3
\$\begingroup\$

For my current project, I need 5VDC rail to power an ATtiny85, an RF module, and an Ultra-sonic range sensor, all of which require 5V. 5V ensure that the ATtiny85 can be operated without crystal at the desired clock frequency reliably, and it also ensures sufficient range for the RF module etc. The total current draw expected, is about 50mA on average, and about 80mA peak.

Currently, I am considering powering the project using 4 AA (alkaline) batteries, in series to get a peak voltage of 6V. Keeping the project low-cost, in low volumes (say, 10 odd pcs), is one of the objectives. Would prefer to keep the batteries until their terminal voltage runs down to about 1.1V, i.e. for series connection, about 4.4V. Are buck-boost switching regulators the only option ?

Edit: Reading around (arduino.cc forums) I found an alternative proposal, which seems to be low-cost and feasible, but would be good to have some community feedback anyway --

Use 3xAA alkaline (4.5V when new but down to 3.3V after run-down), with the DC-DC boost regulator like this one, selling for a what seems like a rather good price (within my project's budget). It can boost the inputs voltage between 1-5VDC, to 5VDC, supplying max current of 500mA (way more than sufficient for my needs). If this works, the only challenge is finding battery holder for 3 AA's. I've only seen those cylindrical, 3xAAA holders, but nothing for 3 AA's.

\$\endgroup\$
10
  • \$\begingroup\$ To get 6 Volts, you would need the alkaline batteries to be in series not in parallel. \$\endgroup\$
    – Anindo Ghosh
    Commented Apr 25, 2013 at 6:53
  • \$\begingroup\$ <palm-face>. Fixed in question. Thanks @AnindoGhosh \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:01
  • \$\begingroup\$ For 80mA peak, you could probably get away with zener regulation. \$\endgroup\$
    – Matt Young
    Commented Apr 25, 2013 at 7:03
  • \$\begingroup\$ Thanks @Matt, but Zener regulation would lead to batteries needing replacement, well before they are depleted, no ? \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:12
  • \$\begingroup\$ @icarus74 The current draw will be a few mA more than your load current. My back of the napkin calculation is about 80mW dissipated as heat vs. 400mW load. ~80% efficiency isn't bad for less than a quarter. \$\endgroup\$
    – Matt Young
    Commented Apr 25, 2013 at 7:26

2 Answers 2

Reset to default
2
\$\begingroup\$

(Question subsequently fixed) First off, to get 6 Volts from the alkaline AA cells, they would need to be wired in series, not in parallel. Paralleling batteries will increase current delivery capacity but keep the voltage the same as a single cell.

While pre-built buck regulator modules are available at the same source (eBay.com) as the boost regulator mentioned in the question, and at a similar price, the following are the trade-offs:

  • These buck regulator modules have a measurably higher efficiency in practice than the boost regulator linked in the question - The boost module gets quite warm (I use that specific module for charging my cellphone from a single AA cell while traveling)
  • The boost regulator is significantly smaller in size
  • The boost regulator works until the batteries are far more depleted, than any boost regulator strategy would allow.

Hence, going by the last two points, the boost regulator module is certainly a preferable approach, if the device running a bit warm is not a concern.

\$\endgroup\$
5
  • \$\begingroup\$ Excellent points there. Running a bit warm may not be an issue, with the only concern being some clock drift (is that the right term?) of the ATtiny85, since I use the internal oscillator, which is temp dependent. The interrupt based codec for RF transmission is timing sensitive, but given how inexpensive the boost modules are, I am definitely going to try it out. Will order some buck regulators as well. \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:11
  • \$\begingroup\$ By the way, would it be possible to check battery level, using voltage divider method and ADC of the attiny, to determine, when to change the battery ? Since my sensor transmits wirelessly the readings, it might as well transmit a low-battery signal. \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:17
  • \$\begingroup\$ @icarus74 Yes, of course. That's a standard technique in battery operated remote sensor devices. \$\endgroup\$
    – Anindo Ghosh
    Commented Apr 25, 2013 at 7:18
  • \$\begingroup\$ Well, what I meant was, whether the act of regulation itself interferes with this measurement ? I guess not, because the voltage divider would be staged before the regulator, but still wanted to be sure. \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:24
  • \$\begingroup\$ @icarus74 That deserves a separate question, but short answer: Use a capacitor at battery level sense input to filter out transient voltage dips on the battery, if any. \$\endgroup\$
    – Anindo Ghosh
    Commented Apr 25, 2013 at 7:27
-2
\$\begingroup\$

You could use a regulator, but a cheaper alternative would be to just use a diodes in series. Although it's not very reliable (voltage goes down as the batteries drain), it should get the job done. If you use 1N4148, you will get a drop of 0.6 volts per diode. The peak would be 5.5 volts. (6.1 = 0.6 volts)

Crude, but works!

\$\endgroup\$
2
  • \$\begingroup\$ Yeah, but I'd have to replace the batteries, when they still have about 70% juices left in them. No ? \$\endgroup\$
    – bdutta74
    Commented Apr 25, 2013 at 7:58
  • \$\begingroup\$ -1, question clearly states good reasons for operation down to an input of 4.4V with a 5V output so this would only make things worse. \$\endgroup\$
    – PeterJ
    Commented Apr 25, 2013 at 10:02

Not the answer you're looking for? Browse other questions tagged or ask your own question.