Heating up a couple of Resistors!

[bugi]

Originally Posted by Digerati

Okay - I see what you are doing now. Thanks for clarifying.

You might have to come up with something that will get a bit hotter as ambient temperatures in some facilities without air conditioning on a hot summer day may approach 90°F. Or the beating sun on an outside wall may obscure the image.

Note that component heating is incremental, not absolute, so higher ambient temperature means that the heating component will end up at equally higher end temperature.

But... If the device is moved from cooler place (say, from a car with air conditioning, or carried outside for a while during a cold day) and put to immediate use in a warmer place, then the "ambient" inside the device (what the heating resistor sees as ambient) is still cool for a while, until the device gets warmed up. This can be solved quite easily with enough ventilation holes in the device, for example on both top and bottom, which lets the air in the environment flush through relatively quickly.

However, even if the device was cooler, a good thermal imager might still show the "hot spot". The imager should show the device as cooler than environment, with a "less cool spot" than the rest of the device. I guess this depends on the imager's capabilities.

[Digerati]

Originally Posted by Digerati

Or the beating sun on an outside wall may obscure the image.

Direct sun on a hot day will wipe out almost all useful data from a thermal imager anyway.

The imager should show the device as cooler than environment, with a "less cool spot" than the rest of the device.

Right. These devices are not used just to look for hot spots, but for cool spots too. So if you live in the Arizona sun, the whole wall may light up on the display, but then the circuit box be a darker cool spot in the middle, with a slightly warmer (and suspicious) spot in the middle of the circuit box.

I guess this depends on the imager's capabilities

Yeah, the better models need a difference of just 2 or 3 degrees to "show" it on the display. And just 1.4°F if you want to spend $2500 or more.

[frogwash]

Good news! We have had 2 - 220 ohm, 1/2 watt, 5% carbon film resistors cooking at the end of 4 AA batteries for about 12 hours now and see no significant loss of temp! Yehaaa .. looks like we might be getting closer. Tomorrow we get the actual Imager to do the real life test. I get what you guys are talking about as far as outside and inside temperatures and it's effects on the components. I am thinking about hooking up 4 batts to 2 - 150 ohm, 1/2 watters to see if we get a slightly higher temperature for a more dramatic effect. More testing to see battery life!

thanks all...!!!!! been great! I will let you all know our results.

[sensiz]

The easiest way to test the thermometer with by using a simple LED, it gets heated up pretty fast. However, for resistors, you can always use the Ohms law to determine how much power the circuit will have and hence the energy that will be dissipated. Please note that once you disconnect a resistor from the battery, it cools down quickly. I’m not very sure about the method of testing that you’re applying here.

[sergen]

If you want to heat up the resistors, you don’t have to connect them in parallel as that would only decrease the resistance. I’d also suggest that get two sets of batteries and connect the send one after a while when the voltage of first set of batteries drops to 4.5V. Since the value of current will lower after a while, the temperature of the heated resistors might fall down because of that. Replace the first set of batteries as soon as the voltage level starts to drop sharply.

[PedroDaGr8]

Another random option, likely way over kill (like squirrel hunting with a 12gauge) but what about a peltier device. Most of the time they are used for cooling but you could exploit the hot side as well for heating.

[Siorus]

Originally Posted by PedroDaGr8

like squirrel hunting with a 12gauge

Meh. That's what .338 lapua hollow points are for. Squirrel? What squirrel?