Ok, so I am trying to power a small RF transmitter remote that originally is powered by a cr2032 battery from the 3.3v pin of the Arduino Uno. Energizer battery specs lead me to believe the short term power draw is rather small but approx. 100ma and potentially as high as 150ma for a short .1-.2 second burst. with the cr2032 battery being 3v, I am thinking the 3.3v pin from my Arduino Uno would be ideal, but specs say it can only provide 50ma. I believe a capacitor would work well for this short term power need, but I am not fully understanding the use of bypass capacitors how it would not short the power source upon initial charging and pull more than the 50ma (albeit for a rather short time)from the Arduino 3.3v pin during its power storing and potentially damage the voltage regulator. Further if the initial charging is not a danger to the 50ma, I would like someone to verify my math on the needed capacitance. 3.3v * 150ma * .2 seconds would be about 0.1 joules correct? If I am right then I will need approx. a 20,000uf capacitor which seems like a huge amount to replace a simple button battery.
Am I wrong with my calculations or assumptions or is there a better route to take to doing this? My end goal is to use Transistors I have soldered on to the RF remote to replace a mechanical button push on the RF remote and thus be controlled by the Arduino. I am using the 5v digital output using a 150 ohm resistor to drop the voltage down to 3v before it gets to base lead of the transistor on the RF remote so that it doesn't create an issue. I originally tried to use the button battery as my ground on the incoming digital base signal, but I seam to not be getting any current flow and such I am now thinking I have to have shared grounds. Could I still share the ground and leave the button battery or something?
I believe a capacitor would work well for this short term power need, but I am not fully understanding the use of bypass capacitors how it would not short the power source upon initial charging and pull more than the 50ma (albeit for a rather short time)from the Arduino 3.3v pin during its power storing and potentially damage the voltage regulator. Further if the initial charging is not a danger to the 50ma, I would like someone to verify my math on the needed capacitance. 3.3v * 150ma * .2 seconds would be about 0.1 joules correct? If I am right then I will need approx. a 20,000uf capacitor which seems like a huge amount to replace a simple button battery.
What math ? I don't see any math.
The battery is rated for 250mAh so if the transmitter drew 150 mA for one hour, it would still last another 45 min.
If it drew 150 mA for 0.5 S, it would last about 12000 presses. Since this seems rather high , I would estimate it actually draws more than 150 mA. (for a very short period
1 hr = 3600 seconds (7200 half seconds)
250mA/150 mA = 1.666
1.666 * 7200 = 12000
The caps aren't going to damage the regulator because it is short circuit protected.
Read the datasheet UNO
Where those simple calculations are wrong is that you assume you can deliver that charge as 3V3, that is full voltage. You can not do that as the voltage will droop long before it is fully discharged.
As you have found even your too small capacitor is way too big to be practical. Forget the capacitor idea and just put an external regulator on the 5V rail.
Jremingrin he is talking about the 3V3 supply not a pin output current.
arduinos website says the 3.3v pin can provide 50ma here under tech specs.
I am not drawing much power from an output pin. i have a 5v output with a 150 ohm resistor. shouldnt that be about 33ma? my battery source is 12v. i was using npn transistor but i do like the idea of the pnp to ground the output. but this still doesnt address getting 3volts of 150ma to the RF remote outside of the transistor circuit. Any ideas on that?
thx grumpy mike. i guess i will have to order a 3vilt regulator. i was hoping to avoid the delay from shipping since my local radioshack didnt have a 3v regulator. i guess i could probably do and andustable one, but that is another thing to learn on a project that already has gotten more complex than anticipated.
again as i am trying to explain. i am powering the remote from the 3.3v power pin and NOT a GPIO.
Wrong. There are several mistakes on this site that should be corrected.
The 3.3volt regulator on the Uno (LP2985-33DBVR) is good for ~150mA.
Other Arduinos could be 50mA.
Don't try to power anything >20mA (or charge a 20,000uF cap) from an output pin.
Leo..
However, there is another issue to consider and that is your assumption of the maximum current draw for TX. You should actually measure it. Due to the high internal resistance of CR2032 cells, I don't think it is even possible to draw 150 mA, and if so, its lifetime would be shortened drastically.
The conventional wisdom is that you should never attempt to draw more than about 30 mA in burst current from such cells and most designers are aware of these limitations. Products using expensive cells must have a reasonable battery lifetime.
Here are a couple of articles on steady state and burst current versus lifetime for CR2032 cells:
The 50mA max current value is from the Duemillenove and prior arduino's that derived the 3.3V from the on board FTDI chip and not from a separate regulator.
pwillard:
The 50mA max current value is from the Duemilanove and prior Arduinos that derived the 3.3V from the on board FTDI chip and not from a separate regulator.
Which would then include all the bogus "UNO"s presently sold which are really not UNOs in any respect, but Duemilanoves with a CH340.
Paul__B:
Which would then include all the bogus "UNO"s presently sold which are really not UNOs in any respect, but Duemilanoves with a CH340.
The Uno "clones" on the market that use the CH340G typically do have a 3.3v regulator on the board, and don't pull the 3.3v from the CH340G - certainly all the ones I've seen have it....
AFAIK there is no usable 3.3volt output on the CH340. There is on the FT232.
I had a quick look at a clone Nano I have here, but it seems the 3.3volt output is connected to the serial out pin. Not sure though. Have to confirm this. If so, the 3.3volt out on the clone Nano is useless.
A clone Uno with CH340 has indeed a dedicated 3.3volt regulator.
Leo..
On my Uno clone it's the small 5-pin SM chip between the 8-pin LM358 opamp and the 1117 regulator.
Just next to a 100uF cap.
Labled as U5. You can just see it here.
Leo..
thanks again everyone. i will have to look up the model of my arduino uno to verify if it has a separate regulator.
jremington good points, unfortunately in my excitement to finish the project, i soldered on the transistors over the manual button leads and have rendered it inoperable without an outside source. i went ahead and ordered a couple replacements and i will use a multimeter for those before continuing hopefully they use the 30mA you mentioned as a rule of thumb and be fine for my Arduino to replace the button battery with the arduino or 3v regulator.