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Powering Fez from a car


#1

I’ve got a power regulator question. I am planning to use a Fez in my oldtimer. Now the voltage coming from the rail is not very stable, somewhere between 11v and 13.5v, depending on the load on the system (e.g. A/C on etc).

I’ve read about standard USB chargers but the may not supply a stable/clean supply. Does anybody know about a schematic that stabilises/filters the input well enough not to fry-my-fez?


#2

Maybe this is something for you: http://www.mini-box.com/micro-UPS-load-sharing

For more like these, google for “car pc”


#3

You can get small DC-DC converters for $10USD or so that would work. Look at Jameco or similar places for brands such as MeanWell or Murata.


#4

You might want to try one of these regulators: http://products.cui.com/adtemplate.asp?invky=931361&brand=v-infinity&catky=328060&subcatky1=521999&subcatky2=527874&subcatky3=.

I’m using one of them to drop the voltage coming from a car’s OBD port down to 5v. The website has some documents that have the schematics. It’s very, very simple to wire up; you just need the regulator and a couple of capacitors, and you can order everything from DigiKey. I’m pretty much a hardware noob and I had no problems getting it rigged up on a breadboard…I’ve hooked the regulator up to power sources anywhere from 11v to 24v, and it always drops it down to exactly 5v. This is working great for me so far to power the Cobra.

Mark


#5

@ Eric: That looks like something to power a complete PC from a Car, and I don’t need the UPS while the engine is off.

@ Jeff: Thanks I’ll look in to that

@ Mark: that thing seems spot on, I’ve look around the web and came with a similar suggestion. My car does not have an OBD of some sorts, just a accessories wire and a negative ground (!)


#6

Yeah, I figured there was no OBD when you called her your “oldtimer” :slight_smile: But it should work more or less the same as my setup since I’m just tapping in to the 12v and ground pins. The regulators are DC-DC converters, the same as what Jeff mentioned. Gus from this forum provided the link to them when I had the same question as you.


#7

Just put some bigass caps and diodes on your voltage rail. I have a diesel which looks like absolute crud on a scope when starting however with a couple of 330uF and a couple of 220uF and a big diode and 3x 330uH inductors it comes right down to almost nothing.


#8

I noticed that the Domino’s external power connector is connected to a 5v regular on the board itself. According to the schematics the connector just puts the 12v on the rail and forwards it to a 5v regulator (LM1117MP-5). Then another regulator kicks in to bring it down to 3.3V, protected by a 10uF capacitor.

Well I don’t want to be rude, the datasheet for the LM1117MP-5 shows me it can take anything from 6.58-20V, with current limitint and thermal shutdown. I won’t be needing the 12v, only the regulated 5v to power my LCD.

Now my question is would adding yet another regulator be overkill? Or just plain paranoia?

@ MarkH: I now understand what you mean.


#9

Regulators lowers voltage making exces of power to heat. So if you hook lcd or whatever to that regulator it will produce more heat.

Now You must ask yourself how you gonna mange that heat. If it will be in some kind enclosure (I suppose since it is a car) I would go with seperate regulator.


#10

Thank you. I know what you mean. I tried using a 12v wall wart for a couple of minutes and noticed the fez getting pretty hot (> 60C using my uncalibrated thermometer)

Looking at other posts on the forum the documentation seems to be off. 12v input is way too much for the fez and imho should not be advertised on the documentation or on the board itself.


#11

I power my FEZ off 9V all the time and it never gets very hot. Are you loading the FEZ down with components or something?


#12

My FEZ is connected to a 5v LCD screen, a shifter and some buttons. The LCD is powered of the 5v rail of the fez. I don’t consider that a serious load.

Now I have a 6v wall wart (300mA) and it doesn’t heat up at all.


#13

“I don’t consider that a seriousl load”. Have you measured it? Screens do draw a lot of current, especially the backlight LEDs.

When I look at the National LM1117 supply characteristics it says max input voltage is 15v. http://www.national.com/mpf/LM/LM1117.html#Parametrics. So max input might depend on what manufacturer is used (ie just don’t assume it’s going to be capable of 20v)

Regulators convert one power supply to a different one. Power conversion generates heat. If you have a higher input voltage, at the same output current consumption, you generate more heat. Draw more current on the output, you need to draw a greater current on input, and you will generate (even) more heat. So if you’re trying to draw a higher current, and you are starting at the upper end of the voltage spectrum, then you’ll generate more heat. The on-board regulators are not designed for high heat dissipation, which is why all the suggestions are to use lower voltages when you’re trying to draw higher currents.

My suggestion for car powered Fez is to make your own step-down regulator to bring the ~14v (when running and the alternator is charging) to ~6.5v, and make sure it has a kick-ass heatsink on it so that it has a way to disspate the heat. Then feed the output of that into the regulator on the Fez. Another approach is to buy a car USB converter that outputs 5v and use a USB cable to power Fez, but make sure its not necessarily a “cheap” one but one that has a good regulator in it (hard to do without laying out the cash first I know ??? )


#14

Now that is a good explanation. It’s hard to get a proper education about this stuff these days. Lots of people on the web assume I know all about this stuff but it’s still a lot of magic to me, and I have no proper resources to ask (hmm yes my only local electronics shop just told me its closing down because of all the webshops).


#15

glad to be of assistance.

Yes, unfortunately there is often no way to know the things that many people see as “foundation” level information. Places like this help so long as others know where you’re starting at.