interference? problem
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I am building a controller for my thermal solar panels.
The controller measures two temperatures and activates pumps depending on the
differential it finds.
I am using two LM35s and on the breadboard things work well, but one of the
sensors needs to be on the roof, at the other end of a 50- to 100 foot cable and
if I hook the sensor to the other end of that long line the readings become
quite erratic. Not totally off, they bounce +- 10 degrees around the real value
but obviously there is something not right.
What is the problem?
Do I need shielded cable?
TIA Uwe
The controller measures two temperatures and activates pumps depending on the
differential it finds.
I am using two LM35s and on the breadboard things work well, but one of the
sensors needs to be on the roof, at the other end of a 50- to 100 foot cable and
if I hook the sensor to the other end of that long line the readings become
quite erratic. Not totally off, they bounce +- 10 degrees around the real value
but obviously there is something not right.
What is the problem?
Do I need shielded cable?
TIA Uwe
Comments
othello159@h... writes:
I am building a controller for my thermal solar panels.
The controller measures two temperatures and activates pumps depending on the
differential it finds.
I am using two LM35s and on the breadboard things work well, but one of the
sensors needs to be on the roof, at the other end of a 50- to 100 foot cable
and if I hook the sensor to the other end of that long line the readings become
quite erratic. Not totally off, they bounce +- 10 degrees around the real
value but obviously there is something not right.
What is the problem?
Do I need shielded cable?
____________________________________________
When porting signals over long distances, one possible solution is to port
current, not a voltage signal. One common range is 4 to 20 mA, where 4 mA is
equivalent to the minimum and 20 mA the maximum.
At your control center and the source, an op amp circuit is used to convert
the voltage signal to current that is ported over the long distance, and
another op amp to convert the current back into a voltage.
Ken
[noparse][[/noparse]Non-text portions of this message have been removed]
othello159@h... writes:
> What is the problem?
>
> Do I need shielded cable
Put a 1.2K resistor in series with the sensing lead, and put a .1uf bypass
right at the pin connected to the sensing lead. This should stabilize your
signal from the LM.
Sid Weaver
W4EKQ
Port Richey, FL
[noparse][[/noparse]Non-text portions of this message have been removed]
I have an LM34 on about 75' of cable, with no problems. I used outdoor phone
cable, unshielded. If I remember correctly, I soldered a resistor on to the
LM34 right onto the lead before placing it in the brass tube that it lives
in. I forget the value and placement, look on Tracy Allens's site for
details, I did whatever it is that Tracy reccomends. It has been keeping the
hot tub temp. perfect ever since.
Jonathan
www.madlabs.info
Original Message
From: <smartdim@a...>
To: <basicstamps@yahoogroups.com>
Sent: Friday, October 17, 2003 5:03 PM
Subject: Re: [noparse][[/noparse]basicstamps] interference? problem
> In a message dated 10/17/2003 4:43:28 PM Pacific Daylight Time,
> othello159@h... writes:
> I am building a controller for my thermal solar panels.
> The controller measures two temperatures and activates pumps depending on
the
> differential it finds.
> I am using two LM35s and on the breadboard things work well, but one of
the
> sensors needs to be on the roof, at the other end of a 50- to 100 foot
cable
> and if I hook the sensor to the other end of that long line the readings
become
> quite erratic. Not totally off, they bounce +- 10 degrees around the real
> value but obviously there is something not right.
>
> What is the problem?
>
> Do I need shielded cable?
> ____________________________________________
>
> When porting signals over long distances, one possible solution is to port
> current, not a voltage signal. One common range is 4 to 20 mA, where 4 mA
is
> equivalent to the minimum and 20 mA the maximum.
>
> At your control center and the source, an op amp circuit is used to
convert
> the voltage signal to current that is ported over the long distance, and
> another op amp to convert the current back into a voltage.
>
> Ken
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the Subject and
Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
>
Shows how to clean up the bouncy signal. He even sells a really nice parts
kit to make them.
>
Original Message
> From: othello159 [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=2OeMUzanJhXTldJkgXvywojVt7ECAMDqbbNCOUww-mPTKHDmQ1LmLjYdtNfvVUCWjNK_CgWV-6S6pRef]othello159@h...[/url
> Sent: October 17, 2003 4:41 PM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] interference? problem
>
>
> I am building a controller for my thermal solar panels.
> The controller measures two temperatures and activates pumps
> depending on the differential it finds.
> I am using two LM35s and on the breadboard things work well, but
> one of the sensors needs to be on the roof, at the other end of a
> 50- to 100 foot cable and if I hook the sensor to the other end
> of that long line the readings become quite erratic. Not totally
> off, they bounce +- 10 degrees around the real value but
> obviously there is something not right.
>
> What is the problem?
>
> Do I need shielded cable?
>
> TIA Uwe
>
>
> To UNSUBSCRIBE, just send mail to:
> basicstamps-unsubscribe@yahoogroups.com
> from the same email address that you subscribed. Text in the
> Subject and Body of the message will be ignored.
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>
> In a message dated 10/17/2003 7:43:20 PM Eastern Standard Time,
> othello159@h... writes:
>
>
> > What is the problem?
> >
> > Do I need shielded cable
>
> Put a 1.2K resistor in series with the sensing lead, and put a .1uf bypass
> right at the pin connected to the sensing lead. This should stabilize your
> signal from the LM.
>
> Sid Weaver
> W4EKQ
> Port Richey, FL
Thanks for all the suggestions, I will try some of them out in the next couple
of days.
But why should this work ( I always want to know why). I can see how the cap
would drain unwanted AC to ground, but why the resistor. Do you know the theory
behind it?
regards
Uwe Langmesser
KB1JOW
Maine, USA
othello159@h... writes:
> But why should this work ( I always want to know why). I can see how the
> cap would drain unwanted AC to ground, but why the resistor. Do you know the
> theory behind it?
>
The cap gets rid of noise and other ibnterference. I don't know what the
resistor does - I just know that it works. I build a 40 LM34 system for a
customer with runs from 5 feet to 36 feet and the resistor/cap combinations
solved
all my instability problems.
Sid
[noparse][[/noparse]Non-text portions of this message have been removed]
> In a message dated 10/20/2003 8:12:51 PM Eastern Standard Time,
> othello159@h... writes:
>
>
> > But why should this work ( I always want to know why). I can see how the
> > cap would drain unwanted AC to ground, but why the resistor. Do you know the
> > theory behind it?
> >
>
> The cap gets rid of noise and other ibnterference. I don't know what the
> resistor does - I just know that it works. I build a 40 LM34 system for a
> customer with runs from 5 feet to 36 feet and the resistor/cap combinations
solved
> all my instability problems.
>
> Sid
>
That is a lot of sensors....
Have you noticed any of this with your sensors:
It takes my sensors sometimes a while to "settle". Debug shows me they switch
back and forth between two temp reading. That leads to my pumps switching on and
off for a number of times before they settle in the on or off state. It is not a
huge problem but I wonder how to stabilize this.
Putting a pause command in the software to read the temp only every 5 sec.
helped a bit but didn't stabilize the system entirely.
Uwe
>> In a message dated 10/20/2003 8:12:51 PM Eastern Standard Time,
>> othello159@h... writes:
>>
>>
>> > But why should this work ( I always want to know why). I can see how the
>> > cap would drain unwanted AC to ground, but why the resistor. Do
>>you know the
>> > theory behind it?
>> >
>>
>> The cap gets rid of noise and other ibnterference. I don't know what the
>> resistor does - I just know that it works. I build a 40 LM34 system for a
>> customer with runs from 5 feet to 36 feet and the resistor/cap
>>combinations solved
>> all my instability problems.
>>
>> Sid
>>
>
>That is a lot of sensors....
>
>Have you noticed any of this with your sensors:
>It takes my sensors sometimes a while to "settle". Debug shows me
>they switch back and forth between two temp reading. That leads to
>my pumps switching on and off for a number of times before they
>settle in the on or off state. It is not a huge problem but I wonder
>how to stabilize this.
>Putting a pause command in the software to read the temp only every
>5 sec. helped a bit but didn't stabilize the system entirely.
>
>Uwe
Hi Uwe,
The LM34 is a micro-power circuit that uses feedback internally to
stabilize its voltage output. It is a common problem with of this
type, that the output will be unstable when it has to drive a cable,
or a capacitor of a certain size. One of the characteristics of this
instability is that it will seem to flop back and forth between two
temperature readings, unpredictably. If you look at the output
signal on an oscilloscope, you can often catch a wild oscillation at
a frequency of several thousand hertz, and 100 mV or more amplitude.
At other times that oscillation will not be there. It may go away
when you touch the output with the 'scope probe or with your finger,
or when you power the circuit up one time but not another.
The combination of resistor and capacitor at the output tames the
oscillation. Intuitively, it works something like this. Say a
fluctuation occurs in the temperature, which internally is a small
voltage change that is amplified to the output. But there is a bit
of a delay and ramp up of the response at the output, and that slow
response at the output becomes more acute when a capacitor or a cable
is attached. Feedback from the output to the input senses that the
output is not changing fast enough and and tries to turn the output
on harder, again with a delay or "lag" to the output. The result is
that the voltage at the output overshoots, and under conditions that
are all too easy to find, the overshoot becomes sustained
oscillation. The resistor you add at the output generates a voltage
proportional to the current flowing out into the load capacitor and
that additional "leading" voltage stabilizes the feedback loop.
You can read about this in the LM34 data sheet, and I also have
information on my web pages at
http://www.emesys.com/OL2heat.htm#TF_LM34
-- regards,
Tracy Allen.
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com
> >--- In basicstamps@yahoogroups.com, Newzed@a... wrote:
> >> In a message dated 10/20/2003 8:12:51 PM Eastern Standard Time,
> >> othello159@h... writes:
> >>
> >>
> >> > But why should this work ( I always want to know why). I can see how the
> >> > cap would drain unwanted AC to ground, but why the resistor. Do
> >>you know the
> >> > theory behind it?
> >> >
> >>
> >> The cap gets rid of noise and other ibnterference. I don't know what the
> >> resistor does - I just know that it works. I build a 40 LM34 system for a
> >> customer with runs from 5 feet to 36 feet and the resistor/cap
> >>combinations solved
> >> all my instability problems.
> >>
> >> Sid
> >>
> >
> >That is a lot of sensors....
> >
> >Have you noticed any of this with your sensors:
> >It takes my sensors sometimes a while to "settle". Debug shows me
> >they switch back and forth between two temp reading. That leads to
> >my pumps switching on and off for a number of times before they
> >settle in the on or off state. It is not a huge problem but I wonder
> >how to stabilize this.
> >Putting a pause command in the software to read the temp only every
> >5 sec. helped a bit but didn't stabilize the system entirely.
> >
> >Uwe
>
> Hi Uwe,
>
> The LM34 is a micro-power circuit that uses feedback internally to
> stabilize its voltage output. It is a common problem with of this
> type, that the output will be unstable when it has to drive a cable,
> or a capacitor of a certain size. One of the characteristics of this
> instability is that it will seem to flop back and forth between two
> temperature readings, unpredictably. If you look at the output
> signal on an oscilloscope, you can often catch a wild oscillation at
> a frequency of several thousand hertz, and 100 mV or more amplitude.
> At other times that oscillation will not be there. It may go away
> when you touch the output with the 'scope probe or with your finger,
> or when you power the circuit up one time but not another.
>
> The combination of resistor and capacitor at the output tames the
> oscillation. Intuitively, it works something like this. Say a
> fluctuation occurs in the temperature, which internally is a small
> voltage change that is amplified to the output. But there is a bit
> of a delay and ramp up of the response at the output, and that slow
> response at the output becomes more acute when a capacitor or a cable
> is attached. Feedback from the output to the input senses that the
> output is not changing fast enough and and tries to turn the output
> on harder, again with a delay or "lag" to the output. The result is
> that the voltage at the output overshoots, and under conditions that
> are all too easy to find, the overshoot becomes sustained
> oscillation. The resistor you add at the output generates a voltage
> proportional to the current flowing out into the load capacitor and
> that additional "leading" voltage stabilizes the feedback loop.
>
> You can read about this in the LM34 data sheet, and I also have
> information on my web pages at
> http://www.emesys.com/OL2heat.htm#TF_LM34
>
> -- regards,
> Tracy Allen.
> electronically monitored ecosystems
> mailto:tracy@e...
> http://www.emesystems.com
Thank you very much for explaining this.
Before I received your mail I had placed the 0.1 mF capacitor and the resistor
in the circuit as was suggested. That had stopped the rather erratic and wide
range change of numbers, but I was still getting the (much more tolerable)
"threshhold jitter " when temperature changes caused the LM 35 to change from
one number to the other.
On a hunch I placed a larger electrolytic capacitor, 10 mF, parallel to the 0.1
and the set up has been working stable as a rock ever since.
Sometimes bigger is better...
Uwe
>the resistor in the circuit as was suggested. That had stopped the
>rather erratic and wide range change of numbers, but I was still
>getting the (much more tolerable) "threshhold jitter " when
>temperature changes caused the LM 35 to change from one number to
>the other.
>On a hunch I placed a larger electrolytic capacitor, 10 mF,
>parallel to the 0.1 and the set up has been working stable as a rock
>ever since.
>
>Sometimes bigger is better...
>
>Uwe
Hysteresis in software can also help stabilize the switching:
IF temperature > threshold THEN
GOSUB gizmo_on
ELSEIF temperature < threshold THEN
GOSUB gizmo_off
ENDIF
That provides a two unit dead band that suppresses jitter.
;-<
;---ON---
___OFF_______|___>___|
T-1 T T+1
-- Tracy