Electric Imp & Batteries

Hello everyone,

This will be my first foray into the world of hobbyist electronics, so I apologise for the newb question.
I am looking at a 3.7V Li-On battery with a capacity of 850mAh to power the imp in my project.

Will it be safe to run this 3.7V straight to the electric imp without any type of regulator? I’m also using other components which say they have a safe limit of 4.5V i think, but I don’t know how much this will affect the Imp itself. I remember reading the imp needs 3.3V so I don’t know if this battery will be too much.

If I do need a regulator, what should I be looking for?

Thanks for the help,
Tom

I should have mentioned: I’m using the SparkFun April breakout board for the imp.

The April board has a building switchmode converter. that will convert it down nicely to 3.3.
Keep in mind that the 3.7 of the battery is nominal, when its fresh from the charger (or even worse, still charging), the voltage can be unto 4.2V

Remco

Thanks for the help Remco; much appreciated!

If it receives a higher voltage, such as 4.2V, is this a detriment to the board or relatively “safe”? Will it cause any long-term damage?
Ideally I’d like to create this project as almost “maintenance-free”, so I can use it whilst charging without having to monitor the charging process.

Thanks again for the info so far; I just want to triple check these details before I go starting a fire!

if you go over 3.6V on the imp, it will die.
the regulator on the April bord is happy upto 17V. altho i dont know if the surrounding components are, but 5V is that USB gives, so thats 100% safe

Remco

Could you elaborate on “the surrounding components” I’d like to power an imp in a sparkfun April board with a 9V supply so that I can also power a 9V relay, and not need a separate power source. Am I good to go there? Does the voltage regulator on the april board not regulate for all the components on the board?

You’re good to go on the april board, yes. Feed your 9v into the VIN/GND pins and it will deal with making 3.3v.

As for the relay, you’ll need a transistor or a FET to drive the relay from the imp’s GPIO - you can’t drive it directly. This article should help: http://blog.makezine.com/2009/02/02/connecting-a-relay-to-arduino/

… R1 should be ~1k and TR1 can be pretty much any NPN transistor eg BC547. D1 use a standard 1N4148 diode.

the ‘surrounding components’ in this case is th reverse polarity protection fet, which can handle +/- 20V between its Gate and source, and the input capacitor, which is listed as 50V in the sparkfun schematic. The limit of the IC itself is 17V, so upto 17V should be fine.

For battery operation, the lower limit might be interesting too.
lets assume a peak usage of 400mA, supplied at the JP3/JP4 pins/pads:
FET: max 90mOhm = 0.036V
Switch mode IC: specifies a 100% max duty cycle, so we only worry about the resistance of his high-side FET: max 600mOhm, at 400mA = 0.24V
Last one: the inductor. another 90mOhm = 0.036V
so to maintain 3.3V, the absolute minimum is 3.3+0.036+0.24+0.036 = 3.612V

Remco

Nice analysis!

We are actually fine right now down to 2.7v, so we can go lower than 3.612v on the input. However, you need to bear in mind things like the current consumption of the DCDC which increases when it’s in dropout.

Thanks for all the info, this is perfect!

Hmm, does it mean that it could be operated by two AA batteries? It seems like borderline voltage…

Two AA lithiums are better than two normal AA’s. See http://data.energizer.com/PDFs/l91.pdf

…though our 2xAA designs have a boost converter to ensure a good 3v.

If I connect a 12v 1amp wall wart to the p+ p- of the april board should I be fine will it automatically scale down to the 3.3v that is needed for the imp without out frying the circuit

Ideally, connect it to the GND/VIN pins, not the P+/P-, but yes April has a DCDC onboard that will take 12v down to 3.3v.

If you dont mind can you explain why I shouldnt connect to the p+ p-

Depending on the rev of April, the P+/P- reverse voltage protection fet was either rated for 12v or 20v. If you have a 12v one, the FET may fail.

The FET is not in the path for GND/VIN.

Can a 9V battery or a 6v (4xAA’s or 4xAAA’s) applied to p+/p- safely power the April, no matter what the revision is?
Thanks

yes

The buck regulator on the April design will handle up to 17V, but optimally you would use a supply around 5V. The more important factor is that your power source can supply enough current. The Imp Card can occasionally pull up to 400 mA, so you need a source that can sustain that for at least brief periods, and still power other connected components.
I highly recommend a read through some of the new documentation, it is invaluable.
https://electricimp.com/docs/hardware/

New stuff is being added all the time.