE: zero-volt-drop solid-state switch for thermocouple
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
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
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