DIY: Built a heater for my LCD - Page 3

nst6563 · 09-28-2007, 09:56 AM

did you check out the link I posted? That looks like a decent option as well (if you're willing to shell out some money for the project). .5mm thick and is a maximum of 6". Accepts voltages of 5-12v. Being so flat, you could lay it inside and not worry about clearance issues or tape.

doesn't say how hot it gets or how fast it gets there either though.

BiG K · 09-28-2007, 11:01 AM

The lithium ion batteries used in electric\hybrid car's have heater pads sandwiched between them. They are typically wafer thin and run on 12v.......might be of use, try googling and maybe getting hold of one? I see it would be more useful, as its thin and can be mounted anywhere!

LCD screens do not typically work in low temperature conditions for a few reasons. First of all CCFL's, which provides the light, does not function well in low temperatures.

Secondly, LCD stands for LIQUID Crystal Display. At low temperatures, the liquid cystals viscosity may decrese, leading to permanent damage. There is a reason why there is a temperature sensor there, if the manufacturers knew the monitor can work in conditions below -10c perfectly with no damage, they would not have bothered putting a sensor!
I advise against altering the temperature circuit!

If you do want to locate the temperature sensor, it will most likely NOT be on the circuit board, as circuit boards can work in much lower temperatures. Most likely it is somewhere on the LCD panel!
I have a temperature probe on my computer, which is accurate to within 1c, the actual temperature sensing chip is so SMALL, I would need a magnifying glass and tweezers to pick it up. It really is as big as a few grains of sand! So looking for the temperature sensor is like looking for a needle in a haystack, it could be anywhere!.

Oh and don't use a peltier device. The hot side gets EXTREMELY hot and will burn and ruin the PCB and whatever attached to it, while the peltier device would most likely fail anyway.

Condensation is caused by heat transfer. Ever wondered how water evaporates? Typically, if the ambient temperature is warmer than a device that is cold, water droplets will form on the cold surface, so its typically from warm > cold

Bugbyte · 09-28-2007, 01:09 PM

I wouldn't worry too much about condensation in the winter, particularly at the temperatures you are talking about. Warm air can hold tremendous amounts of water. As you cool it, it gives up the water. That's why clouds form a few thousand feet up. The war air rises and cools and gives up the water, which forms a cloud. Same reason that when you get fog on the ground (effectively, a cloud at ground level), it 'burns' off as the temperature rises. The water gets absorbed back into the air.

The colder the temperature, the less water it holds. At -30, you are talking about a pretty dry atmosphere.

nobb · 09-28-2007, 02:03 PM

BarryWoods: Nope, Ive already tried building an external blower type (80w) heater and it was useless since the heat would not conduct to the inside of the unit fast enough. And dont worry about the packing tape...its all been replaced by high temp Kapton tape.

nst6563: Yes I have checked your link and I have even seen other heaters which heat the lcd surface itself. Right now, I am not worried about the LCD screen having a slow refresh rate in the cold as I only want it to turn on in the first place. That film is designed only to heat the LCD surface, and not the entire single DIN unit I have.

Bugbyte: Thanks for the clarification of the condensation issue. I forgot about the fact that cold air holds less moisture than warm.

I just got an idea to test out how much condensation will form. I will build another 50w heater and place it in an old single DIN broken radio, then leave that radio out in the cold, turn on the heater, and observe how much condensation forms on the circuit board. Maybe even stick a temp probe in there to see the rate of internal temp change due to the heater. Better to do some testing before ruining my $450 screen from condensation (even if its not likely).

parksgm · 09-28-2007, 03:32 PM

Quote: Originally Posted by turbocad6

hey man, don't know if you thought of this, but do you know what happens to a can of soda when you take it out of the fridge & leave it on the table in the summertime? condensation....

Condensation occurs when water vapor in the air (otherwise known as "humidity") contacts a cold surface. The air contacting the cold surface becomes chilled, and the decrease in temperature reduces the amount of water vapor that the air can hold. As a result, the water vapor in the air changes state from a gas to a liquid and accumulates, or condenses onto the cold surface. If there is very low humidity, then there is very little water vapor in the air, and not much condensation will occur. If humidity is high, then there is quite a bit of water vapor in the air, and much condensation will occur.

A practical illustration: Take a cold can of coke from the refrigerator and place it on a table in an air conditioned room. Relatively little condensation (compared with the second part of the example, described below) will form. Air conditioning lowers the temperature of the air in the room, decreasing the amount of water vapor it contains. Therefore, there is less water vapor that can condense from the air onto the outside of the coke can. Now, take the same can of coke outside on a hot summer day. The relatively high air temperature (when compared with the air conditioned room you just left) allows relatively more water vapor to be dissolved in the air which can then rapidly condense onto the cold surface of the coke can.

Condensation might form on or inside your LCD after you start your car because the LCD plastic and metal components of the LCD have a higher specific heat (retain more heat energy per unit mass) than the air inside your car, and as a result, remain colder than the ambient air temperature in your car after you turn on your heater. To restate this, the heater in your car very rapidly warms the air inside your car, while the LCD remains relatively cooler. Condensation could then occur from the warmed air inside the car onto the relatively cooler LCD. If you drive your car for an extended period of time, the warmed air in the interior + internal heat from the operation of the LCD will eventually cause the LCD temperature to slightly exceed the ambient air temperature, and no more condensation will occur from the air to the LCD. In fact, the reverse will occur, as any water that had previously condensed onto the LCD from the air will now evaporate (the reverse of condensation) back into the relatively cooler air.

Practically, however, I doubt this is much of an issue, because the very low ambient air temperature in the winter means that humidity is also usually very low. So, there isn't much moisture in the supercold air in an unheated car to condense onto the LCD. However, depending on the size of your car and your activity level prior to entering the car, you yourself can be a significant source of moisture than can result in some condensation. Each time you breathe in a closed car, the air you breathe is warmed to body temperature by contact with your respiratory tract and lungs. The warmed air can hold more moisture, and so water rapidly evaporates from the surface of your mucous membranes into the air contained within your lungs. When you exhale, that warmed, humidified air is expelled back into the cold environment of the car. Wherever that warmed air comes into a contact with a cold surface, condensation will form as the air temperature drops. If you drive an SUV, the moisture you exhale will be relatively diluted into large volume of the car interior, and with less moisture per volume of air (lower humidity), condensation will not form quickly...if you drive a very small car (I'm thinking smart car, 350Z, Mini Cooper), then the moisture you exhale will not be diluted as much, and higher the higher humidity will result in relatively more condensation.

Finally, there is a practical way to limit the amount of condensation that occurs in your car...use the defroster! In modern cars, turning on the defroster actually turns on the air conditioner as well as the heater. Before coming out of the vents, air is first cooled, to dehumidify it, and then warmed, to increase its water carrying capacity. The warm, dry air is then blown across the cold glass surfaces in the car where condensation is most likely to have occured. By turning on the defroster, you increase the ability of the air inside your car to absorb moisture that you exhale, reducing condensation.

Hope this helps...

Gregory

nst6563 · 09-28-2007, 03:40 PM

so to sum it up...

Heat up your LCD components faster than the rest of the car and you won't have a problem

parksgm · 09-28-2007, 03:44 PM

Quote: Originally Posted by Bugbyte

I wouldn't worry too much about condensation in the winter...

Kudos Bugbyte...you managed to explain the same thing I just did with 1/4 the words. :-)

parksgm · 09-28-2007, 03:57 PM

Quote: Originally Posted by nst6563

so to sum it up...

Heat up your LCD components faster than the rest of the car and you won't have a problem

Quote:

What do you mean direct the heater to my car screen, could you clarify? Relying on the car's heating system alone is not enough to heat up the LCD because it takes a very long time for the heat to be transfered from the cabin to the screen (takes an hour when the temperature is -30C).

Well, really, you need to heat your LCD components faster or at the same rate as the ambient air in the car. A better solution than a direct heating element might be to simply direct air from a heater duct onto/inside the LCD. That way, as the engine slowly heats, the air from the heater will slowly increase in temperature, which will slowly increase the temperature of the LCD. The slow rate of increase will allow the LCD temperature to remain more or less identical to the ambient air temperature within the car until condensation is no longer an issue.

BarryWoods · 09-29-2007, 02:15 AM

I really wouldn't worry too much about condensation. At the temps we're talking about, it doesn't really happen. Unless you have a dog, then all bets are off! My freaking dog can fog up the windows with the defroster on high and the windows down. I still think the ducted hot air from the heater is the best idea though. It will also keep any condensation that may happen at bay. Really all you need to heat is the glass anyway.

spmclaugh · 10-07-2007, 03:49 PM

Haha, thanks parksgm, I was about to type up a little thing on the condensation, but that was a good writeup.

Forget the resistors, seriously! Just use a solid (rectangular) piece of thin metal! Don't use copper, use something with less conductivity. Stainless steel is only about double the conductivity of nichrome (which is what is used for heating elements). Titanium isn't too far behind. You get ideal distribution of heat if you use a sheet like this! No hot spots, and no messy tape-job.

For thin plates, try www.allmetalsinc.com for small quantities. I calculated about $10 for a custom-cut sheet of 304 stainless 20 gauge. Or just use your local metal shop and ask for the thinnest stainless they have.

There is a foil called "stainless tool wrap" / "stainless heat foil" that seems like a good choice. Search for that on google, or search for:

.002" stainless

This is a site that comes up:

http://www.maximum-hobby.com/store-p..._30445281.html

This maximum-hobby place sells small sheets for under 4 bucks.

You can cut holes or bend things around however you like. Use an external current limiter if you feel that you need to (TEST IT). The limiter could be as simple as a regular resistor, or you could go all-out and use a thermistor to control a PWM current source.

Also, don't even consider using a Peltier, there is absolutely no benefit. The only time you would *ever* use a Peltier to heat something is if you got one efficient enough to use it like a "heat pump", to save electricity. That's definitely not the case in -30C weather.

spmclaugh · 10-07-2007, 04:04 PM

Oh, and your chemistry teacher was right. And about your toast: if there was condensation, it was from the water stored INSIDE the bread, not water vapor from the air in the room. It was the cooling of the toast on the glass table, not the heating of the air in the room. And if you burnt your toast, it wouldn't have even happened at all, because there wouldn't have been any moisture inside.

nobb · 10-26-2007, 11:53 PM

Grrrr...sorry to have to bump this thread again. But this morning was -5C and my LCD monitor refused to open due to the annoying temp protection circuit. So I turned on my DIY 50 watt heater and it still took like 10 minutes for my screen to heat up and open. It seems that my heater is pretty much useless.

The resistors are on the bottom on the unit and I would have thought that by convection...the heat would rise and heat the rest of the unit up. Obviously my heater is not sufficient (would be even worse in -40C)...so I was wondering if you guys have any tips or advice?

Should I eBay some 10w resistors and build a 100 watt unit? How can I improve the efficiency? I remember autonode had some rugged displays that were rated down to -40C and they only had 10watt heaters.

nobb · 10-27-2007, 02:12 AM

UPDATE: I think I just figured out how to hack the temperature sensor!!! Other k301 owners might be interested to know how I did it....so here is what I did:

I applied power to the screen. Once the screen is at a horizontal position (right before it flips up), cut the power. Then I unscrewed the screws at the back of the screen and took off the plastic. There is an exposed circuit board. Look for a very small blue rectangle thing (thats the thermistor). Right beside it on the circuit board should be labelled J3 RT1. Desolder the thermistor and in its place...solder on a 10kohm resistor. I tested the k301 by putting it in the fridge (-15C) and it powered up!