How much current is left to draw from the 3.3V power supply on the FEZ Cobra without blowing things up? (When powered from an external 6V DC adapter)
Enough to run plenty of sensors, GPS, etc.
No motors, no servos, etc. If you have to question whether it’s going to pull too much current, it probably is.
I have no display, but want to add a EIA232, EIA485, CAN transceiver and
an additional µC.
The EIA485 and the CAN transceiver can both draw up to 200mA on shorted lines.
Let’s say the whole thing needs 500mA.
The regulator can take 800mA I think so you will have about 500mA for your use. Now, I still do nto recommend not going that far. Maybe 200mA is good max value… which is enough to power all you need, as far as transceivers.
With drawing that much current, I would not “try if it works”, I would just pick a voltage regulator, a capacitor and make a separate power supply.
I have put another 3.3v reg off of the 5v rail that gives me another 300-400mA. Its the best way for me…
Cheers Ian
Ok, I’ll take the advice and add a seperate switchmode supply.
Thanks!
Do you connect both 3.3V supplies together? Or just connect grounds?
You should never connect supplies together, only grounds
Then what happens if the supply of the EMX falls away. But the supply of the receivers stays. The receivers will then f.e. keep an IO of the EMX high on 3.3V. (or reverse, when the supply of my transceivers falls away).
I thought you should never apply any voltage on inputs when there is no supply voltage on that IC.
(In absolute maximum ratings of inputs they often refer to f.e. Vdd + 0.3V)
Generally, power supplies are powered from the same source. They are all on or off
This is never the case in practice. One capacitor holds its value longer than the other + a regulator can fail.
I think the only valid way is to remove the regulators (IC1 & IC2) from the FEZ board and use larger regulators to deliver 5V and 3.3V.
Erm,
You are really over engineering this. I have never damaged an IC due to pushing voltage to a signal pin without having voltage on Vdd and I don’t ever anticipate doing so.
Removing the Vregs on the Cobra isn’t going to do a single good thing for you. All you need to do is have a separate SMPS if you really think you will be drawing too much current from the onboard linear Vregs. 
@ IanR " have put another 3.3v reg off of the 5v rail that gives me another 300-400mA. Its the best way for me."
That sounds like it would make a good hw sample. With description, parts, and video. Wonder if things like this could go on fezzer? Even without code, it would seem to be a good place to post pictures and a vid.
[quote]@ IanR " have put another 3.3v reg off of the 5v rail that gives me another 300-400mA. Its the best way for me."
That sounds like it would make a good hw sample. With description, parts, and video. Wonder if things like this could go on fezzer? Even without code, it would seem to be a good place to post pictures and a vid.[/quote]
This probably isn’t a very good idea, either. You are just going to end up pulling too much from the Cobra’s 5V linear Vreg.
There’s no free lunch here, you can’t just chain Vregs and expect to get boots in the amount of current you can pull.
Not really… The 5v at approx 500mA is 2.5 watts, (I wouldn’t draw much more than that), that equates to 750mA available on the 3.3v rail I split that so the cobra has 300 mA and I can have another 300 mA easily on another 3.3v reg.
I’ve run the 5v reg at 1/2 an amp and it seems to be ok with it.
Cheers Ian