Project Mentorig Question: Powering a project?

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air.command
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Project Mentorig Question: Powering a project?

Postby air.command » Thu Jan 13, 2011 7:01 pm

Okay, I’ll get the ball rolling.

I wanted see if I could get some suggestions on powering my current project. It’s a small PIC based timer for controlling an RC servo motor for parachute deployment. I want to give people the option to power the timer from as many possible power sources as possible. The circuit itself will probably draw 20mA at most, but since people will supply their own RC servo motor they could potentially draw up to 1 Amp, though 100-300mA would be more common for the small servos.

The timer can run from 2 to 5.5V, but the standard servo requires 4.8 - 6V. I would like to be able to power the whole thing from anywhere between 6 and 9V (perhaps 12V?). That allows people to use a heavy but simple 9V battery or a lighter 6V battery (such as 2 x 3V CR123As) or a pair of small and very lightweight LiPos at 7.4V. etc.

I normally use a 7805 regulator with a 9V battery (as with my previous project ) or optionally 6V battery with a simple (but inefficient) power diode to drop the voltage. I couldn't use a 7805 due to its drop out voltage.

My main question is this: Is there a surface mount regulator that I could use over the input voltage range (6-9V) to give me something in the 4.8-5.5V range with the 1amp capability? Ideally the target voltage would be 5V. I have tried looking at LDO regulators, but I don’t have any experience with them, or what their limitations are at higher currents, or whether they are even right for the job.

Should I leave the voltage regulator off and let people make up their own power supplies? Though I want to make it as simple for the user as possible.
If it's not possible to do it with one solution should I let people order different versions for different voltages?
What other considerations should I take into account in the power supply such as filtering, polarity reverse protection?
What voltage do common rocket electronics normally run at?

Any other suggestions would be most welcome.

The project:

It is a simple delay timer that activates a single RC servo motor to be used for parachute deployment on water rockets. The timer allows you to configure the start and end servo motor positions (stores them in the EEPROM internally) and you can select a delay as needed.

The main motivation is to keep it as inexpensive as possible as many water rocketeers are really on shoe string budgets. Small size and weight are also important, not just for flying on rockets, but also shipping through the post. It will also have a small footprint.

The system just self arms 10 seconds after power-on and detects launch to start the timer. I’m also giving the user a number of options for triggering the timer either through a built in G-switch, or external make or break contacts.
You can select the delay in 1 second increments up to 13 seconds, where a 0 delay can be used when the trigger comes from apogee detecting sensors such as the uMAD, barometric sensors or other flight computers. The 0 delay can also be used with burnout detecting sensors when the timer is used to activate a staging mechanism in multi-stage water rockets.
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strud
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Re: Project Mentorig Question: Powering a project?

Postby strud » Thu Jan 13, 2011 8:02 pm

What voltage is you PIC running at ?

There are some rather 'grunty' small LDO SM regs about.

Here's an example :



However, they are not that LDO really ie this one needs a 1.3V overhead for 5V @ 1A....

Better option is to use a reg for the PIC and run a 6v supply straight thru for the servo.

CS

PK
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Re: Project Mentorig Question: Powering a project?

Postby PK » Thu Jan 13, 2011 8:43 pm

To answer your question directly
http://search.digikey.com/scripts/DkSea ... P-5.0CT-ND

Looks like the best compromise between 6-9V input and 1A output.

However..... being able to power the circuit from a range of different power sources may not be all that you think it is.

LiPo batteries have the best current density of any available rechargeable battery.
LiPo batteries have the best energy density of any available rechargeable battery.
LiPo batteries have the highest cell voltage of (almost) any rechargeable battery.
LiPo batteries are the cheapest batteries available.

So, for any project, you really do need to ask yourself, "Can I run straight from a LiPo?"
It'd be an interesting experiment to take one of your timers, hook a servo up and feed 3V in to the downstream side of the 7805.
If it works, then the real answer to your question is: "You don't need any regulator, just run from a single LiPo".

I'd imagine there might be some variation between servo's, but if you can pull it off then your circuit just go way simpler...

Better yet, switch to a "nichrome wire melts fishing line" setup. There's no shortage of current available from a decent battery.


Is the circuit for your timer online somewhere? Happy to help drive the component count/cost down..

PK

ogivemeahome
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Re: Project Mentorig Question: Powering a project?

Postby ogivemeahome » Thu Jan 13, 2011 9:05 pm

You might consider investing in a Jaycar catalogue (a few bucks). They also have a half decent website.
Somewhere in all that you'll find variable regulators - the output voltage is set by the value of a resistor
which you put in the feedback loop. With multiple resistors and one of those fiddly little rotary switches,
you could even provide user selectable voltages (if it needs to be that versatile?)

I'm intrigued by the idea of using capacitors as a power supply - downsides;
- sufficient charge likely to be bulky
- requires switchmode buck/boost regulator (likely to be bulky)
- as complexity increases, reliability suffers (or cost goes up)
It would be interesting to see a weight comparison.
A smooth sea never made a skilled mariner. (English Proverb)

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Re: Project Mentorig Question: Powering a project?

Postby PK » Thu Jan 13, 2011 9:40 pm


air.command
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Re: Project Mentorig Question: Powering a project?

Postby air.command » Thu Jan 13, 2011 10:16 pm

Crop Circles: ... just a bunch of guys looking for their rockets ....

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Re: Project Mentorig Question: Powering a project?

Postby PK » Thu Jan 13, 2011 11:04 pm


Andrew Burns
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Re: Project Mentorig Question: Powering a project?

Postby Andrew Burns » Fri Jan 14, 2011 5:18 am

If you're worried about charging the lipo you could build the charge circuit into the board. There are a few single IC lipo charge controllers for mobile applications that don't cost a whole lot and take up practically no room, for example: the LM3658 http://search.digikey.com/scripts/DkSea ... 8SD-BCT-ND

Supply the IC with 4.5 to 6V and it will charge at a user selected current from 50mA to 1A, it's really designed for USB charging so perhaps just slap one of these and a mini USB socket on the board and let people charge it from their computer (or any number of cheap wall socket to USB adaptors).

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Re: Project Mentorig Question: Powering a project?

Postby PK » Sat Jan 15, 2011 10:18 pm


flyonline
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Re: Project Mentorig Question: Powering a project?

Postby flyonline » Sun Jan 16, 2011 8:35 pm

Would it be possible to use something like ? He also has others with lost alarms in them.

I have also run 2 micro servos off very small 1s Lipo for long(ish) periods of time, say 15-20min with no troubles at all. Speed and torque where down but it's certainly do-able and with cheap Lipos costing peanuts these days seems a good option. Only downside is the possible fire risk in a crash, but the LiFe's etc offer a safer alternative.

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Re: Project Mentorig Question: Powering a project?

Postby PK » Sun Jan 16, 2011 8:56 pm

That's what AC is doing now. It's a 5 volt linear regulator. The problem it creates is that you need double the weight of batteries to get the same result. A boost converter and a single LiPo is an option, but the parts cost jumps a bit.
PK


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