Voltage Booster supplier?

[gambit7]

Ok... I picked up a handy-dandy adjustable switching voltage regulator from Dim. Engineering to step-down a 7.4V 300mah lipoly pack to basically between 5v-6v. Going this route saved me 10 grams, I'm definitely happy about this.

My question NOW is, given I can still get the CG right on the glider I'm building, where can I find a voltage BOOSTER that will take a SINGLE-cell 3.7V 300 or 400 mah lipoly up to 5V? I've seen inklings of them mentions for micro-indoor flyers, but my applications is a Hitec Micro 555 running 4 Dymond D47 servos.

Anybody know any suppliers?? If not, I guess I'll need some kind of schematic. :(

Anyways, if all goes well I wont be sacrificing too much mah and I'll be saving overall around 22+ grams... almost an OUNCE.

[TimAuton]

I've been shopping for a DC-DC converter recently (to power a 12V camera on my 6V robot), so I had a look at the links I'd found. I couldn't see any packaged DC-DC coverters which take 3.7V in and give 5V out with decent current (I found one that would do 100mA), but there are loads of ICs which form the heart of what you need. You have to add a few discretes to finish the circuit but that shouldn't be too hard, the datasheets tell you what other components you need and give you a circuit diagram.

Search for 'dc-dc converter' on Digikey (or your favourite electronics bits emporium), plug in the right parameters and you'll find a nice selection.

[Comatose]

We've got one coming out in about 2 weeks. Its about the same size as our adjustable voltage regulator (an inch tall by half an inch by half an inch.) Its a SEPIC converter that will step up or step down. Input will be 3-12v and output will be 3-12v. Its rated for half an amp max in or out. Three terminal device with input, output and ground pins, and a potentiometer to adjust.

It'll probably be priced around $25. Honestly its not powerful enough to do 4 servos, but applications like cameras or charging batteries off solar cells works well.

For an even lighter weight rx pack, why not use a 2s kokam 145 mAh pack and the step-down regulator? The 145 pack is only 9 grams, and will give you the same total power and flight time as a one cell 300mAh pack.

[gambit7]

We've got one coming out in about 2 weeks. Its about the same size as our adjustable voltage regulator (an inch tall by half an inch by half an inch.) Its a SEPIC converter that will step up or step down. Input will be 3-12v and output will be 3-12v. Its rated for half an amp max in or out. Three terminal device with input, output and ground pins, and a potentiometer to adjust.

It'll probably be priced around $25. Honestly its not powerful enough to do 4 servos, but applications like cameras or charging batteries off solar cells works well.

For an even lighter weight rx pack, why not use a 2s kokam 145 mAh pack and the step-down regulator? The 145 pack is only 9 grams, and will give you the same total power and flight time as a one cell 300mAh pack.

Ahh see that's the thing I was confused about... if you step down to a lower voltage how do you calculate exactly the capacity you're going to have assuming an 80% efficiency? And in vice-versa, if you step UP.

Comatose; as per what you're building, you could make a killing selling just 1.5A step-up regulators if you could keep them under an inch square.

As for the 145 pack, what I was worried about is actually getting TOO light :rolleyes: yah, go figure. Preliminary looks seem to point to even my CURRENT setup being too light, so there's almost no point going for the lower weight. Also, my current battery is 2 cell 7.2v 300 mah, if I wanted to just halve the weight all I'd have to do is separate the cells.

[ZAGNUT]

lots of people are getting away with using unregulated voltage from two li-pos for the flight pack. most receivers can easily handle this but servos are the grey area.

i'm doing this with an FMA FS8 and two ancient airtronics 401 mini servos but being a chicken i added a diode to each servo lead. this way they're seeing about the same voltage that a freshly charged 5 cell pack would put out.


dave

[Comatose]

Calculating capacity:

What you're really dealing with is watt-hours in both cases. Your energy is your mAh * avg voltage. Assuming a 300 mAh, 3.7v cell, you end up with 1.11 watt hours. two of them would have twice that much, 2.22 watt hours.

If your converter is 80% efficient, then the energy availiable at the output is .8 times the energy in the battery, or .888 and 1.776 watt hours.

Now, you can convert back to amp-hours. That's just the calculation in reverse: Watt Hours/Voltage = Amp Hours

So at 5v out with an 80% efficient converter you end up with the equivalent of a 175 mAh battery with one 300 mAh cell feeding a boost converter, and the equivalent of a 350 mAh cell with two 300 mAh cells feeding the step-down converter.


Yeah, we'll probably end up making a boost regulator at some point. SEPIC is a really versatile topology, though. For example, if you have a solar array that's rated at 12v full sun, and it puts out 4v when there is cloud cover, and are powering a 5v load, with a buck regulator you'd shut down below 6v in, and with a boost regulator you'd shut down above 5v in. With a sepic you can get a nice, steady 5v out with an input ranging from 3 to 12.

Or 6v out, or 9, or whatever you're going for.

The real rub with boost converters is that everyone always thinks in terms of output current. A 1.5A step-down converter is easy enough to design - you can have up to 1.5A out and that means you have 1.5A or less in.

A boost isn't as simple. If it was a boost converter from 3-15v in and 3-30v out, rating it gets hard. If you need 1.5A out at 30v with a 3v input, that's 18 or so amps at the input. If you rate by input current then you only get 150mA out (less, really) at 1.5A input going from 3v in to 30v out.

Maybe with a nice big chart or little web calculator everything would work acceptably.

[gambit7]

Sweet spot seems to be single-cell 3.7 V 400mah battery OR 2 cell 7.4V 190mah. Both are about the same weight and yield about a 250mah pack.
Of course, we're assuming the regulators/boosters would be the same weight as well. :rolleyes:

Anyways, good to know I can shave another 5 grams off if I need to... although I can fly all day on this setup... tee hee

Kinda trick having a 19 gram 388mah battery pack... which is what my 300mah pack is at 90% efficiency. Geez, i love lipoly.