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doctor electrons

Sun Dec 23 2012, 06:42PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Is it possible to feed multiple devices with a single thermistor? Let me Explain a bit more about what i am trying to accomplish. ( went google crazy with this to no avail )

I have a thermoelectric cooling (and heating) system i am gathering parts for. I have found a controller for the peltier
chips that will reverse polarity to heat or cool. The controller also has a fan control output and a thermistor input (10k).
The application requires the use of four 90 watt chips. The chips are to be attached to aluminum blocks that are thermal epoxied to a stainless steel vessel. This application also requires things to be kept sanitary.

The problem : Each controller has its own thermistor and thermistor input. The less holes for thermowells drilled
into the vessel the better!!
The real question : Can one thermistor be wired to all four controllers without loosing any accuracy? Basically paralleling
the four controller inputs together with the thermistor wired to one of them?

I do not have the controllers yet so i cannot take any resistance measurements at the sensor terminals. I guess i am hoping one of the great members of 4hv has stumbled across something like this before!

Thanks in advance for any input!!

Ash Small

Sun Dec 23 2012, 07:12PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I don't see why not. A thermistor is just a 'resistive load'. It would be exactly the same as paralleling four power supplies to feed a common load.

You'd need to take into consideratoin though, that, say each controller sends 1 volt to the thermistor, the thermistor would only pass the same current as if it was connected to one controller, so each would read 1/4*I. You'd need to choose your thermistor accordingly.

doctor electrons

Sun Dec 23 2012, 07:31PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Thanks Ash!

I will have to wait until i have the controllers in hand to find out exactly what the excitation voltage is.
I actually overlooked the fact that the current would be split! Glad you pointed that out! That would surely
affect the operation of the setup!
I am trying to find a way to run all four controllers with 1 thermistor instead of four. Seems easy but it has been
just the opposite! Looked into splitters and other devices but had no such luck.

I believe there is an offset in the controller that could possibly make up for the current difference. It is a hysteresis
error setting. Hopefully that has enough adjustment, that would make things real easy!

Edit:

What about using one leg from the thermistor on say the + of the first controller, and the other on the - of the last.
The rest would have to be in series with a resistor or two to drop the voltages back down to normal. I am going to try
to model the circuit in multisim. Good thing about multisim is if you let the factory installed smoke out in there
it wont break the bank!

Ash Small

Sun Dec 23 2012, 07:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You should be able to get a thermistor with 1/4 the resistance of the value of the one the controllers are calibrated for.

That is all you need. It will pass 4*I, which will be shared by the four controllers.

doctor electrons

Sun Dec 23 2012, 07:56PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Appreciated!

I am going to search for it right now!! That's why i asked the question, there is always
an answer, i just didn't know it! Thanks again Ash, you rock!

Dr. Slack

Sun Dec 23 2012, 08:52PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Without knowing how the controllers excite and measure their thermistors, you may or may not be able to parallel them.

If each has a high impedance floating current source (which can be paralleled without problem), and a floating voltage measurement, then it will work.

I can dream up a few methods of measuring thermistors that will fight each other if you simply parallel them.

Caution

Ash Small

Sun Dec 23 2012, 11:38PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Slack wrote ...

Without knowing how the controllers excite and measure their thermistors, you may or may not be able to parallel them.

If each has a high impedance floating current source (which can be paralleled without problem), and a floating voltage measurement, then it will work.

I can dream up a few methods of measuring thermistors that will fight each other if you simply parallel them.

Caution

Yes, but the most common/most reliable method is to measure current from a fixed voltage source?

Dr. Slack

Mon Dec 24 2012, 11:21AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Ash Small wrote ...

Yes, but the most common/most reliable method is to measure current from a fixed voltage source?

Which bit of Caution don't you understand? Like check first, then parallel?

Actually, the most common thing for signal-level terminals is that the thing won't burst into flames, or even be damaged by wrong connections, so why not parallel, then test? However, it might be confusing.

If I was building cheap micro-controller based Peltier controller, I would probably measure the resistance of the thermistor by building it into an RC delay circuit, or an RC oscillator, and measuring the delay/frequency with an internal counter. In an analogue one, I'd may well use a Wheatstone bridge connection, with a low impedance at one end of the thermistor.

I would set up two controllers seperately, measure the voltage at the thermistor terminals, then start experimenting with them. Even if they both work by injecting current and measuring voltage, a difference in voltage on a sense terminal could cause one to be fully on, the other to be fully off.

But it's not my money or my gear, so please feel free to connect first and debug/repair later.

Steve Conner

Mon Dec 24 2012, 11:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've designed several Peltier controllers for use with 10k NTC thermistors. They all worked as follows:

One terminal of the thermistor went to signal ground. The other terminal was connected to a reference voltage via a 10k precision resistor. The voltage across the thermistor was read by the microcontroller's built-in ADC, relative to the same reference voltage. The result was a kind of bridge circuit that linearised the thermistor around 25 deg.C.

If you just paralleled these, the thermistor would get more current than it should. For four units you would need a 2.5k NTC. There would also be issues with ground loops, on my design at least, as signal ground was connected to power ground.

Elegant solution: Modify one controller or build an amplifier for it so it can drive 4 TEC modules.

Ghetto solution: Stuff the 4 thermistors into one thermowell.

Ash Small

Mon Dec 24 2012, 12:11PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Dr. Slack wrote ...

Which bit of Caution don't you understand? Like check first, then parallel?

I would assume that went without saying....ALWAYS check first.

Steve Conner wrote ...

If you just paralleled these, the thermistor would get more current than it should. For four units you would need a 2.5k NTC. There would also be issues with ground loops, on my design at least, as signal ground was connected to power ground.

You would need a thermistor of the correct rating.

I would suspect that the controllers might require 'slight' re-calibration.

These things are generally designed to be 'as idiot proof as possible'.

I also can't see how 'measuring the voltage across a resistor' would make any difference in this situation. It's still a case of 'four parallel power supplies powering a common load'. You won't get the problems you sometimes get when paralleling some SMPS's.

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