| Author |
Message |
tgq
Joined: 15 Jul 2002
Posts: 305
Helped: 2
|
 02 Oct 2002 18:18 Passive thermometer with diode |
|
|
|
hi
is it possible to get the temperature with a simple diode and a multimeter, diode test enabled ?
i only read xxx mV and about - 2 mV/°C , how can I read xx °C with passive components ?
thx
|
|
| Back to top |
|
 |
flatulent
Joined: 19 Jul 2002
Posts: 4850
Helped: 285
Location: Middle Earth
|
| 02 Oct 2002 18:43 two choices |
|
|
|
One choice is a thermocouple which also requires a cold junction compensation otherwise you are measuring the temperature difference between two places.
Another method is to use a material whose resistance changes with temperature and measure the resisitance. Varisistors was a trade name of such a two terminal device.
|
|
| Back to top |
|
|
Roscoe
Joined: 03 Feb 2002
Posts: 61
Location: Germany
|
| 02 Oct 2002 22:08 |
|
|
|
Hi,
i think a simple way is to use a platinium resistor (eg. PT100 or PT1000). Measure the resistance with your ohm-meter and use the R vers. Temp. chart to read the temperature.
The PTxx resistors have a good linear R/T range, and they are not expensive. For better reading of small temperatur changes you should use the PT1000 type.
roscoe
|
|
| Back to top |
|
|
RegUser_2
Joined: 24 Dec 2001
Posts: 223
Helped: 2
|
| 29 Nov 2002 13:03 |
|
|
|
| You have to pass a constant current through the diode and measure the voltage drop. The current have to be low enough not to heat the sensor itself. Usualy about 10 uA DC current source is used to bias the diode. I don't know if the multimeter uses a constant curren to measure the voltage drop of a diode junction, so you will need external power and a JFET/resistor pair to provide constant current. In case You have a high-voltage batery, you may use a large value resistor to emulate a current source. Naturally your multimeter have to be very high impedance to read the diode voltage without shunting the diode.
|
|
| Back to top |
|
|
sofer
Joined: 20 Dec 2001
Posts: 23
|
| 30 Nov 2002 17:39 |
|
|
|
| You should use thermistors (and not varistors as someone suggested. Those are for overvoltage protection). There are 2 kinds NTC (negative temperature coefficient) which means that as the temperature goes up the resistance goes down, and PTC (positive temperature coefficient) where the resistance goes up with temperature. They are cheap, they come in every concievable package you might imagine. They are not that linear, so it this is important to you, there are ways to linearize them or just measure the table and use it. This the only solution for passive components. For active components, except using the diode approach, which is very good you can also use a lot of ready made temp sensors like LM50C etc.
|
|
| Back to top |
|
|
RegUser_2
Joined: 24 Dec 2001
Posts: 223
Helped: 2
|
| 28 Mar 2003 4:21 |
|
|
|
| The thermistor is not quite easy to read because of their nonlinearity, and also they tend to self heat if too large bias current is applied, what aditionaly scew the correct temperature reading.
|
|
| Back to top |
|
|
