Power and Heat considerations (enclosures, etc)

[dcb]

I finally fixed my milliamp meter and did some measuring. I have not tackled enclosures yet due to the added costs and heat dissipation concerns, but wanted to discuss it anyway.

With the LCD off, the unit (prebuilt) uses 20ma. Each level of brightness adds 20ma so that it is ~80ma when full on.

So the guino creates ~1/4 watt of heat with the lcd off, and ~1 watt with it on. The lowest rated component is +70C (158F), so some consideration needs to be given to not trapping all that heat. I don't think it magically stops working at that temperature, just that you shouldn't plan on going above that.

Since we are going from 14 volts to 5, the regulator itself is dissapating .18 watts w/backlight off and .738 watts w/backlight on setting 3. The regulator is easily the largest generator of heat here, and the backlight is the largest user of the remaining power.

Simple strategies:
Consider not mounting the mpguino in the sun. I usually set it in front of the instrument panel where it gets a bit of shade usually and is in a "natural" reading location.

Crack car windows on hot days to reduce interior temperatures.


If adding an enclosure:
Drill plenty of ventilation holes on top and bottom.

Turn off the backlight during day use.

(advanced) Consider adding an 8 volt regulator (and a couple small capacitors) to the power line away where power is tapped, this will reduce the amount of heat created by the internal regulator to 1/3 of what it would be at 14 volts.

(advanced) Remove the lcd transistor and power the lcd backlight from the cars panel dimmer via an externally mounted ~270 ohm resistor. This is a pretty elegant approach actually, the resistor won't contribute to the in-case temperatures, the backlight will be "in synch" with the dashboard lights, the internal regulator will be limited to .18 watts, and it is dirt cheap and not hard to circuit bend onto an existing mpguino.


Note: When/if I can, I will add a temperature readout to the cpu monitor display (apparently the 168P I'm using has an internal temperature sensor), that should help folks determine the state of cooling.

[dcb]

Here is another possible option (for prebuilts anyway) put a 6 watt 8 ohm resistor in series with the positive lead to knock down the peak current
8-Ohm Non-Inductive Resistor - RadioShack.com

Though I think an external 6-8 volt regulator would be much more effective for the same price.

[Froggy]

That's interesting about the chips built in temperature monitor. According to sources on the Net, it seems it will require the user to calibrate each unit though as results vary from chip to chip (even within the same chip batch). It would be a good addition (and it's free.)

[electromike]

My only additional finding:
The temp inside a car parked in the summer sun is higher then the melting point of hot glue.
Do not use hot glue on this project at all.

[brucepick]

I remember seeing high and low temperature hot glues and guns. Sorry I don't recall the melting temperature of the hotter stuff but that might do it if you want to use hot glue.

Might have to go to a hobby store or maybe a hardware store to find them though. The last time I looked at glue and glue guns in a big box store they were all "uniersal", whatever that is.

[mirkwood]

Originally Posted by electromike

My only additional finding:
The temp inside a car parked in the summer sun is higher then the melting point of hot glue.
Do not use hot glue on this project at all.

I can confirm this.. apperently the hot glue that i have used, have a low melting point...

black vinyl -> metal plate -> mpguino -> attached with hot glue.. get very hot..

bright side tho, is that i now can do some changes to the design of my casing while it is out.

[Execut1ve]

Hey dcb, I know this thread is an oldie but can you provide more info on how to tie the MPGuino backlight to the car's panel lighting? would it be possible to get a preassembled unit ready for this option? thanks!

[dcb]

with revision 1.3, the low power lcd backlight is powered by 12 volts, and controlled by an npn transistor on the ground leg. So assuming you have a 0-12v signal for the dash lights, you can break one trace on the guino and use that varying source for controlling the backlight at a high level, and still be able to adjust the brightness within that level using the guino middle button.

The low power lcd has reduced generated heat considerably. By by powering it upstream of the diode, if the power dips the unit will use the large capacitor to power the cpu (brain) and not the backlight, and thus should keep it "alive" longer. Also, I'm using 470uf caps these days.

[Execut1ve]

I see, so this is pretty much a do-it-yourself deal. Is there a compatible LCD with black numbers on a blue field? and how are the LCD's for visibility during the day with the backlight off? unrelated to the topic of the thread but I am curious

[dcb]

I'm happiest with the green, folks have retrofitted other displays though.

The standard display is transflective, meaning that it has a backlight and can reflect ambient light too so is still readable, but moreso with a tinge of backlight.

I've only seen white letters on blue or white on black, not black on blue. Blue is about the worst color for nighttime use fyi, and red makes me think my oil light is on.