What can I do to let a diode experience a range of temperatures?

Question

Hi,

I want to immerse a diode in various water baths but I'm not quite sure how I can do this as I can't insert a diode directly into a water bath. At the same time though, I want the diode to be able to respond to the change in temperature surrounding the diode.

I had a look at heat shrink tubing on the internet but that will "shrink" the heat. Also, another way (thanks to amansscientiae) is to place the diode directly in oil inside a test tube. Would it be ok for me to do this though? (As I will need to connect wires on both sides of the diode.)

Answer 1

Immersing circuitry in mineral oil is routinely done for high-voltage or high-power circuitry. The transformers and capacitors you see on utility poles are often oil-filled.

It is also possible to insert the diode directly into water if the water is pure enough and you don't leave the diode in the water long enough for the body material to soak up a significant amount of water. You don't have to worry about glass or hermetically sealed diodes, but watch it with epoxy-encapsulated packages. You will have some current leakage around the diode, but this will only be a problem if you are trying to measure the leakage current of the back-biased diode. This leakage current will otherwise be significant only if impurities increase the conductance of the water too much.

Answer 2

As long as the diode is forward-biased, it will have only about 1V across it (0.7V from the junction voltage drop, plus a bit more from the contact resistance -- this actually obeys Ohm's law and so will vary with current). This potential difference should not be enough to electrolyse water, especially if you use demineralised water (it's the dissolved minerals that make it conductive).

Alternatively, use a liquid known to be non-conductive -- any kind of oil will be fine, but cooking oil is readily available and non-toxic. You can make certain the liquid really is non-conductive by dipping two wires in it without a diode connected to them; and also by first placing the diode in the test tube, with voltmeter and ammeter ready to go; then pouring in some oil (which has been allowed to reach room temperature) and making sure the meter readings stay the same.

Answer 3

Place the diode (connected to the circuit, or the whole circuit) and a thermometer inside a cooler. Heat the air in the cooler with a hair dryer, or cool it with dry ice. Close the lid of the cooler and give time for the temperature inside the cooler, the diode, and the thermometer to equalize.

Another option is to place the diode inside a test tube and cap the test tube. Then place the test tube in the water bath and give time for the temperature in the test tube to equalize the temperature of the water bath.

Answer 4

I actually applied HooKooDooKu's cooler & oven idea to select zener diodes for use in analog-to-digital and digital-to-analog converters, back in the 1970's. The diode was plugged into a socet at the end of awand, so I could move it in and out without opening the door. There was a little fan to stir the air inside the cooler and a hole to insert the mercury thermometer. Today's digital thermometers are much better.

I graphed the voltage v temperature curves, which were always cubic curves, like ax^3 + bx^2 + cx +d. One thing to watch out for is historesis; The voltage might not follow the same curve every time.

Answer 5

Just put the diode in a weighted plastic bag
(and lower it into the water, keeping the end of the bag aboive the water line)

or a 'fridge or freezer

or an inverted cardboard box with a lamp bulb
(if you want 80 C you could change the number of light bulbs or put a soldering iron in ...but you must be in attendance in case something moves )


(additional info : A diode does NOT obey Ohms law )
every 80 mV DOUBLES the currrent
every one degree C lowers the voltage by 2 mV