Power and Heat considerations (enclosures, etc)
 

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.

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.

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.)

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 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.

I can confirm this:rolleyes:.. 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.

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!

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.

http://ecomodder.com/wiki/images/9/9e/Dimmerguino.PNG

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 :)

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.

I'm interested in using an external regulator and the capacitor(s). I could put them under the dash or in the engine bay.

dcb,

Can you give some details on the capacitor(s)? I'm guessing they are wired across the + and ground, both before and after the regulator?? What rating caps?

And how to read the three "legs" on the regulator? I've seen pics of them but don't see markings I understand.

I put the suggested 8 ohm resistor in line with the 12V power. It does help the Guino stay reasonably cool.

However, after serious EOC, I've seen the Guino reset itself. (Gallons used etc. go to zero, and the Guino goes to the home screen which I think is "Custom".) I'm theorizing that with battery power dipping into the 10V range or lower, the 8 ohm resistor is too much, and it doesn't get the power it needs.

Thanks!

If the Guino uses 20 ma with the lcd off and I put a 3-pin VR in the power line, will the VR now consume additional milliamps when the lcd is off? How much?

I'm a little confused about the setup. was still thinking about the resets. Do you use the starter? That can make a large voltage drop as well.

you are using an external 8v regulator and an 8 ohm resistor? That might be overkill, dunno.

but just an 8v regulator should not add any noticable milliamp drain.


FYI I am currently using a 470uf cap and two .1uf caps with the regulator, and low power backlight, which is not fed through the regulator (schematic in post 8), but rather is fed 12v through a resistor and has the bottom leg pwm'd. So it is a sort of switching power supply just for the lcd backlight.

Voltage regulator not yet bought. Probably today. Need the caps too but which ones and where do they go? Polar or non polar??

I plan to ditch the resistor when putting in the regulator. I'm not up to wiring inside the guino assembly. It will all be external to it.

Usually when doing eoc I bump start it but i will use the starter after sitting at a light. Definitely there's a voltage drop with starter use and that could be when it resets due to the 8 ohm resistor.

Sorry I don't follow your last paragraph about the latest guino components. I can solder OK. The 8 ohm resistor is in a 4x2x2 project case with a major vent hole so I think I'll put the VR and heat sink in there instead of the resistor. Or maybe screw to the firewall for a heat sink.

Thx.

You have a prebuilt, no? do you know what version is it? 1.1 or 1.3?

Prebuilt from you. Sorry don't know the version. I remember I sent it back once and you replaced either the LCD or the whole thing. You recall?

ok, probably 1.1. lemme see if I can whip up a diagram.

OK. I'm headed out to get the 8V regulator at an electronics store here and other errands. I'll check later for any caps info.

I *think* this should do it using radioshack parts. The cap is kinda expensive, I just grabbed the biggest one they had to help address the resets.

You probably won't need an extra regulator with this, as you have a voltage drop in the diode and the 8 ohm resistor.

So when you start the car, the idea is that this will help maintain a higher voltage for the guino. Dunno if it will help though. If you can salvage a 25v (or higher) large cap that would be cheapest.

4700µF 35V 20% Axial-Lead Electrolytic Capacitor - RadioShack.com
4700uf axial cap

1N4001 Micro 1A Diodes - RadioShack.com
1n4001 diode

8-Ohm Non-Inductive Resistor - RadioShack.com
8 ohm resistor

http://ecomodder.com/forum/attachmen...1&d=1295108192

OK - I could add this to my existing setup.

I was all set to ditch the resistor and put in the 8V regulator.

You think the big cap + diode + resistor is better than using the 8V regulator, without the resistor? Is there a down side to the regulator??? I mean a negative aspect. I mean, this d**n English language is a minefield for double meanings. Is the VReg a worse plan?

I think the extra regulator could compound the resetting on restart problem. The above is a supersized version of what is on the 1.3 boards, which, knock on wood, has seemed pretty solid.

Sounds good.
Radio Shack, here I come.

Issue resolved.

Cause: faulty 4-pin computer connector that I put in, to give a way to unplug the Guino. Not the tiny ones for floppy drive, the larger ones that powered hard drives and plenty other stuff 'till the SATA drives came out.

I never could determine for certain if it was the male or female connector, or was it the pin/tube fit or the crimping of the wire to the connector contact. But I did determine the +12V wire passing thru the connector was intermittent.

Cure:
Severed the wires on either side of the connector. Inserted 8-10" of new wire in line. Twisted bared leads, soldered, then covered with heat shrink tubing and shrunk it. Seems to be fine now. I hope I didn't jinx it by saying so.

I got the cap and diode from Radio Shack but didn't bother putting them in. I think I'll return them. This fixit job was enough of a PITA as it was, without opening up the project box that holds the existing 8 Ohm resistor and redoing all that. It was worth it though.