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FEZ Cobra power supply


I read the brochure. It said if i dont use a TFT display i can get away with up to 9 V. I Have an RC 7.2 V pack (7.8 without load), and want to drive one servo, Wifi, and a webcam (optionally sometimes a gampad for the camera and Wifi) will i GET AWAY WITH THIS :stuck_out_tongue: ?


It should work. Give it a try.


I have one question also about power supply.
I power cobra direct from 5V line and use 3.5" LCD and work ok. Can be bigger(4,3" and 7") powered from cobra if I power it with 5V on 5V in pin(5V In/Out)? Or power supply on board do not have enought output?
I asking this because I will design own PCB and would like to know if Cobra’s power supply is powerfull enought…
Also what is power usage of:
1.)3,5" LCD
2.)4,3" LCD
3.) 7" LCD
4.)Cobra/EMX (Alone)

Have this info in Specification will be great/must to have!!!


The Cobra power supplies are limited, and that is why there are warnings about how to power it when used with a LCD in particular (the LCD draws a lot of power). Specifically the heat dissipation of them is not that great in higher current draw and higher input voltage situations, so if you have too high an input voltage with big current requirements then you risk damaging the VREGs or the board.

@ Dejan, I don’t understand what you’re saying about powering directly from the 5v line? Are you saying you use a regulated 5v power supply and apply that power to the 5v-IN ? If so, then the Cobra regulators are not in the equation and you will be limited by the heat dissipation and current capability in your power supply. Needless to say, if you’re designing your own board you should be considering your own VREG and power supply considerations including heatsinking, and building that onto the board from the outset.


@ Brett: Yes I use regulated power supply and connect it to 5V In/Out pin(first pin). Is not EMX powered from 3.3V? As Im quick read specification then EMX module and ALL LCD’s use 3.3V for powering so regulator on cobra must power them no mather which input I use(regulated or unregulated socket).
This is reason why I asking for power usage for all 4 items that I know how to design power supply and which one put to my board… I will not put 5A 5V to 3.3V regulator on board if there is no need for them if will be 3A enought…
Strange thing is that in specification is not rated power usage for LCD’s and modules and I think a lot of customers ask same question…


ah good point about 3v3 - the input voltage to 5v reg has always been the one that will take the bigger hit as the input voltage and current draw increases, so I don’t think you are stressing the 3v3 VREG limits.

Given the previous sugestions are in the order of 1A input only, I don’t think you need 5A but 3A is a healthy margin. You have all this kit already, don’t you? Why don’t you measure actuals?? If you want to get to the point where you know this stuff with any accuracy, you may have to approach GHI directly - that means jumping on a call to them. As has also been mentioned elsewhere today, since you can look at the schematics and even the Eagle layout of a Cobra, you can see what capacity the GHI parts use and plan for your own level of overkill :slight_smile:


I have only Cobra and 3,5" so I can mesaure only this two but I will need in near feauture also 7" support so I don’t want redesign board again :smiley:
2A 3.3V I need for UMTS/HSDPA modem so I think 3A will not be enought for all things :slight_smile: Maybe I will put 2x 3A regulator to seperate Modem and EMX…
I will use Cobra Eagle design as start point and will make mesaurements with items which I have in next weeks when complete programming :slight_smile:


Ok Im little research:
1.)3,5" LCD Panel: PT0353224T-A802
Panel: 10.1 mA
Back-Light: 20 mA
Touchscreen: ??

2.)4,3" LCD Panel: ??
Panel: ?? mA
Back-Light: ?? mA
Touchscreen: ?? mA

3.)7" LCD Panel: PT0708048T-A302
Panel: max 260 mA
Back-Light: 160 mA
Touchscreen: ?? mA

160 mA current consumption with everything enabled


Hey i got it working That is servo (Draws voltage from vin/out ) and control over that servo + control over another one (PWM) but the thing is that when i accelarate (i control an ESC on RC) rapidly the ESC looses the signal for a second (or perhaps Fez Cobra resets ? ) if i Accelarate gradually everytrhing is ok . The same thign happens when im full speed and then suddenly stop . The energy source is a battery Pack 7,2 V 1400 mAh. What is happening ? any idea ?


HELP me Out guys - maybe i should use a condensator ? or something on the PowerPlug input Cables ?


You need to ramp speed or have a very high power source. Ramping is better.


BUt when i gradually increase everything is ok and i even think when i had the power supply + debugged from PC USB i could increase speed then suddenly stop - this is important as the ESC offers breaking - what should i Do ? Will a step down voltage regulator help or do i need to increase the mAh in my pack ?
OH and the pwr led does not get dimmer, or blink for a second - it is like continously powered ? Any solution ?


Stopping should be different to ramping up speed. Do you have problems in both directions or just when increasing speed?

The way you’ve described the problem is that when you apply a rapid change that corresponds to a large change in ESC speed you have weird stuff happen - that is most likely because you are seeing a voltage drop when a very high instantaneous current demand is applied.

What you need to do is ascertain if the Cobra is in fact browning out and resetting. That will help you understand whether the current demand is too large for the pack to handle.

Please also tell us about your battery pack, and what ever else you have connected to the device!


Well then I have

USB GamePad Receiver hooked up,
Servo for direction control (PWM + Vin/out drawn from the board)
ESC (only PWM from Board)

The pack is 1400 mAh, 7,2

I debugged it once with USB ( probably gave it a little power boost)

Yes it happens when braking ( braking in means of speeding up and suddenly throwing reverse - the controller then brakes the wheels) I thinkg it worked when i Debuged and stoped when i used just the pack energy. I will try today with just the speed control (without the directional servo drawing energy from the board).

I will try it for reverse today - i was a little frightened of high current getting into my fez… but it will shut down then right ? It has this safety measure :stuck_out_tongue: ?


What is on the end of the ESC?

Tell us about your battery pack - what kind of battery is it, from a reputable maker? What chemistry? New or old? Does it’s reference have any information related to current draw - like a C rating? Full or partial charge - measure it’s at-rest voltage.


Ok on the End there is a 380 Motor for RC cars

Battery: Ni-MH 2/3A 1100mAH 7,2 And there is a warning that the ESC is designed for 1000-1500 mAH Ni-MH and using an incorrect type or higer capacity might damage it :confused: It is a standard green batery pack - no manufacturer printed.

Now the voltage is 7,5 V The ESC can make a squeeky sound at first - the voltage drops to 3,4 for a second (without the FEZ COBRA)

MAJOR UPDATE GUYS - The Ethernet LED blinks for a second - i guess the voltage drops to low for the controller … what to do ??


new battery time - or better still, power the ESC from that pack and power the Fez from the new pack ! Just tie GNDs together.

PS: dropping to 3v4 at startup is a very bad sign for any battery, but to go from 7–>3 is huge voltage sag.


The thing is … it is a new battery from a new MODEL,
I would like to measure the battery charge in the RC Car - can i do that if i Have 2 seperate batteries ? -no i see that the pwr led can even blink

Again my main concern now - can i measure the charge of a seperate battery (like you mentioned one powering the ESC- the one i want to measure, and one for FEZ - BTW what kind of pack ?)


I have designed many things for the RC industry. Just seeing this post now.
I would first do what Brett suggested. Power the cobra by its own power source and the ESC with its own and tie the grounds together. Though you need to pay attention to the voltage levels for your PWM signal to the ESC. If its 3.3v your ESC may not like that as it will be looking for a typical voltage of 5v. Servos have typically been less sensitve to a lower signal that what they are powered from.

Also, more and more ESC’s are being made with VERY poor BEC nowadays. So it would not surprise me that the BEC is the problem. Do what Brett says and it will lead you to a better conclusion on what to do next.


@ jdal

Thx for your advice, as well as Bretts

Here is where im at.

1 I got an RC something happend with the ESCs BEC and it doesnt give the DC level it should( but i dont need it :P).

2 I have charged the battery pack - it does not have that big drop it had.

3 The RC Car can go backwards and brake BUT when i go forwards and brake instantenously i get a spark from the RC engine (well after a while even going backwards is an issue -> and after a real while just steereng can have this effect.

4 Get it i can get another power supply for the FEZ Cobra (the PWM is Now 5V

5 I observe the Vin/out -> GND and its 4.99 -but when that spark happens it drops to around 4.96~7 for a second and the blink occures to the LED and FEZ resets (i guess )

6 I would just love to have it all compact and stuff … i will add WiFi to it probably it would be great if i could just increase the mAH to 1500 ( it says i cant more :confused: ) .

7 Will giving it a regulator like LM2576 help ? ( it says that it is often used before a linear regulator ) ??

Will be grateful for any advice Thx