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Best Way of Boosting 7.4VDC to 12VDC? Answered

I need to power two stepper motors off of 7.4VDC, and am trying to figure out the best, most cost efficient way. 
What type of DC booster circuit would be best for this application?
Do I need any noise filter? If yes, what kind?
If I understand correctly, Stepper motors need a constant current. Does this limit my selection of DC boosters/step up devices?

Discussions

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Downunder35m

Best Answer 2 years ago

Yet another topic to the same question won't give you different answers ;)

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RocketPenguinDownunder35m

Answer 2 years ago

Well, considering I've gotten zero answers so far on any of the other questions that related to the same project (yet are not the same question. Yes, it may contain this question, but this post is specifically asking for one thing, and one thing only.) I am rather saddened that something that isn't all that difficult (or, at least I'd imagine; I don't actually know the answer) cannot be answered.
Furthermore, I've asked this same question on multiple other forums, all with no answers thus far. If there is something I am missing from the question, some information that would lead to the answer, please do inform me.

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-max-RocketPenguin

Answer 2 years ago

I'll try to give you a better answer!

What you need is a boost converter. The simplest way is to go on ebay or amazon and buy a brick or a PCB with whatever chip that claims that it boosts voltage. Typically you search for terms like "Booster" "Boost converter" "Boost regulator" "Flyback converter" "switch mode buck boost" Stuff like that.

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If you cannot find a device that meets your criteria, then you may need to go all-out and actually make one yourself. Go to Digikey or Mouser and use the parametric search in the voltage regulator section. Make sure to narrow your search to switching regulators and DC-DC converters. Use the parametrics to further narrow down your search and you can organize the listings based on any criteria (price, efficiency, etc.)

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Select a converter rated 20% higher than your expected maximum load, and use the datasheet to see the efficiency curves. Typically the higher the boosted voltage and larger the difference is between input and output, the less efficient the converter will be, and the closer you are to zero power draw and maximum power draw the less efficient the converter will be.

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Lastly, layout of the circuit is CRITICAL for good performance. Breadboarding the circuit will probably work sorta-kinda (with the regulators that operate under 100 KHz) but you do need to pay attention to the layout. The circuit to build is offered in the application note, usually along with a recommended PCB layout. Make sure all traces are as short and thick as possible, and surrouned by ground plane.

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RocketPenguin-max-

Answer 2 years ago

Thank you for posting an informative, helpful, and topic related answer. However, I've come to find that the simplest solution is not to boost the voltage, but to find an equivalent part that runs at the current voltage.

Thanks again for providing a legitimate answer!

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-max-RocketPenguin

Answer 2 years ago

No prob. Im pretty sure 7.4Vdc comes from a 2S lithium battery, which can vary from 8.4V down to 6V. You may consider testing the performance of your circuit over that range.

Ideally there should be a safety cuttoff at below 6V, and the performance should remain acceptable over the full range.

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RocketPenguin-max-

Answer 2 years ago

That's what I will be using as a power source.

The Arduino and the servo shouldn't have a problem. The only main concern is the Stepper motors. Not so much for the over voltage, but rather for the undervoltage. I don't foresee it to be a big problem though. The batteries we purchased for our project exceed the capacity requirements by 250%, so I'm hoping the voltage will remain at or above the 7.4v rating for the 20 minutes the robot will actually be in operation.

We don't have a safety cuttoff, but we do have a module that notifies us when one of the cells is low.

Thanks for the tips!

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-max-RocketPenguin

Answer 2 years ago

Margin in the capacity is good, hopefully the manufacturer or seller of the battery was not lying, which is very common with those xxxFire 18650 batteries on eBay and Amazon. Any one rated over 3500mAH is definitely a lie.

This graph should give you a better idea of what to expect, it shows discharge curves at a few different rates. the C rating is based on the nominal capacity of the battery (the AH rating). You can see that at slow discharges you should be fine, as the whole curve is higher. But as you draw more current, the overall voltage sags.

https://www.maximintegrated.com/en/images/appnotes...

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RocketPenguin-max-

Answer 2 years ago

It's a Turnigy 2S 30C 5000mAh LiPo off of HobbyKing. Should theoretically power the device for around 1.5 hours, which is well over what we need. Rather keep it safe than sorry!

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Downunder35mRocketPenguin

Answer 2 years ago

I answered a related question of yours....
As Iceng already pointed out it is always best to have all connected devices matched to the battery voltage.
And a simple Google search gives this for example.
Not 7.2V but should be close enough to work with unless you need a lot of torque...
You can even motors for less voltage, but mostly for small things like in DVD players or the old floppy drives.

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RocketPenguinDownunder35m

Answer 2 years ago

I didn't realize 7.4v Stepper motors existed. Problem solved, thanks!

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Josehf Murchison

2 years ago

Well you have three choices:

A jewel thief, small loads and low current.

A buck boost converter, not much better than a jewel thief.

And a DC to DC power inverter capable of handling large loads.