On Friday, December 17, 2021 at 2:32:16 AM UTC+11, jla...@highlandsniptechnology.com wrote:
> On Thu, 16 Dec 2021 01:20:00 -0500, Spehro Pefhany 
> <spef...@interlogDOTyou.knowwhat> wrote: 
> 
> >On Mon, 13 Dec 2021 22:40:09 -0800 (PST), Anthony William Sloman 
> ><bill....@ieee.org> wrote: 
> > 
> >>Back when I did one, back in 1979, you needed a lot of gain in that one op = 
> >>amp. I ended up going with four - it made the low pass filtering a lot easi= 
> >>er. My bosses got tetchy, so I had to draw the one, two and three op amp op= 
> >>tions. Electrotherm had a bulk deal on the uA715, so the one amplifier opti= 
> >>on wasn't all that expensive, but nothing else around at the time had that = 
> >>much gain. 
> > 
> >Odd, RTDs have such high output even a single LM358 should be about 
> >good enough for *most* purposes unless you're running them at much 
> >lower than normal current or looking for off-label uK performance 
> >(which I remember you were doing with thermistors). 
> > 
> >At 0.5mA you get ~200uV/&deg;C with a Pt100 ohm DIN RTD. Interchangability 
> >is maybe 1/3&deg;C for inexpensive ones (at room temperature) and a degree 
> >or two at extremes, so for normal purposes- any modern op-amp assuming 
> >you don't mind trimming the offset. 
> > 
> >Coincidentally, about the same output as you get measuring temperature 
> >with silicon BJT(s) operated at 10:1 current ratio. 
> > 
> >>The linearising involved a smidgen of positive feedback which fr= 
> >>ightened the guy who took over the project, and nobody could make him see s= 
> >>ense. 
> > 
> >Shouldn't be too hard to show mathematically that the net feedback is 
> >strongly negative for all sensible RTD values. And that break 
> >protection is in the safe direction. 
> > 
> >He probably would be terrified (with some justification) by an 
> >enhanced Howland current source. 
> > 
> >The cool thing is that the same resistor that linearizes the RTD can 
> >be used to avoid having an active current source for excitation. Just 
> >a resistor from a reference voltage will do for excitation- there is 
> >no advantage to using a current source.
>
> I thought I invented that! One resistor to the + supply and one more 
> for positive feedback from the opamp output, RTD to ground. 

When? I didn't think that what I was doing in 1979 was any kind of invention, and I'm fairly sure that Honeywell Pt-resistance sensor option that came out within a year or two used the same trick - not that you can entireily rely on what you read in the trade literature.

> I did invent a 3-wire version too. Can't remember that circuit now. 
> Nowadays we just use a mux'd differential-input delta-sigma ADC and 
> measure the RTD ratiometrically against a good resistor; don't need a 
> very good reference.

I had to point out - to the software guy - that we were using the same voltage reference for the A/D converter and the bridge  excitation in the circuit that I publlshed in 1996. 

-- 
Bill Sloman, Sydney

On Thu, 16 Dec 2021 01:20:00 -0500, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

>On Mon, 13 Dec 2021 22:40:09 -0800 (PST), Anthony William Sloman
><bill.sloman@ieee.org> wrote:
>
>>Back when I did one, back in 1979, you needed a lot of gain in that one op =
>>amp. I ended up going with four - it made the low pass filtering a lot easi=
>>er. My bosses got tetchy, so I had to draw the one, two and three op amp op=
>>tions. Electrotherm had a bulk deal on the uA715, so the one amplifier opti=
>>on wasn't all that expensive, but nothing else around at the time had that =
>>much gain. 
>
>Odd, RTDs have such high output even a single LM358 should be about
>good enough for *most* purposes unless you're running them at much
>lower than normal current or looking for off-label uK performance
>(which I remember  you were doing with thermistors). 
>
>At 0.5mA you get ~200uV/&#4294967295;C with a Pt100 ohm DIN RTD. Interchangability
>is maybe 1/3&#4294967295;C for inexpensive ones (at room temperature) and a degree
>or two at extremes, so for normal purposes- any modern op-amp assuming
>you don't mind trimming the offset. 
>
>Coincidentally, about the same output as you get measuring temperature
>with silicon BJT(s) operated at 10:1 current ratio. 
>
>>The linearising involved a smidgen of positive feedback which fr=
>>ightened the guy who took over the project, and nobody could make him see s=
>>ense.
>
>Shouldn't be too hard to show mathematically that the net feedback is
>strongly negative for all sensible RTD values. And that break
>protection is in the safe direction. 
>
>He probably would be terrified (with some justification) by an
>enhanced Howland current source. 
>
>The cool thing is that the same resistor that linearizes the RTD can
>be used to avoid having an active current source for excitation. Just
>a resistor from a reference voltage will do for excitation- there is
>no advantage to using a current source. 

I thought I invented that! One resistor to the + supply and one more
for positive feedback from the opamp output, RTD to ground.

I did invent a 3-wire version too. Can't remember that circuit now.
Nowadays we just use a mux'd differential-input delta-sigma ADC and
measure the RTD ratiometrically against a good resistor; don't need a
very good reference.


-- 

I yam what I yam - Popeye

 Sylvia Else wrote:
=================
>
> As I've indicated in my reply to Jan, I believe that the circuit in my 
> UPS is not providing anything like the required compensation. 

**As if a lying, autistic tenth wit like you would know . 


> the designer's remit was to provide some temperature compensation, so he 
> did, without caring whether it was the correct amount. 

**  Maybe he, she, it had brain tumor   -  big as yours. 



......  love,  Phil 

On Thursday, December 16, 2021 at 5:20:17 PM UTC+11, Spehro Pefhany wrote:
> On Mon, 13 Dec 2021 22:40:09 -0800 (PST), Anthony William Sloman  <bill....@ieee.org> wrote: 
> 
> >Back when I did one, back in 1979, you needed a lot of gain in that one op amp. I ended up going with four - it made the low pass filtering a lot easier. My bosses got tetchy, so I had to draw the one, two and three op amp options.

> Electrotherm 

Actually Eurotherm. I was working for Chessel Recorders at the time (which was part of the Eurotherm group.

 > had a bulk deal on the uA715, so the one amplifier option wasn't all that expensive, but nothing else around at the time had that much gain. 
>
> Odd, RTDs have such high output even a single LM358 should be about 
> good enough for *most* purposes unless you're running them at much 
> lower than normal current or looking for off-label uK performance 
> (which I remember you were doing with thermistors). 

It certainly wasn't for micro-Kelvin applications. Chessel Recorders were manufactured in high volume for low end applications like bakeries and breweries.

> At 0.5mA you get ~200uV/&deg;C with a Pt100 ohm DIN RTD. Interchangability 
> is maybe 1/3&deg;C for inexpensive ones (at room temperature) and a degree 
> or two at extremes, so for normal purposes- any modern op-amp assuming 
> you don't mind trimming the offset. 

If you wanted 10 degrees to fill the 0 to 10V span of the chart recorder that's a gain of 5000, which is quite high. It's forty years ago, so the details are hazy.

> Coincidentally, about the same output as you get measuring temperature with silicon BJT(s) operated at 10:1 current ratio. 
> 
> >The linearising involved a smidgen of positive feedback which frightened the guy who took over the project, and nobody could make him see sense. 
> 
> Shouldn't be too hard to show mathematically that the net feedback is  strongly negative for all sensible RTD values. And that break  protection is in the safe direction. 

He seemed to think that any positive feedback was risky

https://en.wikipedia.org/wiki/Geometric_series

makes the point that the sum with any r less than one is finite, which isn't exactly higher mathematics, and r was about +0.003
 
> He probably would be terrified (with some justification) by an  enhanced Howland current source. 
> 
> The cool thing is that the same resistor that linearizes the RTD can  be used to avoid having an active current source for excitation.

That wasn't the approach I was using, We just pushed the current through the sense resistor up a bit as it got hotter to compensate for the slow decline in sensitivity with rising temperature. Very crude.

> Just a resistor from a reference voltage will do for excitation- there is no advantage to using a current source. 

There is a bit. A Wheatstone bridge throws away a bit of the sensitivity - in a symmetrical bridge, half of it. It certainly isn't worth the trouble of going to current source excitation.

--
Bill Sloman, Sydney

On 15-Dec-21 3:06 am, Rick C wrote:
> On Friday, December 10, 2021 at 9:02:59 PM UTC-4, Sylvia Else wrote:
>> On 11-Dec-21 11:30 am, David Eather wrote:
>>> On 10/12/2021 9:50 pm, Sylvia Else wrote:
>>>> Anyone aware of such a thing?
>>>>
>>>> Clearly, one can achieve that just by putting a thermistor in series
>>>> with an ordinary resistor, but my UPS seems to contain a single
>>>> component that behaves that way - perhaps less than 1% at 20 Celcius -
>>>> it's in the circuit that controls the battery charging float voltage.
>>>>
>>>> Did they perhaps exist 20 years ago (when my UPS was made), for some
>>>> reason?
>>>>
>>>> Sylvia
>>>
>>> lead acid batteries are temperature sensitive when recharging
>> Yes, this is presumably the reason for the NTC resistor in the charging
>> circuit. But if it has a typical thermistor characteristic, then in this
>> particular implementation, the compensation will be out by a factor of
>> ten, which is probably worse than no compensation at all.
>>
>> I may have to remove it from the circuit to better characterise it.
> 
> I worked on a design last year where the charging circuit had a thermistor to adjust the batter voltage.  I pointed out the resistor was on a board near a motor and other power circuits that would certainly make the PWB hotter than the battery at the other end of the cabinet.  The designer didn't care.  He would often wax on about the various nits he had picked up over the years and yet could not understand something so simple as a temperature sensing thermistor needed to be thermally coupled to the battery rather than the motor the battery would be driving.
> 

As I've indicated in my reply to Jan, I believe that the circuit in my 
UPS is not providing anything like the required compensation. Perhaps 
the designer's remit was to provide some temperature compensation, so he 
did, without caring whether it was the correct amount.

Sylvia.

On 14-Dec-21 8:04 pm, Jan Panteltje wrote:
> On a sunny day (Fri, 10 Dec 2021 22:50:31 +1100) it happened Sylvia Else
> <sylvia@email.invalid> wrote in <j1gt8aF63iuU1@mid.individual.net>:
> 
>> Anyone aware of such a thing?
>>
>> Clearly, one can achieve that just by putting a thermistor in series
>> with an ordinary resistor, but my UPS seems to contain a single
>> component that behaves that way - perhaps less than 1% at 20 Celcius -
>> it's in the circuit that controls the battery charging float voltage.
>>
>> Did they perhaps exist 20 years ago (when my UPS was made), for some reason?
>>
>> Sylvia
> 
> Used a simple Si diode as temp sensor in some equipment.
> -2 mV / degree C or there about, linear at that.
> 
> 
> 

In the end, it became apparent that this thermistor is in parallel with 
an ordinary resistor far away on the board.

Also, that the temperature compensation it provides is much too low. 
That will be fixed, along with putting the thermistor on the batteries.

Sylvia.

On Mon, 13 Dec 2021 22:40:09 -0800 (PST), Anthony William Sloman
<bill.sloman@ieee.org> wrote:

>Back when I did one, back in 1979, you needed a lot of gain in that one op =
>amp. I ended up going with four - it made the low pass filtering a lot easi=
>er. My bosses got tetchy, so I had to draw the one, two and three op amp op=
>tions. Electrotherm had a bulk deal on the uA715, so the one amplifier opti=
>on wasn't all that expensive, but nothing else around at the time had that =
>much gain. 

Odd, RTDs have such high output even a single LM358 should be about
good enough for *most* purposes unless you're running them at much
lower than normal current or looking for off-label uK performance
(which I remember  you were doing with thermistors). 

At 0.5mA you get ~200uV/&#4294967295;C with a Pt100 ohm DIN RTD. Interchangability
is maybe 1/3&#4294967295;C for inexpensive ones (at room temperature) and a degree
or two at extremes, so for normal purposes- any modern op-amp assuming
you don't mind trimming the offset. 

Coincidentally, about the same output as you get measuring temperature
with silicon BJT(s) operated at 10:1 current ratio. 

>The linearising involved a smidgen of positive feedback which fr=
>ightened the guy who took over the project, and nobody could make him see s=
>ense.

Shouldn't be too hard to show mathematically that the net feedback is
strongly negative for all sensible RTD values. And that break
protection is in the safe direction. 

He probably would be terrified (with some justification) by an
enhanced Howland current source. 

The cool thing is that the same resistor that linearizes the RTD can
be used to avoid having an active current source for excitation. Just
a resistor from a reference voltage will do for excitation- there is
no advantage to using a current source. 



>--=20
>Bill Sloman, Sydney
-- 
Best regards, 
Spehro Pefhany

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1jqv3YkW8CYw

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-- 
Edward Hernandez <dtgamer99@gmail.com> wrote:

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> From: Edward Hernandez <dtgamer99@gmail.com>
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> The John Doe troll stated the following in message-id 
> <sdhn7c$pkp$4@dont-email.me>:
> 
>> The troll doesn't even know how to format a USENET post...
> 
> And the John Doe troll stated the following in message-id 
> <sg3kr7$qt5$1@dont-email.me>:
> 
>> The reason Bozo cannot figure out how to get Google to keep from 
>> breaking its lines in inappropriate places is because Bozo is 
>> CLUELESS...
> 
> And yet, the clueless John Doe troll has itself posted yet another
> incorrectly formatted USENET posting on Tue, 14 Dec 2021 06:40:44 -0000
> (UTC) in message-id <sp9e9b$q3d$1@dont-email.me>.
> 
> This posting is a public service announcement for any google groups
> readers who happen by to point out that the John Doe troll does not even
> follow it's own rules that it uses to troll other posters.
> 
> mFpwbmxNA8bW
> 
> 
> 

On Friday, December 10, 2021 at 9:02:59 PM UTC-4, Sylvia Else wrote:
> On 11-Dec-21 11:30 am, David Eather wrote: 
> > On 10/12/2021 9:50 pm, Sylvia Else wrote: 
> >> Anyone aware of such a thing? 
> >> 
> >> Clearly, one can achieve that just by putting a thermistor in series 
> >> with an ordinary resistor, but my UPS seems to contain a single 
> >> component that behaves that way - perhaps less than 1% at 20 Celcius - 
> >> it's in the circuit that controls the battery charging float voltage. 
> >> 
> >> Did they perhaps exist 20 years ago (when my UPS was made), for some 
> >> reason? 
> >> 
> >> Sylvia 
> > 
> > lead acid batteries are temperature sensitive when recharging
> Yes, this is presumably the reason for the NTC resistor in the charging 
> circuit. But if it has a typical thermistor characteristic, then in this 
> particular implementation, the compensation will be out by a factor of 
> ten, which is probably worse than no compensation at all. 
> 
> I may have to remove it from the circuit to better characterise it. 

I worked on a design last year where the charging circuit had a thermistor to adjust the batter voltage.  I pointed out the resistor was on a board near a motor and other power circuits that would certainly make the PWB hotter than the battery at the other end of the cabinet.  The designer didn't care.  He would often wax on about the various nits he had picked up over the years and yet could not understand something so simple as a temperature sensing thermistor needed to be thermally coupled to the battery rather than the motor the battery would be driving.  

-- 

Rick C.

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