Remote controller battery power stabilzation

[fax8]

Hi everybody,

I'm designing a a remote controller which will be powered by a standard 9 Volt battery. The remote controller will have various buttons, an ATMEGA 328p microcontroller (will use my Femtoduino), 3-axis gyroscope accelerometer and magnetometer (as in my FreeIMU), LDR sensors, an RGB led and a pager motor. Communication will happen using a Bluetooth Mate Gold.

I never designed anything battery powered and my education is not in e.e. ..
Do I have to implement some kind of battery power stabilization for the 9V battery?
Note that the FreeIMU, Femtoduino and Bluetooth Mate all have their respective 10uF caps with their voltage regulators.

Thanks,

Fabio Varesano

[Simon]

providing your parts can take 9V and won't mind the voltage slowly dropping to 7V or lower you will have no problem using the battery directly

[fax8]

Thank you Simon, yeah the voltage regulators on the various parts are all 9V tolerant and as the higher voltage I have is 5V I think it's ok if power drops to 7V. Thanks!

[Simon]

well most regulators drop out at 1.7-2V although you can get some regs that are as low as 0.2V but these are specific. You need to look at the datasheets for the devices you are using

[Mechatrommer]

stabilizing with just a simple bypass capacitor can save you from a rusty contact or quick bump on the unit.

[fax8]

Thanks guys.

My voltage regulators are MIC5205 which should have a low drop out voltage, so I think I'm ok on this.

Regarding the bypass caps.. what about one 10uF eletrolitic and one 100nF ceramic in parallel?

[Alex]

You dont need a tank capacitor, the regulators have plenty of on-board capacitance plus they will regulate even if the voltage drops to 7V (assuming standard regulators).  Use a good quality battery clip as you should anyway.

On a different note, I dont know if you are aware of this but the bluetooth board has a maximum voltage spec of 6V. All boards appear to have on board regulators. Power consumption will be quite high with all the gizmos you mentioned. I would suggest you use 4x AA batteries or higher (depending how long you want it to last) and not use any regulators or tank storage capacitors.

[fax8]

Thank you Alex for your hints! Yeah, I'm feeding power to the bluetooth module through the voltage regulator on the Arduino .. Maybe it's better to use an external voltage regulator as the MIC5205 will feed only 150mA..

Would you please explain why it would be better to use 4 AA batteries and why tank caps are bad?

Thank you.

[Alex]

A useful thing to do is a power budget diagram. Draw little blocks on a piece of paper representing all of your system's components including motors etc. Note down the power supply requirements of each block and its peak power consumption. Work backwards towards the power supply source and see if the requirements for each block are met. For example you connected the bluetooth board to the arduino regulator - is that really necessary?

Sure, I like the curious type.

4x AA batteries will provide a nominal 6V with an energy storage capacity of roughly 2.3Ah or 13.8Wh.
Because the total voltage of the battery pack will never exceed 6V (under some load) you need not use a regulator to powwr a device with a maximum rating of 6V.

On the other hand a 9V alkaline has a capacity of roughly 0.5Ah or 4.5Wh. This is much lower than 4x AA batteries.

You might say that since you are using a higher voltage your system will draw less current therefore the 9Vbattery will last nearly as long as the 4x AAs. However, linear regulators are used so the current demand from the power source is equal (plus some extra) to the current demand of the load. In other words the additional voltage is 'dropped' across the regulator and dissipated as heat. Therefore for battery powered applications that use linear regs it is best to use a voltage source with enough voltage to power all devices and you would choose the maximum rating for the devices so that the system keeps on working as the batteries are drained.

I felt that a 9V battery would not be sufficient because I read words like leds, motors and wireless in your original post. 4xAAs are not that much bigger than a 9V battery, twice as big.

Tank capacitors are not bad. In E&EE you very often need to make a decision after weighting several factors. In this case the benefit of adding a tank capacitor does not justify the hassle. If you use a decent quality battery holder - and by decent I mean one that doesnt disconnect the batteries when you move it around! - then the batteries will be able to supply the loads constantly. The increased cell size will offer a rather low ESR so the voltage will not drop considerably during peak current demands (you need to verify this).

Keep any high power devices like leds and motors directly powered from the battery in your application and any selsitive silicon like the MCU behind a voltage regulator (the on-board ones).

Alex

[Mechatrommer]

stabilizing with just a simple bypass capacitor can save you from a rusty contact or quick bump on the unit.

i was assuming you dont have a regulator. even if you do, you'll need at least two cap for the regulator one before and one after the regulator, use the value recommended in the datasheet is sufficient, no need large tank cap.

[fax8]

Thank you Alex for your explanation. Everything sounds good, I'll check if 4 AA batteries will fit into the remote controller case I'm designing.

Thanks Mechatrommer for your hint.

I'll let you guys know how it goes. Thanks again.

[cyberfish]

If you want longer battery life, a switching regulator may be a good idea. You can find ones that are pin-compatible with 7805.

Something like this -
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=102-1709-ND

At 9V to 5V, the savings could be substantial. Theoretically you'll get about 80% more battery life. In reality probably more like 50%.

[Zero999]

Another option is four AAA batteries which will still give better life than a 9V battery and 5V linear regulator.