Hey guys, I am haven't quite a bit of trouble boosting a single nimh
cell to 3.3V at 300mA. At first Maxim looked great and user friendly.
So i order a few samples and built a few boost using the MAX1797,
MAX1724 and MAX1674. I am getting 3.3V out but as soon as the current
increases the voltage drops to 2V. I was just wondering someone could
give me some pointers because im out of ideas. I was thinking about
using mosfet driver boost regulator instead of boost ICs. But my search
has been unsuccessful. Thanks for your time.
-Rich
What kind of battery contacts are you using to connect to your AAA battery?
What is the brand of battery you are using and do you have a datasheet for
it (which we could see)?
What you are trying to achieve is probably possible (1W of useful energy
from a mostly discharged AAA NiMH cell), although extreme vigilance will be
needed in trying to achieve maximum efficiency.
You aren't using an ordinary spring contact based battery holder are you?
The spring contacts so often used on hobbyist battery holders look like they
should be of low electrical resistance, but looks can be strongly deceiving.
If you uncoil the spring, you may find the steel wire is surprisingly long,
perhaps 30 centimeters or so. Typically the wire diameter is roughly that
of 22 AWG copper wire, however, the steel they make them from has
dramatically higher resistivity to that of plain copper, perhaps in the
neighborhood of ten times as much. All this results in adding a good 100 to
200 milliohms of resistance or so directly in series with each battery (when
used in multi-cell holders). In your application, every milliohm counts, so
you must use a battery holder with superior contacts.
Aside from this, you also need to make sure to use adequately large wiring
for the given wiring distances. On breadboards or other non-permanent
prototype connection schemes, it is not unusual to end up using surprisingly
long wire runs perhaps over one meter using thin wire (IE: 22 AWG). This
can easily end up inadvertently adding tens of milliohms or so of extra
resistance which will negatively effect circuit capability.
This utility may help in selecting the proper wire sizes for practical
resistances in your application.
http://www.mogami.com/e/cad/wire-gauge.html
Additionally, your power switch needs to be of good construction with very
low resistance. A small primitive switch could easily add a few tens of
milliohms, which again will compromise circuit capability. Ideally you
would use a gold plated switch with large contact surface area.
