Re: [bascom] 10 SPI temperature sensor <--> 1 AVR
Hello.
For what it's worth, I use lots of the AD590 type temperature sensors.
They have many advantages one of which is a temperature range of -55 to
+150 C. I've actually used them at even lower temperatures for short
periods of time.
They "pass" a current directly proportional to absolute temperature so you
can use something as simple as a single resistor to ground to scale that
current up to the voltage range you need. Also, since the output would
theoretically go to zero current at absolute zero, you end up with a
positive voltage from your "sense" resistor even at negative temperatures.
This is very handy for interfacing to a single ended (positive supply only)
A/D converter with minimum parts and still being able to see temperatures
down to -55C or lower.
Using computer calibration tables we get accuracy of around 1/100th of a
degree C on our systems. At that level, it's darn difficult to even do the
calibration! There is also an electrically identical part, the AD592 which
is in a different package with a smaller temperature range.
Oddly, the592 in the plastic TO-92 case is often more expensive than the
hermetic packaged 590! Very strange. Still, these are wonderful little
sensors. I especially like the extremely high output impedance which
allows us to have very long runs of cable (1000s of feet in some cases) and
still get very low noise, accurate operation. They are sort of like a
miniature 4-20mA two-wire transmitter. In this case, it's a 218-423uA
current loop, I guess. (1 microamp per degree K).
If you've got an A/D that's able to read 0 to 5 volts then you can use a
10K resistor to sense the current. That will give you something like 2.18
Volts at -55C and 4.23 Volts at +150C. You do need to have something like
5 Volts across the sensor, however, so you'd want to have at least 9.23
Volts to apply to the positive side of the sensor.
Jim
At 04:23 PM 10/7/2003 +0200, you wrote:
>The max6675 is indeed nice, but I can't find a reseller anywhere. I've been
>using the LT1025 for my high-temperature applications. I'd also like to use
>the negative range of the thermocouple, but dual supply A/D converters are a
>rare breed I imagine. (and the 10-bit ADC of the ATMEGA128)
>
>The thermistor one (max6682) is not all that interesting, because of the
>fact that most thermistors do't have an extended range. I was looking ath
>the LM83 (if I remember correctly, haven't got the datasheets near me), it
>does about the same, over a wider range using 2N3906 PNP or NPN transistors;
>3 at a time, but what I was asking for is a temperature sensor with
>integrated I2C ADC, like the LM73/75 or such.
>
>Thanks for the advice,
>
>
>Steven
>
>
>>From: Marvin Dickens <mpdickens@tlanta.com>
>>Reply-To: bascom@grote.net
>>To: bascom@grote.net
>>Subject: Re: [bascom] 10 SPI temperature sensor <--> 1 AVR
>>Date: Mon, 06 Oct 2003 11:54:30 -0400
>>
>>Steven Engelen wrote:
>>
>>>I'd use (and I am using) I2C sensors, but unfortunately they don't go that
>>>far in their measuring range (I'm looking for -55 to 110°C), but then
>>>again, very few sensors do.
>>
>>If you go spi you have a wide range of choices in the maxim product line.
>>The max6682 will
>>do -55c to +125c and is designed for use with a thermister. For high temp
>>stuff (Which is
>>mostly what I do with temperature) a good choice is a max6675 (But it's
>>cold junction type-k
>>thermocouplers).
>>
>>Best regards
>>
>>
>>Marvin Dickens
>>Alpharetta, Georgia USA
>>
>>
>>
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