When the car heats up under the sun the in-dash radio works fine but when it cools down the radio will not power up. I tried replacing fuse check the wire harness etc. but can resolve this issue. Any advice will be highly appreciated.
Thanx.
Please drop me a line at vocalglobe//at\\hot mail//dot\\com (remove the //\\)
Where do you live? In extremely cold conditions transistors & IC's can become "Cold Soaked" that is, the components have become so cold that their internal resistance has risen to the point that they can no longer conduct electricity.
On Sat, 03 Sep 2005 16:34:55 +0000, ironrod rearranged some electrons to form:
Ummm... where did you get that information from? Oh, I see, you pulled it from your a**.
Electrical resistance of metals and most semiconductors goes DOWN with temperature, not up.
Commercial semiconductors will perform within specifications from 0-70C. Industrial rated semiconductors will perform within specifications from -40 to 85C; automotive electronics should operate within that range.
More likely the problem is a bad connection on the unit.
Well lets see, I've been pushing 'trons through wires for almost 30 years now, so I'm reasonably confident that I've got the basics right. In extremely cold conditions, approaching absolute zero, resistance drops to near nothing. In the real world the hotter the junction of the semi- conductor the greater the current flow. Conversely, the lower the temperature the less current flow across the junction. Now in the real world there is a condition designers have to deal with called "thermal runaway" and it is a real problem in high powered circuits that get too warm. Basically the higher the current the hotter the junction, the hotter the junction the higher the current flow. Without any checks in place the device will get so hot it melts. The most simple solution, of course, is to limit the output of the power supply. No matter how hot the junction gets it can't flow any more current than the power supply can output. Now jump to the other half of the spectrum where the junctions are abnormally cold. Your power supply with its limited current might not be capable of generating enough power to meet the greater demand caused by the colder junctions.
Now while semi conductors are capable of operating across a wide range of temperatures, the device itself might not be. Most electronic devices are designed to operate at "room temperature". Devices that are intended to be used in extreme conditions are built much differently. In some cases, I've seen environmental design problems where the best solution was to create an artificial environment and use "off the shelf" components.
Now, you never did mention if you lived in a cold climate or not, if you don't then obviously the problem can't be cold soaking. If you do, re-create the conditions where the radio doesn't work, power it up and wait. Even under the coldest conditions there will be some limited current flow which should cause the electronics to warm up and begin to work. Also, you might try re-creating the problem and then using a space heater to warm the interior of the car.
On Mon, 05 Sep 2005 07:05:39 +0000, ironrod rearranged some electrons to form:
That's a different mechanism... the increased temperature knocks more electrons loose from the semiconductor. OK, technically I guess that would qualify as "lower resistance" (at least at your meter test points).
But that would not occur unless the temperature is decades below zero.
Most automotive electronics is designed to handle a much wider range of temperature than consumer electronics.
I'm not the OP. But I would still suspect a simpler solution, like a loose connector contact (heating up and expanding). Connectors have a higher failure rate than semiconductor junctions.
Now you begin to see the difference between a tech (me) and the engineer. Because car radios have to live in the enclosed environment of a car, complete with their heat catching, solar panel, greenhouse glass window arrangement, things getting to hot are much more likely to occur than things getting to cold, (which could only happen during certain times of the year in certain geographic locations.) Heat rejection would be a design priory, whereas cold performance would be just an afterthought. I agree that cold soak probably isn't the most likely cause, however the symptoms given by the OP were a text book example of the phenomenon. Which is why my first question was, 'were did he live?'.
You stated "But that [cold soak]would not occur unless the temperature is decades below zero." This isn't necessarily true, remember most IC's operate at 5 volts, every time you cross a junction you drop between .2 to .5 volts, it would only take a very slight increase in resistance of several components before the overall load was so great that 5 volts wasn't enough to get the job done anymore.
Here is an interesting test, if you live in one of the colder areas of the country. Take a cheep transistor radio, place it inside a plastic bag then leave it outside, in the shade, overnight when the temperature is expected to drop below zero. Turn it on and count the seconds from when you hit the switch until it starts playing.
On Mon, 05 Sep 2005 17:25:11 +0000, ironrod rearranged some electrons to form:
Hmmm... I guess you haven't heard of IC's with half a million gates and operating at 3.3V (like the CPU in your computer). The junctions in an IC for the most part are not in series. And they are most definitely not all forward-biased.
The I/V relationship of a forward-biased doped PN junction in silicon is an exponential relationship that also has a temperature dependance, unlike a resistor which is a linear relationship. I is proportional to exp(V *(q/kT) - 1) where q = charge of an electron and k = Boltzman's constant, T= temperature in degrees K. Room temperature (25C) is 298K. Cutting the temperature in half (149K or -124C or -191F) theoretically drops the PN junction turn-on voltage by a factor of ln(1/0.5) = .69 Assuming you could actually get that cold, a 0.7V junction would actually be turning on at more like 0.4V. At -50F (227K) the change is less than 0.2V.
Can't do it, since it doesn't get down to -191 degrees here.
Thanx all for replying. I live in Texas where its in 100s now. The Radio will only power up in extreme heat. I just dont get it. Checked wiring harness and fuses no issues there still in normal temp the radio just will not power until in extreme heat. Any suggestion will be much apprerciated.
You might try asking on rec.audio.car.
I would try powering it from a 12v power supply and following the power bus. Use a heat gun and gingerly heat small areas until it works, then use a circuit cooler spray to cool it off in various areas until I could reliably make it start and stop.
Sounds like a poorly solder connection somewhere such that the conductors move/expand connecting when hot. That type of problem is more common than you might think. I used to see it at times when I was working in electronics. Especially component leads going through a soldered connection hole in a circuit board. The connection visually looked fine but when moved with an insulated tool or heated and cooled as I described would fail.
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