Single cell boost help.

I can only use the provided AAA 1.2V nimh cell.

Then build the booster to charge as much as you can while the motor is not active. Down convert while running the motor. Forget about buying, you need to get into the circuit to build it. Be prepare to burn out some parts, the point of this exercise is to see how far you can push on the chips.

I think I'm am allowed to use anything as long as i don't charge it before the run. Would switching the voltage high before bringing it back down hurt my efficiency? I really thought long and hard before asking. My classmates and and I are truly stumped, next week is block integration no one i know has anything reliable. Thanks for your reply

-Rich

Here's a little more detail. Sorry about being to vague earlier. For my Jr. design i have to build the power block to our solar powered hen. The power block has to power the main control block which has back lighted LCD and a motors and sensors block that consists of a 2 parallax servos and 3 sharp IR sensors with 3.3V. The "hen" gets its power from a 100W flood lamp through a solar panel that provides 3V at

50mA. The "hen" get one hour to charge a depleted nimh battery and find a foil target in a electrical tape bordered box without coming in contact with one of the other 5 "hens". The motors block is also required to have a fast mode where the provide 3.3V is boosted to 5V for high speed.

The main controller block should use a low amount of current around

50mA peak. While the motors run 250mA in there slow mode (i don't even want to think about fast) and the sensors run 3x30mA (I'm sure they can be multiplexed). Finally my block uses runs about 3mA for my PWM micro that sends battery level.

After some research i have found the LT1700 its DC to DC controller that uses 2 mosfets. I will give it a try after some more research, I am reluctant to order since all these attempts are costing me arm and a leg. I will also look into super caps, 10F cap sounds fun. Thank you all

-Rich

Let's separate the booster in two parts. First part is to oscillate to high voltage (as high as the parts can handle, probably 10s of V). Second part is to regulate it down. High voltage will minimize resistive lost, especially at the high current input stage. With a good switching regulator, there should not be too much lost due to higher voltage.

You would need the micro to oscillate/monitor the supercap voltage, then turn on the motor for the movements. You should keep a separate bootster for the micros/sensors. So, you only need 250mA for the booster. 250mA sound possible with commerical units. But as you are finding out, manufacturers don't always tell you everything.

One suggestion/optimization, if you have the time at this point: your NiMH cell will charge faster and extract substantially more energy from its solar panel if you use a maximum power point controller (MPPC) to optimally load the solar array. As an example, Stephen Woodward published a simple Design Idea entry in EDN magazine some time ago.

Supercaps are fun, but their 2.5v ratings are pretty limiting.

That looks like a good choice. Beware of the start-up requirements

-- see the datasheet's "Start Up Load Current," Fig. 4.

Best, James Arthur

Correction: "Maximum-power-point-tracking solar battery charger," Woodward, Stephen, Electronic Design, Ideas For Design, Sep. 14, 1998.

Also, MPPT seems to be the standard abbreviation, not MPPC.

offers a few more links.

Best, James Arthur

Hi,

This app note from Maxim addresses the step-up conversion of very low voltages by having a separate converter provide the drive for one that provides the main power. This way an external FET can be used with very low on-resistance requiring higher gate drive.

This design can produce 2.5W at 5V with inputs down to 0.8V. It can be easily adapted to 3.3V out by changing the feedback reistor on the output stage.

kevin

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.

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.

I tired building another boost using a MAX1709 last night. I will test it this afternoon.

Look up "beam" robots... you have the same problem, which is a voltage source which won't power your motors. They charge a cap, and then burst the motor for a bit. The *average* power is thus decreased, and the momentary power while the thing is going can be arbitrarily large.

You won't need supercaps for this, just normal caps. In fact, smaller caps will allows the voltage to recover much faster, so you might be able to keep the robot moving, thus working around the additional current required to restart the motor if it stops.... Note that the actual power you can pump into the motor is the same regardless of the cap size; all the size does is select the frequency (vs pulse width) of the supply.

However, I have little johnson motors that will easily run down to 1V. A gearbox could decrease the torque load on the motor sufficiently to allow it to move, albeit slowly. You can get gearboxes online.

--
Regards,
  Bob Monsen

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revolutionary way. The legal way involves the offer of alternatinve
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