## Power harvesting cap

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Hi Freaks,

Have a pump that shoot back power on supply line.
My intention is to harvest this power in a cap.
Reason for this is the device runs on battery and I want to save battery life by collecting the energy in the cap and then feed it back again on pump supply lines.
The back voltage reach 30V at peak (says my work mate)
Any considerations what cap to use? Size?

Have a weak memory reading some other post that low ESR caps are needed if you want to suck up large amounts of power fast.
Or was that a dream??

4-9VDC      0--------->|------------0
|
=          PUMP
|
GND        0-----------------------0

Explain how a pump "shoots back power on the supply line"? Surely the pump uses more power than it "shoots back"? Otherwise I would love to have a schematic for it... Unless you are talking about the huge spikes an inductive load produces?

I sounds to me like the OP is seeing a voltage spike due to a load dump. When the power is shut off to the pump motor it is still spinning. It then acts as a generator with no where for the power to go. The voltage jumps. How much actual power can be saved in a cap is anyones guess. The OP seems to desire the equialent of regenerative braking on cars.

Someguy22 wrote:
The OP seems to desire the equialent of regenerative braking on cars.
I have worked on some supplies that use this principle. Good analogy.

Lennart, How about providing some timing values. What frequency? How wide are the 30V Spikes.

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Last Edited: Thu. Aug 11, 2011 - 06:37 PM

Yes, I guess voltage spikes are the correct term.

Done some testing today.
Looking at the supply line after the diode in the schematic from my OP with a scope show 16V spikes when the pump is fed with 8V. They come at a rate of appx 60Hz (the speed of pumps piston).
Adding 330Âµ low ESR cap as in schematic lower the spikes to appx 9V.

Connecting an ampmeter to the supply before diode show that the pump draw 32mA with no cap and 27mA with the cap.
In the pumps normal working environment it will also at times be subjected to resistance from small water droplets in a 6 feet tiny tubing connected to suction side. This raise the spikes to 30V+ with no cap and only appx 12V with the cap added.

So "harvesting" the spikes into the cap should make the battery last quite a bit longer.
An unexpected bonus is that it also makes the pump strokes less noisy.

Still have the question to the sparkies in the forum for any considerations what type of cap to use.
Material or other characteristics.
The pump is speced to run at least 10.000 hrs at appx 40 degrees C. The cap must last at least this long.

Have you considered getting rid of the blocking diode and just letting the current go back into the batery? That's if the battery's chemistry allows recharge.

Quote:
Have you considered getting rid of the blocking diode and just letting the current go back into the batery? That's if the battery's chemistry allows recharge.

This is how the system works today.
I have to dig deeper to be able to tell if the battery can benefit from these spikes.
After all there is a circuit responsible for recharging the battery but I suspect that there are current limiting involved that would prevent too much current to flow into the battery.

High current and high voltage charging spikes on a normal battery chemistry, as far as I know, will ALWAYS adversely affect the battery lifetime.

That said: Step1: Use solid capacitors anywhere you want maintenance free, low-ESR, long lasting operation.
The best would be ceramic NP0 dielectric capacitors, but I think your budget might not support that decision.
After that it's hard to predict whether X7R ceramic or solid Tantalum low-ESR wins. Depends on the exact specifications and the amount of caps you can put in.

After that anyone's guess is as good as mine as to what ESR value would be useful and what is pointless, just on the basis of "there is a spike, which is 30V undapened"

The fact that the drive current is 30mA makes me wonder if you really need low-ESR to make the difference. After all a normal solid Tantalum can also be found with 0.6ohm. 0.6 * 0.03 = 0.18.

Of course, the lower your ESR the lower that voltage waste.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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You better first take a look at balancing the motor. You did it already partially, but it seems that the motor has no rectifying cap what soever....

You could use a flyback/feedback circuit to harvest the energy that is stored in the motor when you shut it down.
that could save you power. Also indeed a cap should balance the motor as the indictive spikes that you have when powereing the motor will be more or less counter acted by the caps you installed. this will save batteries as the spikes will be less violent on the batteries.

is it a free running motor or one that immidiately stops when de-powered?

@Asmyldof
Thanks for the suggestions about what cap would be suited.

@meslomp
The "motor" is constructed like a solenoid.
An electromagnet surrounding the piston turn on and pull the piston in one direction.
A spring pushes it back when the magnet is not powered.
So I guess you might say that it is turned on/off at appx 60Hz and stops immediately when no power is applied.
I'm examining the possibility to harvest power btw every on stroke.